AN INVESTIGATION AND ANALYSIS OF RATIO

' WAVES OF EXTRATERRESTRIAL ORIGIN

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

By HSIEN-CHING KO, B.S., M.S.

The Ohio State University 1 9 #

Approved by:

_ Adviser Department of Electrical Engineering ACKNOWLEDGEMENTS

The research presented in this dissertation was done at the

Radio Observatory of The Ohio State University under the supervision of Professor John D* Kraus who originated the radio astronory project, designed the radio telescope and guided the research.

The author wishes to express his sincere appreciation to

Professor Kraus, his adviser, for his helpful guidance, discussion and review of the manuscript. For three years he has continuously- provided the author with every assistance possible in order that the present investigation could be successfully completed. Dr. Kraus has contributed many valuable new ideas throughout the investigation*

It is a pleasure to acknowledge the work of Mr. Dorm Van

Stoutenburg in connection with the improvement and maintenance of the equipment. Thanks are also due many others who participated in the construction of the radio telescope. In addition, rry thanks go to Miss Pi-Yu Chang and Miss Justine Wilson for their assistance in the preparation of the manuscript, and to Mr. Charles E. Machovec of the Physics Library for his cooperation in using the reference materials•

The radio astronony project is supported ty grants from the

Development Fund, the Lovejqy Memorial Fund, and the fund for the basic research of The Ohio State University and also ty a grant from the National Science Foundation* TABUS OF CONTENTS

CHAPTER

I INTRODUCTION

1.1 Historical 1.2 Radio Astronomy at The Ohio State University 1.3 Statement of the Problems

II APPARATUS AND THEORETICAL CONSIDERATIONS

2.1 Quantitative Measurements and Units 2.2 Limit of Detection 2.3* Attenuation between Antennaand Receiver 2.U Celestial and Antenna Coordinates in Radio Astronomy 2.^ The Antenna Relationship 2.6 The Ohio State University Radio Telescope

III OBSERVATIONAL PROCEDURE

IV LOCALIZED SOURCES OF EXTRATERRESTRIAL RADIO RADIATION

U.l Introduction i|. 2 Detection of Radio Sources 14.3 Distribution of Radio Sources l+.U The Radio Magnitude, Color and Spectrum Index U.5 Indentification of Radio Sources

V COSMIC RADIO BACKGROUND RADIATION

5.1 Introduction 5.2 Construction of a Radio Map of the Sky 5.3 Discussion of the Distribution of Cosmic Radio Background Radiation

VI A RADIO MODEL OF THE GALAXY

6.1 Introduction 6.2 Galactic Structure 6 . 3 The Center of the Galaxy 6.U Construction of a Radio Model of the Galaxy 6.5 Comparison with the Andromeda Nebula

VII SUMMARY

BIBLIOGRAPHY CHAPTER I

INTRODUCTION

1.1 Historical

The young science of radio astronomy is coining of age. Using

radio antennas and receivers instead of the conventional optical

lenses and eyepieces it has opened a new window on the universe.

Although Sir Oliver Lodge speculated on the possibility of de­

tecting solar emission at the beginning of this century, the fundamental

discovery in radio astronomy was made in 1932 when Karl Jansky (37) of

the Bell Telephone Laboratory found the first evidence for the existence

of radio radiation of extraterrestrial origin. These accidental obser­ vations by Jansky opened up a new era in astronomical investigations,

since our knowledge of the universe had previously been gained only by observations of the radiation in a narrow range of optical wave­ lengths. To these light waves from the stars and galaxies for the astronomer's telescope there have been added radio waves for the radio astronomer's antennas and receivers. These radio waves can penetrate the earth's atmosphere and ionosphere over a wide range of frequencies.

As seen in Figure 1, the optical window covers the visual and near -k -3 visual wavelengths from about 3 x 10 cm to about 10 cm (about five octaves), while the radio window extends from about 1 cm to at least

20 m (about 11 octaves) which is restricted by the ionosphere on the long wave side and by molecular absorption bands on the other side.

Between 1937 and 19U8* Reber (66, 67) studied the cosmic radio radiation with apparatus especially designed for the purpose and mapped the distribution of the galactic radio radiation brightness with some Ui ' »- ID >

I £ I o OZONE o MOLECULAR IONOSPHERE ABSORPTION ABSORPTION RADIO REFLECTION I I < o I- Q_ O I J__ 1 I -10 -8 -6 - a -2 10 10 10 10 10 I 10 10 10 WAVELENGTH IN cm

Fig. 1. The electromagnetic spectrum showing the regions of transparency of the earth's atmosphere. detail. He also attempted to detect radio radiation from the sun and extragalactic nebulae without success.

In 19U2 and 19k3 Southworth (?U) succeeded in detecting radio radiation from the sun at wavelengths from 1 to 10 cm. Appleton and

Hey ( 1 ) investigated observations recorded during the second world war with radar equipment and found a significant relationship between intense solar radio outbursts, sunspots and flares. But the emergence of radio techniques as a powerful tool in astronomical investigation has only oceured during the past ten years.

During the work of Appleton and his colleagues on the nature of the ionosphere, they observed the short period sporadic reflection of radio waves which has since achieved such importance in the study of meteors. This has opened the field of "Meteor Astronomy".

The next major discovery was that of the localized radio sources or so called "radio stars". In the flilky Way radio survey made by Hey,

Parson and Phillips (36) at 6i| Mc/s, they noticed that the radiation from the direction of Gygnus showed fluctuations in intensity and suggested that at least part of the galactic noise was due to a very intense discrete radio source. Afterwards a detailed study of this region by Bolton and Stanley (ID), using, a Lloyd-mirror interference '1 technique, confirmed the existence of a discrete radio source less than

8 minutes of arc in diameter in the constellation of Cygnus. Ryle and

Smith (69) found another intense discrete source in the constellation of Cassiopeia, using the radio version of Michelson's stellar inter­ ferometer. At .the present time more than several hundred radio sources have been found and the number increases as more sensitive h radio telescopes are brought into use. Some radio sources have been identified by Baade and Minkowski (5>6), using the 200 inch Mt. Palomar

telescope, with interesting optical objects such as colliding galaxies, peculiar nebulosities and remnants of supernovae. In 195?1, Hanbury

Brown and Hazard (28) succeeded in detecting radio emission from extra- galactic nebulae such as the Andromeda Nebula and from an aggregate of

such nebulae. The distribution of radio brightness across the Andro­ meda Nebula has recently been studied by Professor Kraus with the narrow beam of The Ohio State University radio telescope. Radio radiation from the supergalaxy was detected by Professor Kraus and Ko (1+6) in 1953*

The possibility of radio astronomical spectroscopy was first con­ sidered by van de Hulst (82) in 191+1+ but it was not until 19$1 that the

21 cm hydrogen line was detected by Ewen and Purcell (22) at Harvard.

This result was confirmed only a few weeks later by Muller and Oort (5>8) at Leiden and a little later by Christiansen and Hindman in Australia

(19). This achievement has provided a new way to study the spiral structure of our own galaxy. The contribution of radio astronomy to the knowledge of our galaxy may be greater than to the knowledge of the sun and meteors, van de Hulst, Muller, and Oort (83) have recently publish­ ed charts showing the structure of the spiral arms using their 21 cm observations.

Investigations at centimeter wave-lengths have recently been start­ ed at the Naval Research Laboratory (Washington, D.C.) and many ionized hydrogen nebulae and H II regions have been detected by Haddock, Mayer and Sloanaker (26) at a wave-length of 9.1+ cm and by Hagen, McClain and

Hepburn (27) at 21 cm.

$ 1,2 Radio Astronomy at The Ohio State University

A radio astronomy project at The Ohio State University was initiat­ ed in 1951 by Professor John D. Kraus who designed the radio telescope and has directed the program. In the summer of 1952, a radio telescope consisting of an array of 2h helical antennas was completed and put into use ibr the preliminary survey of the galactic radio radiation at

250 Mc/s, In the winter of 1952 the antenna size was enlarged to I4.8 helices and the galactic and localized source observations were begun.

In this survey 207 localized sources were detected and a radio map of a part of the sky was obtained. Radio radiation from the supergalaxy was also detected. In the summer of 1953, the radio telescope was again increased in size to 96 helices and the receiving system was improved considerably. With this new equipment, the detailed structure of the cosmic radio radiation was studied and many localized and extended sources were detected.

Many articles have been published during the past three years (3 8 —

53) and in addition there have been 6 master's theses and two Ph.D. dissertations including the present one completed on this project. 6

1.3 Statement of the problem

Several radio surveys have been made over various parts of the

celestial sphere between frequencies of 9.5 and J4.8O Mc/s by many

observers. However, the resolving power of the antennas employed in

these earlier observations was insufficient to reveal much detailed

structure of the cosmic radio radiation. The radio isophotes were ade­

quate to establish the general features of the galactic radio radiation,

but more detailed maps obtained with a high resolution antenna are

needed in order to make a thorough study of our galaxy and the origin

of galactic radio radiation. This has been done in recent surveys with

The O.S.U. radio telescope. A description of this work and its inter­ pretation constitutes a portion of this dissertation.

It has been known that the investigation of the galactic radio rad­

iation may yield information on the shape of our own galaxy. Ihe major parts of our galaxy are hidden from visual observation, even with the

largest optical telescope, owing to the presence of interstellar gas

and dust. This interstellar matter, however, presents little obstruc­

tion to the penetration of radio waves and hence it should be possible

to obtain a model of our own galaxy by radio observations.

The first task was to conduct a radio survey of the celestial

sphere with The O.S.U. high resolution radio telescope. A detailed radio map of the cosmic radio radiation was then prepared. During the

observations, a number of localized and extended sources were detected

and their distribution in space, spectra and possible identification with optical objects were studied. 'With the new and accurate data

available, a model of the radio galaxy was then constructed. The correlation between radio and optical observation is discussed and the origin of cosmic radio radiation is considered. CHAPTER II

APPARATUS AND THEORETICAL CONSIDERATIONS

2.1 Quantitative Measurements and Units

It is the purpose of much radio astronomical observation to mea­ sure radio brightness from different directions in the sky. The bright­ ness , B, at a specified frequency is defined such that the power, dp, flowing into the solid angle dAli subtended at the observer and incident upon a normal surface of area dA in the frequency interval (f, f + df) is given by

dP = B dlldA df. (2.1)

Thus the power within the receiver band width, df, at the input terminals of an antenna, due to randomly polarized radiation, is

B dlLdA df (2.2)

The power received wtien an antenna is pointed in a given direction is determined not only by the brightness in that direction alone, but also by a weighted mean of the brightness in all the directions con­ tained within the antenna acceptance pattern.

In optical astronomy the terms "surface brightness" or "specific intensity" are used to describe the radiation emitted by the source.

2 2 2 Radio brightness has units of ergs/sec/cm /sterad/cps, watts/m /deg / cps, etc. Astrophysicists, however, prefer to express the results in terms of the brightness temperature ( or equivalent black-body temper­ ature) because of its probable relationship with the electron temper­ ature of inter stellar gas.

The available power from a resistance at a temperature T is given by Nyquist (60) as KTaf, where ii is Boltzman's constant and the band width. This idea was extended by Burgess (18) to an antenna contained in a black-body enclosure at temperature T. From the

Rayleigh-Jeans formula the power in a uniform temperature black-body enclosure at temperature T is given with sufficient accuracy at radio frequencies by

8TTKT A f (2.3) A1

If p A f is the power incident on the antenna over a unit solid angle, then

UirpAf = A f ano

P = M . 2h £ T = 3.07 X 10-28 f2T (2.14) where

p = W/m^/cps/sterad

f = Mc/s

T = °K

Since an antenna responds to only one of two polarized components of the incident radiation, the power collected by the antenna is spAAfAift., where A is the effective aperture of the antenna as given by

where D is the directivity. Therefore the power received is

2 2K A D AfAirL= DKT Af aJL (2.5) for the direction in which the antenna, directivity is D. By integrating 10

over all directions, the available received power reduces to

£££- A f e|JL- KTAf DdJh UT lllT llTT

it TT h r

= KT Af (2.6)

Equation (2.6) shows that the power available from an antenna in

a black-body enclosure at temperature T is also given by KTAf. There­ fore it would, theoretically, be possible to measure the temperature of

the enclosure by putting a resistance in place of the antenna and ad­

justing the temperature of the resistance until equilibrium is establish­ ed.. A comparison of the power available from an antenna with that from a heated resistance of the same impedance thus makes it possible to deter­ mine an equivalent black-body temperature of the source. The state of

affairs is not altered if the antenna is directive and receiving radi­ ation from a diffuse source which subtends a solid angle greater than

the beam area of an antenna SL . However, if the antenna is receiving radiation from a source which subtends a solid angle oo that is smaller

than Sh , the equivalent black-body temperature becomes

(2.7) and T is usually referred to as the equivalent antenna temperature. A The equivalent antenna temperature represents the temperature of the attached reference resistance when equilibrium is established and does not represent the equivalent source temperature Ts.

In practice, antenna beam widths are usually large and have se­ veral side lobes which are directed to different brightness levels of 11 the sky. Therefore the temperature of the heated resistance at equili­ brium again represents the weighted mean of the temperature in all the directions of the antenna lobes and is referred to as the equivalent antenna temperature. A certain correction is of course necessary to obtain the brightness temperature of the sky in the direction of the

antenna main beam.

Another case occurs in an expression of radio radiation from a

small source which subtends an unknown solid angle. Most of the local­ ized radio sources are much smaller in angular extent than that of exist­ ing radio telescope beams. In such cases, the intensity of the radio radiation can only be described in terms of total power flux received at the earth within a given frequency band, and usually referred to as the "flux density" or "intensity". In this case

Flux density S = BdAL Source extent and x-

Total collected P • £* f A / B dlh = | ^ f AS (2.8) power in the J receiver where A is the effective area of the antenna system and is related to Az ? the antenna beam area JL by the expression A = . The unit Watts/m / cps is usually used for the flux density and the unit jansky (» 1 watt per square meter per cycle per second) will be used in this dissertation. 12

2.2 Limit of Detection

In order to measure the small noise power which is collected by an antenna, it is necessary to employ an amplifier whose input circuit itself gives rise to a random noise pox^er inherent in the input circuit of the receiver that may be consid.era.bly greater than that from the antenna. In the absence of an input from antenna, even though the re­ ceiver be perfectly stable, the output of the receiver fluctuates in a random manner about a mean value which corresponds to the noise power generated in the input circuit and which may be denoted by T^. It may­ be shown that if the mean reading of the output corresponds to the total input power of KT A f = K(ri'A + TR ) A f then the mean value obtained by n instantaneous observations deviates from the true mean value by an amount given by T/ \Tn. If the band width of the circuits preceding the output recorder is A F it is then possible to obtain effectively A F independent observations per second. If the time constant of the out­ put recorder is t seconds, then it will record effectively the aver­ age of /\Fft independent observations. Therefore the standard devi­ ation of the output fluctuations corresponds to T/yAF*t . The smallest measurable power increment discernible by the receiving equipment must therefore be larger than the standard deviation of the output fluctu­ ation, or

* ta >- t VAF-t _ (2.9)

Since T = T^ + T^ and T^ = (N-1)T0 where N is the noise figure of the receiver and T0 ambient temperature, we have 13

AT a > Ta + (N-l) Tq (2.10) /I f T t

Inserting practical numerical values, t * 60 seconds, a F = 1 Mc/s,

N «* 1.5» Ta ® T » 300°, we obtain a minimum detectable temperature 28 T^ = 0.058 K. This corresponds to an intensity of 3.U x 10“ janskys/ sq. deg. at 250 Mc/s. In practice the receiver gain and noise figure do not remain constant but vary slightly with the time. Thus a change -i _ O in the gain or noise figure of the order of — (in practice 10 -

1 0 ”^ are usually used) would produce a change in the output reading corresponding to the minimum detectable temperature. They can be mini­ mized by taking special precautions to stabilize the receiver power supply and by calibrating the apparatus at frequent intervals. In

Dicke*s system, an antenna is periodically replaced by a load resistance at constant temperature Tg, and the receiver is made to respond to the switching frequencyj so that the output fluctuations are now proportion­ al to the difference T^ - Tg rather than to the total temperature TA +

(N-1)T0. It is easily seen that Dicke system is most sensitive when measuring an antenna temperature that is equal to the temperature of a standard resistor. However, the output is still proportional to tie gain of the receiver and therefore gain stability of the receiver is important.

The detection of a localized radio source of angular extent oJ much smaller than the antenna beam area SL is adversely affected by the masking effect of the general background radiation. In order to detect radiation from such a localized source, the equivalent antenna temper­ ature of the source should be comparable to that of the general background radiation. The equivalent antenna temperature T^ of a radio source at 1U temperature Tg is given by Eq. (2.7) as T^ = ^sTriT ' ^ ^-oca-^-ze^ source is in the direction from which the general background radiation has an equivalent antenna temperature of Tg, it is difficult to measure radiation from the localized source unless Tg-^; is comparable to Tg.

Moreover, the detection of localized radio sources is limited not only by the sensitivity of the receiving equipment and by the masking effect of the general background radiation, but also by confusion with near-by radio sources. It is impossible to resolve localized sources into individual ones unless the beam width of the antenna is narrower than the angular separation of the radio sources. Since the directivity 4ir of an antenna is related to the beam area iL of an antenna by D = , the ultimate number of localized sources which could be resolved by the pencil beam is approximately equal to the directivity D of an antenna.

Thus, the directivity of an antenna is an effective indication of the ultimate number of radio sources which a radio telescope is capable of resolving. 15

2.3 Cable alternation between Antenna and Receiver

The power collected by the antenna is passed through an element such as a coaxial cable or a transmission line from the antenna to the receiver. We shall consider effects of such a transmission element.

The cable has two effects; attenuating the power which passes through the cable, and generating noise power within the cable. Thus, the available power at the output of the cable will consist of the noise power generated within the cable and the attenuated input.

Antenna. _r\ Receiver A T A (I x ->|f dx-M

Fig. 2,

If oi. is an attenuation constant per unit length of cable at temperature Tc, the power available at the output of the cable due to a thermal noise generated within an incremental length dx, of the cable is dl' = T e_0

= T oC e-G< xdx (2.11) 16

The total power available from the cable of length L then is

• L

To = J dTc 0

f L = WT e" “^d x Jo c

= Tc(l-e_o/L) = Tc(l-r) (2.12) xvhere T = e“

Therefore, the total noise power Tr at the output of thecable is

T.r = TA + Tc (1" T:) (2*13) where T^ represents an equivalent temperature corresponding to the in­ put power to the cablefor

'A - Tr ~ (2.llj.) cn For a lossless cable, = 0, and TT = 1. The contribution of power from the cable is zero and the total input power passes through the cable without alternation.

The effect of the cable loss on the measurements of the input power from the antenna is illustrated in Fig. 3. As *tr-»l, the line

(a) approaches to the line (b) where the output is always equal to the input power.

It is also obvious that the output from a cable exceeds tiie input to the cable when T, is lower than T . Therefore, the accurate measure- A c ’ ments of an antenna temperature which is much lower than the cable temperature becomes more critical and difficult.

Sometimes a transmission system consists of more than one cable 17

■with different temperatures. For example, in the case of the Ohio State

University radio telescope, there are three types of cables,- one exposed in open air, one underground and the third at room temperature. It can CT> be shown that n elements of cables with attenuation constants of a,,

--•JT.n.and temperatures of T^, T2 , will have the total available power of

Tr = *Cta + Tl(1~ ^ ' ^ + T2(1" ^

+ V-j/l- T*-,) t n + Tn (l- t n ) (2.15) where

*L - "" ^

T T r

P Ph a

Input T A Fig. 5* Cable attenuation between antenna and receiver 18

2.U Celestial and Antenna Coordinates in Radio Astronomy

In radio astronomical observations, the presence of a localised

radio source is registered as a deflection on the record when the

source passes through the main beam of an antenna. However, similar

deflections mag- also appear on the record when an intense source passes

through a minor lobe of an antenna. As improvements are made in the

sensitivity of a radio telescope, the spurious deflections caused.by

minor lobes of the antenna become more pronounced. These may cause

confusion in the interpretation of the data and may result in ghost

radio sources which do not exist.

It is therefore desireable to investigate thoroughly the locations

of all minor lobes of an antenna,. As the altitude of an antenna main beam changes, the positions of minor lobes in the sky' also change. The

characteristic of an antenna system is usually described by means of

antenna polar coordinates, while the observational results in radio

astronomy are described in celestial coordinates.

W e shall consider the following problem:

Given the antenna polar coordinates Q (polar angle) and ^ ( a z ­

imuth angle), to calculate its celestial coordinates cT (declin­

ation) and H (hour angle) for any given altitude of the antenna

beam corresponding to the celestial declination S'o .

That is, we wish to find

S = (f ( G , , £ ) (2.16) H = H ( 0 ,

S o ) (2.17)

In ’the spherical triangle PZX (Fig. U), we have, by the cosine formula, since two sides PZ and ZX and the contained angle PZX are given, 19

Fig* 4* Celestial and antenna Coordinates* 20

cos PX = cos PZ cos XZ*sin PZ sin XZ cos PZX or

sin & = cos (£o cos Q + sin sin 0 cos (2.18)

Thus £ can be calculated directly from Eq. (2.18).

By the sine formula

sin XPZ _ sin PZX sin XZ sin PX or

sin H = sin (b -S---n■ (2.1?) T cos^ x

Thus II can be calculated from Eq. (2.19) by substituting cT obtained from Eq. (2.18) into Eq. (2.19). For a particular case, where

Eqs. (2.10) and (2.19) reduce to

sin c?T = cos 0 (2.20)

sin H = sin (2.21) 2.5 The Antenna Relationship

The distribution of cosmic radio radiation received at the earth is conveniently described in terms of the celestial coordinates: right ascension OL and declination cT . The intensity distribution of the

, ).

However, the antenna polar coordinates are more convenient in specify­ ing the antenna characteristics. The antenna polar coordinates define a direction in space provided the orientation and the locations of the antenna are specified. Hie directions are indicated by a polar angle

9 and longitude angle

When the radio telescope is used to collect the radio waves from the sky, the output reading of the radio telescope does not tell us the distribution Tg ( c( , S ) of the sky temperature, but a weighted mean of the values of Tg ( o{ , cT ) within the antenna acceptance pattern. The finite beam width of the antenna always produces a smoothing or averaging effect on the true distribution, lowering and broadening the peaks, and reducing the amplitude of the rapid variation.

We denote the observed distribution T0 ( o(0 , S 0), when the antenna beam is oriented to the directions corresponding to ( olt , £,). Then the functions Ts ( d , § ), T0 ( , S 0 ) and A ( ^ , 9 ) are connected by the intergral equation,

T0( ^ , £ ) = K J Tg( , 6 ) A ( , 6 ) d JL (2.22) i^TT where K is a constant depending on the radio telescope. Therefore, 22 there remains a difficult problem of restoring the true distribution

T ( o( ) from a knowledge of the antenna polar diagram A (

Let us now turn to the study of the following integral equation Oo co

TQ (x, y) = ^ Ts(x-u, y-v) A(u, v) du dv. (2.23)

— oo -Co This expresses the observed distributions in two dimensions measured by the apparatus which has an instrumental function A(u, v).

For the case of one-dimension, Eq. (2.23) reduces to oo

TQ (x) = J" t s (x -u ) A(u) du. (2.2U) — 90 Let us assume that T , Ts and A are all Fourier transformable. Then by definition of the double Fourier transform, 0° OO

F(p, q) = yv j / Tc (x, y) e ^ P x + ^ d y (2.25)

— Oa -oo The inverse is 0° oo

Tc(x, y) = irr J j F(p, q) e“j(Px + ^ dp dq(2.26) - O O - oo

Let T0, Ts, and A denote the Fourier transforms of T0, Ts and A re.- spectively. Then we have, from the two-dimensional convolution theorem

Tc (p, q) = 2~ir Ts (p, q) A (p, q). (2.27)

This leads at once to a formal solution of the true distribution in terms of Fourier transforms, (see Burr (17) and Bracewell and Roberts (16))

Ts (p, q) = To (P> I? (2.28) 2t t i (p*q) 23

In the one dimensional case, this reduces to

T (p) = fo (P) (2.2?) 2 A(p).

If A * 0 for all p and q, the solution Ts(x, y) exists and is unique.

However, if A ■ 0 at certain points, then TQ ought also to vanish at these points from Eq. (2.27), and Ts becomes indeterminate at these points. It is thus clear that the nature of A plays an important role in our problem.

It has been known that in any practical antenna, the Fourier trans­ form of its polar diagram vanishes outside a finite region. The effects are to eliminate a semi-infinite band of high frequencies and alter the relative amplitude of the lower frequency components of the Fourier transform of the true distribution. That is, some of the detail in the true distribution is irretrievably lost, and only the modified low fre­ quency component may be recovered. Therefore, any method to restore the true distribution is in any case only partial. Furthermore, there is considerable labor involved in calculating the correction. It should be mentioned that the distributions discussed above are over a plane, not a sphere. In the observations of the galactic background radiation where the distributions are over a sphere, the problems are of course much more complicated.

Therefore, radio surveys of the sky with high resolving beams have an increasing importance. Such attempts have recently been made by

Hanbury Brown and Hazard (30), McGee and Bolton (!?!;), and by us (1*7) in the observations of the cosmic radio background radiation. Several interesting detailed structures of the background radiation have been 2k found with such high resolution .beams. These detailed structures have not been revealed by the radio surveys with wide beam antennas even though corrections for the antenna smoothing have been made.

The effects of the antenna smoothing have recently been discussed in detail by Matt and Kraus (53), Burr (17), and Bracewell and Roberts

(16 ). 2.6 The Ohio State University Radio Telescope

The Radio Observatory is located at a north latitude of I4O0 l 1

0.2" + 0.5",, and a west longitude of 83° 2' 33.6". The radio telescope was designed by Professor John D. Kraus who initiated the project and has directed the research: of radio astronomy. The details about this new radio telescope have been described (hi, 51)- The radio telescope consists of two chief parts, the antenna and the receiving system.

Both antenna and receiving equipments were under continual development during the period of the observations.

The antenna system used in radio survey described in this disser­ tation is shown in Fig. 5. The antenna is an array of $b right handed helical beam antennas in h groups of 22+. The antennas are mounted on a ground plane which is 160 feet long by 22 feet wide with its axis about 12 feet above the ground. The entire array pivots on a horizon­ tal east-west axis over a range in declination from about -U0° to +90°.

It operates as a meridian transit-type telescope. The antenna collects and focuses the radio waves like the lens of an. optical telescope. The helix axes point in the direction of maximum response of the antenna.

The antenna pattern is fan-shaped, with a half power beam width (equiv­ alent to resolving power) of about 1°.2 in right ascension and 8° in declination at a frequency of 25>0 Mc/s. With diurnal rotation of the earth, the antenna beam sweeps across a narrow strip of the sky and the intensity of the received signal is recorded by a pen on a moving paper tape.

Measurements of the antenna pattern were obtained by using the un­ disturbed sun and other intense localized radio sources as a signal Fig. 5. The Ohio State University radio telescope. (The Ohio State University Photography) 27

source. The antenna pattern was found to consist of a main lobe,(1°.2

in right ascension and 8 ° in declination between half-power points), 8

principal minor lobes and many smaller minor lobes. The presence of

the principal minor lobes results from the fact that the spacing be­

tween antenna elements of the array is greater than one wavelength.

Of the total power, about 50 per cent was concentrated in the main lobe

and the rest in the minor lobes.

Fig. 6 shows the antenna power pattern of the main lobe. The best

estimates of the antenna constants are as follows:

Directivity 2800

Beam Area 11+. 5 sq. deg. 2 Maximum effective aperture 320 m

Assuming 1+0 per cent loss in the antenna and cables, the effective 2 aperture comes to about 1 90m .

The receiving system is of total power type. Analysis of the in­

dividual system components of the receiver is given by E. Ksaizek (52.)

and by Don Stoutenburg (77). Two types of operation of the receiver

were used in the observations. The ordinary superhetrodyne receiver was used for the survey of the summer sky made in the spring of 1951+.

In order to eliminate the receiver drift and other equipment instability,

the power received from the antenna was compared at intervals with the noise power from the standard resistor. This was done at intervals of

about two minutes, with the receiver disconnected from the antenna and

switched to the standard resistor for about 10 seconds. ro co ro VJ\ Relative power Relative ro

§ Hj H* 3 Cfl fu cf 3 > ® o CD TO P3 c+ O Hj C+- o CD 3 CD 3TO (D 3 o c + H >d o 03 o o vn o o \n Angle of elevation fron center of pattern of center fron elevation of Angle H O o o VJt I s 9 (5 4 Pd (0 >d

(->• o Antenna pattern TO 29

For the survey of the winter sly made during November 195k tlirough June 1955, the Dicke differential system was used. A block diagram of the system is sho'vm in Fig. 7. In this system., the antenna signal is compared 26.5 times per second with the thermal-noise output of the standard resistor at .room temperature by means of a rotating capacitor switch. The receiver is made to respond, to the switching frequency, so that the output reading measures the temperature differ­ ence between the sky and the standard resistor.

The sensitivity of the receiver has been improved considerably during the last year. The present minimum detectable signal of The -°6 Ohio State University radio telescope is about 10 janskys. ANTENNA STANDARD RESISTOR

\ ROTARY CAPACITOR SWITCH 1 26.5 cps

Radio Frequency Pre-Amplifier

Super-Heterodyne

A.O. Selective Amplifier

A.C. Filter (26.5 cps)

Phase Sensitive Detector

D.C. Amplifier

Recorder

Fig. 7. Block diagram of receiving system 31 CHAPTER III

OBSERVATIONAL PROCEDURE

Our observations of the cosmic radio radiation were made at the

Radio Observatory of The Ohio State University (Longitude W 83° 2 1 +

33.6"$ Latitude N 1*0° 1 ’ 0.2" + 0.5") between February 1953 to June

1955. Both antenna and receiving equipments were under continual

development during this period. The following is a list which shows

the period of observations and the equipment used:

Frequency Receiving (Me/s) SystemResolutionAntenna

1 Feb. 1953- June 1953 1*8 helices 19.2x16° 250 Dicke

2 Oct. 1953- June 195U 96 helices 250 Straight

3 Nov. 195U- June 1955 96 helices l°.3x8 ° 21*2 Dicke

The observations were carried out by fixing the antenna beam at a

number of different elevations corresponding to the required declina­

tions. For each diurnal rotation of the earth a narrow zone of declina­

tion was swept by the antenna beam and the power intercepted was re­

corded as function of right ascension. The measurements were repeated

at four degree steps in declination until a sufficient number of good records were obtained. The sky surveyed extends from about Dec. -1*0°

to + 60°. Although records were taken continuously twenty-four hours a day, usually only those obtained from midnight to 6 A.M. were suf­ ficiently free from solar and man-made electrical disturbances to be usable. The frequency was changed from 250 Mc/s to 21*2 Mc/s in the 32 winter of 19$$ to avoid interference from local radio and television stations.

Fig. 8 is a sample record obtained on December 1|, 19$b with the antenna beam directed to Dec. + 22°. The pronounced deflections due to the transit of the Crab Nebula (Ml) and 1CUU3 are shown superposed on the gradual rise due to the general background radiation. Since sidereal time gains about four minutes a day on mean solar time, a de­ flection on the record due to the radiation from a fixed position in celestial sphere must move forward at this rate.

A gradual decrease of receiver sensitivity, an order of $ to 10 per cent, has been experienced everyday. This is probably due to de­ tuning of the equipment and aging of amplifier tubes. Over-all sen­ sitivity calibration is made once per day using a diode noise genera­ tor. Radio noise due to the sun, distant atmospherics and man-made electric disturbances sets a practical limit to the sensitivity of the receiver. During the day sun and near-by radio and television stations control the noise level of the receiver. During the night the noise level falls off gradually and increases again at sunrise. The most favorable observing period is between midnight and sunrise, when the theoretical limit of the receiver sensitivity may be approached. Prac­ tical experience shows that the rapid change in temperature of the equipment and cables, especially in the summer, is also a limiting fac­ tor on the performance of the equipment.

The intense localized radio sources, such as Cassiopeia A, Cygnus

A, Taurus A and Virgo A have been used to calibrate the antenna pattern, sensitivity, and over-all equipment time constant. Fig. 9 shows tran- 53

DEC. 4, 1954 RIGHT ASCENSION DEC.+ 22° 1950.0 242 MG 96 HELICES 06 00 05 30m :'

5

TAURUS A

GALACTIC RISE IG443

E MARKS

Fig. 8. Sample record of the transit of the Crab Nebula and taken with The Ohio State ■ University r-dio telescope. Fig. 9. The times of transit of intense radio sources and times of sunrise and sunset. sit (EST) of intense radio sources and of sunrise and sunset at the

Radio Observatory of The Ohio State University. The observing hours are shortest around June when the sun is at the highest altitude and longest in the winter when it is at the lowest altitude. About 8 to

10 observing hours are usable in the winter while this drops to 6 to

8 hours in the summer.

Three major radio surveys have been made at the Radio Observatory of The Ohio State University since the first completion of the radio telescope. (See Table I).

The first survey made with an array of U8 helices was mainly de­ voted to the detection of the radio sources. Two hundred seven radio sources were located in this survey and the general features of the background radiation were obtained (I#). The second and third surveys were made to explore the detailed structure of the cosmic radio back­ ground radiation using the improved resolution of an array of 96 helices.

A radio map of the summer sky (i|?) was made from the results of the second survey. The third survey was of the winter sky. Many localized and extended radio sources were also detected during these surveys. 36

CHAPTER IV

LOCALIZED SOURCES OF EXIRA-TERRESTRIAL RADIO RADIATION

Ii-l Introduction

The discovery by Hey, Parsons and Phillips (3d) of short period fluctuation in the intensity of galactic radio radiation from the direction of Cygnus led to the discovery of new typos of astronomical objects. Subsequent measurements by Bolton and Stanley (10) showed that this region contains an intense discrete source with an apparent angular diameter of less than 8 minutes of arc. The discovery of the

Cygnus source was followed by the detection, by Ryle and Smith (69), of a more intense radio source in Cassiopeia. A search for other radio sources was continued in Australia, England and the Uni.ted States.

Steady improvements were made both in the resolving power of the antenna systems and the sensitivity of the receivers. There have been seven major surveys of localized radio sources published during the past few years. They are shown in Table II.

Among the surveys shown in Table II, there is good agreement be­ tween the positions and power flux densities of the intense sources, and also good agreement for some of the fainter sources which are relatively isolated. However, the agreement is far from satisfactory for fainter sources and especially those sources in the regions of high source density. This may indicate that these surveys are incomplete.

Some of the positions listed actually refer to blends of objects or ghost sources rather than to actual sources. The problem of "blends" or "ghost" radio sources is still important and further improvement of radio telescopes is desirable. The number of localized sources which 37

has so far been detected has been limited by the sensitivity and re­

solving power of the radio telescope. There is no doubt that improve­

ments in the equipment would lead to the discovery of more sources.

Table II

Surveys of Localized Sources

No. Observers Fre- Limit of Beam Region No. quen- Sensitivi- Reso- covered of cv ity 10"26 lution Sour­ Mc/s Janskys ces

1. Stanley and Slee (76) 100 100 9 x 17* +50 to -5o° 22

2. Ryle, Smith and Elsmore 81 30 1.5 x 90* +90 to +10° 50 (70) 3. Mills (55) 101 50 11+ x 21+ +50 to -9 0 ° 77

h. Brown and Hazard (31) 158 5 2 x 2 +70 to +1+0° 23

5. Shain and Higgins (72) 18.3 3000 17 x 17 +10 to -90° 37

6. Bolton, Stanley and Slee ioo 5o 8 x 12* +50 to -5o° 101+ (11) 7. Kraus, Ko and Matt (1+9) 250 100 1.2 x 8 +50 to -5o° 207

8. Ko, and Kraus (1+0) 21+2 50 1.3 x 8 +60 to -50° 19

* interferometer technique 38

i|.,2 Detection of Radio Sources

The presence of a radio source is detected as a rise in the inten­

sity on the recorder. If the source is a point source, the signature

on the record is a direct measure of the antenna pattern. For an ex­

tended source, the broadening of the antenna pattern is observed. -This

is shown in Fig. 8 of Chapter III in 'which the signatures of the radio

sources Taurus A and IC 11U3 are presented. The effective angular

size of Taurus A is about 3*5' x 5 ’ and it is practically a point

source. The signature of IC hh3 shows a broadening effect due to its

angular extension,. about 1°.5 which is comparable to the half power beam width of the antenna. When the source signature is of much great­

er width than the true antenna beam width, it indicates that either the

source has an angular width comparable to the antenna beam width or

there are several localized sources close together. The apparent an­

gular size of the source can be determined from the broadening of the half-power beam width of the antenna pattern. The effect of the source extent on the antenna pattern has been analyzed by Matt and Kraus (53).

The detection of radio sources becomes very difficult if the sources are superposed on a general, background rise, especially if the gradient is steep. This occurs near the galactic equator and the grad­ ient becomes steepest near the galactic center. A result of this effect is a decreasing concentration of radio sources in the galactic plane and towards the galactic nucleus.

The position of a radio source is found in terms of its right as­ cension (R.A.) and its declination. All positions in this dissertation are Epoch 1950.0. Galactic Coordinates are based on Ohlsson's (Lund) 39

table.

The right ascension of any source is determined by observing the

time at ■which the received intensity reaches a maximum. For a right

ascension determination it is necessary to know the time delay between

the -instant of the transit and the maximum deflection. The time delay is determined using Gygnus A as a calibrating source. The time delay

is partly affected by the collimation error of the antenna, and large­

ly due to the time constant of the receiving equipment. To increase

the sensitivity of the receiver, a time delay circuit is introduced preceding the output recorder. A time delay of from 15 seconds to 1 minute was used for the present investigation. The probable error in

the right ascension determination is + 20 seconds for intense sources

and + 2 minutes for weak sources. More accurate determinations could be obtained by a faster tape speed of the recorder.

The determination of the declination of any source is made by

comparing observations made at several declinations in the vicinity of

the source. The time required for the source to transit the antenna beam changes with the secant of the declination of die source. This

can also be used to determine the declination of the source. As sec

only to sources at higher declination. The accuracy of the declination

determination is low because of the small resolution of the antenna beam in declination. For an intense source the estimated probable

error is + 0.5 degree and for the weaker sources + 2 degrees, when ob­

served at more than two adjacent declinations, and + 4 degrees when observed at only one declination. 1*0

The intensity of the source is found by extrapolating the back­ ground radiation over the period of transit, and difference between the observed intensity and the estimated background level represents the excess radiation due to the transit of the source* The intensity of the source is determined by measuring the maximum rise in intensity above this background radiation* This method is susceptible to error if the gradient of background radiation is steep. The weakest radio source which can be detected by the above method is limited by several factors:

a) the amount of fluctuation due to receiver instability,

b) the gradient of the background radiation,

c) receiver sensitivity,

d) interference.

By averaging several independent records the sensitivity can be in­ creased by a factor of where n is the number of records avail­ able. However, this process requires considerable time to obtain sufficient records and it is not yet feasible to apply it to the whole region of the sky. The equipment was operated with a sensitivity such that it is possible to detect a source of intensity greater than 20 x -26 10 janskys on a single record when the source is located on the flat part of the background radiation.

Table III is a list of radio sources detected during the survey of the winter sky with The O.S.U. radio telescope.

The list contains only the intense radio sources readily detected with a single record. The flux density is twice the maximum observed in one polarization and has not been corrected for the finite size of T A B L E D I

L ist of Radio Sources

Flu^^Densi^y No. R. A. (1950) Dec. (1950) (10 Wm (C/S) ) Other Catalog Numbei R e m a r k s h . m . d e g .

1 02 19 ±2 + 4 4 t2 30 R02.01, BH4.BH5 N G C 8 9 1 ? 2 02 4 8 ± 2 +31 ±2 90 KKM AriB l°Extended Source 3 03 16+1 +42 ±2 60 KKM PerB, R03.02, N G C 1 27 5 M03+4, BH6, BSS40 4 03 2 O il -3 7 + 2 200 M03-3, BSS10, S03-4 N G C1316 5 03 2 7 .5 + 1 .5 + 56± 2 80 B H 7 6 03 5 9 -2 + 6 i2 70 KKM TauE 1°.5 Extendedsouroe 7 04 35 ± 1 .5 +3 0±2 90 KKM TauB, R04.01 ? M04+3, BWSS-B 8 04 5 7 i l +46+1 90 KKM AurA? M05+4 Auriga nebulosity BH9, BSS76 1.7 Extended source 9 05 3 1 .5 + 0 .5 +22+1 800 KKM TauA, R05.01, ML, Crab Nebula M0 5+2, BSS2, HMS2 10 05 4 2 1 3 0 t 3 KKM OriA, BWSS-D Orion Complex 9°Extended source 11. 06 15 i i + 2 3 ll 160 KKM GemB, HMS22, IC 4 4 3 HMH13, Baldwin and 1?2 Extended source D e w h ir s t 12. 06 3 0 .5 ± 0 .5 +5±1 160 KKM Mon A Rosette Nebula, N G C 2 2 4 4 l?5Extended source 13. 07 38+2 +42+2 4 5 R 0 7 .0 2 2°Extended source 14. 08 11+2 +48+2 60 R08.01, BHU, BSS84 1. 5Extended source 15 08 1 8 i2 + 3 2 i2 45 KKM LynA, R08.03 16 08 2 0+2 + 8 l4 2 0 K K M H y a P Fluctuating source 17 08 2 4 + 3 -4 4 ± 4 KKM PupA-(VelA), H II region BWSS-F 5° Extended source 18 09 15.5+1 -1 2 i2 170 KKM HyaC, M09-1A BSS 26, S09-1 19 10 12+2 + 4 8 -2 ' 1*2

the source. The errors in the flux densities are estimated to be + 25

per cent for sources of intensity greater than about 60 x 10“ janskys

and + $0 per cent for weaker sources. The position of a number of

sources agrees satisfactorily with that given by other observers and in

these cases their catalogue numbers are shown.

The following abbreviations are used: M for Mills (55), R for

Ryle, 3nith, and Elsmore (70), BSS for Bolton, Stanley, and Slee (11),

BW5S for Bolton, Westfold, Stanley, and Slee (15), KKM for Kraus, Ko,

and Matt (1*9), BH for Hanbury Brown and Hazard (31), S for Shain and

Higgins (72), HMS for Haddock, Mayer, and Sloanaker (26), and HMH for

Hagen, MacClain, and Hepburn (27). Optical objects (5) are also given where their identifications have been well established.

To eliminate ghost radio sources which may be produced by the minor lobes of the antenna, a special precaution has been taken in the

interpretation of our data. This has been done with a convenient instru­ ment showing the positions of the important minor lobes of the antenna on the celestial sphere for any antenna elevation.

One of the striking features in Table III is the existence of a large number of sources close to the galactic plane having angular o diameters of 1 or more. The existence of such galactic radio sources of large angular extent has been also reported by Bolton, Westfold,

Stanley, and Slee (15), and Hanbury Brown, Palmer, and Thompson (33).

It is interesting to point out that the optical identifications have been made for 8 out of 19 of the sources in Table III. Two are abnormal extragalactic nebulae, NGC1275 (No. 3) and NGC1316 (No. !*)• The others include the Crab Nebula Ml (No. 9), ICl+l*3 (No. 11), Auriga Nebulosity ('No. 8), Rosette Nebula (No. 12), and two large H II regions (No. 10 and No. 17). These 6 sources are known to be in our own galaxy. Five out of 10 extended sources have been identified with the galactic nebulosities and H II regions, and this suggests that the remaining unidentified extended sources may belong to such galactic objects. kh

k-3 Distribution of the Radio Sources

The radio sources detected during the present radio survey at 2i|2

Mc/s have been given in Table III. The galactic distribution of the radio sources is presented in Fig. 10. It indicates that both intense

sources and extended sources show a concentration in the galactic plane.

The actual concentration must be more pronounced, since the present observational method tends to mask such concentrations along the galac­ tic plane. A similar concentration has been noted by Mills ($3), by

Hanbury Brown and Hazard (31), and recently by Bolton, Stanley and

Slee (11).

Until more accurate observational data and more complete theories of the emission from the radio sources is available, it is impossible to derive values for their space density, mean distance of separation and absolute luminosity with accuracy. However, it seems worth attempt­ ing to deduce the order of the space density and absolute magnitude even though approximate, from the present data. It will be assumed for simplicity that the sources are uniformly distributed throughout the galactic disk and that all have the same absolute intensity. The mean angular displacement of the class I source (sources within 15° from the galactic equator) is about 2.5°. If their mean distance from the plane is assumed to be about $0 psc then it follows that they must lie at approximately 1000 psc. The majority of the sources have a power flux density within the narrow range 30 x 10“2^ to 160 x 10-2^ janskys. (See

Table III). If we assume an average flux density of 10“^ janskys, then the mean radio luminosity will be . g i F ar si ndi s as shown. s e n i l n e k o r b e h T . n w o h s s a ls o b m y s t n e r e f f i d y b d e t a ic d in e r a s e iz s r la u g n a d n a s e i t i s n e t n I he boundari t on surveyed. d e y e v r u s n io g e r e th f o s ie r a d n u o b e th t n e s e r p e r 10. GALACTIC LATITUDE ' 0 5 - c 0 4 - ' 0 3 - - I 0° + I O' + - + 30' + 50' ° + 0 4 acti i o sources i he regi . r e t n e c - i t n a c i t c a l a g f o n io g e r e th in s e c r u o s io d a r e s n e t in f o n o i t u b i r t s i d ic t c la a G e h T 20 20 c o ' '

4° 2° 0° 8° 6° 4° 2° 100° 120° 140° 160° 180° 200° 220° 240° 4° 2° 0° 8° 6° 4° 2° 100° 120° 140° 160° 180° 200° 220° 240° °\ \ O' \ OAIE SOURCES LOCALIZED XEDD SOURCES EXTENDED + + 1 J AATC OGTD I LONGITUDE GALACTIC ---- "V" ANTD ^ MAGNITUDE 3 > 3 MAGNITUDE ^ 1 ---- A + 1 ____ + I ____ + L • 0 & / x/ 1 I O / 1 ° 0 5 - 40° - ° 0 3 - + I 0° I0° + + - ° 0 I - +30° + 50° + 40° 20 20 45 0 ° ° ° °

1*6

Lf = k irr2 Pf

- 1.2 x 103ii r2 Pf W/cps where r = mean distance of radio source in psc.

Pf = mean power flux density of radio source at the frequency f

(janskys)

Inserting 1000 psc for r and 10-2^ janskys for P^, we arrive at the mean radio luminosity of 1.2 x 1 0 ^ W/cps or 1.2 x 1023 ergs/sec/cps. o It also follows that the mean space density is of the order of ij. x 10 source/psc^ which corresponds to an average separation of the source of about 290 parsecs. These figures are in excellent agreement with those obtained in Hanbury Brown and Hasard's analysis in the region be­ tween galactic longitude I4.O0 and llj.00 (31). lull The Radio Magnitude, Spectrum Index and Color Index

In optical astronomy apparent magnitude of a star m represents a

logarithmic measure of the intensity of light energy received from a

star. It is defined by the equation

m = - 2.5 log 1^ (U.l)

where 1^ is the intensity of light energy received by a light-sensitive

apparatus used to determine the magnitude of a star. To find the rel­

ative luminosities of two stars of different magnitudes, m and n, let

us consider the ratio of luminosities of these two stars.

hlL = 2.512 (n“m) (lj.,2) h.

or

log - o.li (n-m) (iu3) Ln

from which

m = n - 2.5 log ^ ([)..[;)

A similar magnitude system is used in radio astronomy to afford

a convenient comparison of radio data with the optical measurements.

The apparent radio magnitude of a radio source is related to its power flux density as we observe it, and depends on the radio luminosity of

the radio source and on its actual distance.

Similarly, we have the relation for apparent radio magnitude, where

Pjjj = flux density of a radio source of magnitude

Pn = flux density of a radio source of magnitude rij.

By assigning a certain radio magnitude to an arbitrary radio flux

density, the reference of a magnitude scale is gpecified. Eq. (Iu5)

then becomes

nip = (nr + 2.5 log Pn ) - 2.5 log

= K - 2.5 log Pm (I4 .6 )

The apparent radio magnitude is thus defined by i

Several radio magnitudes have been used by different observers in the literature. They are shown in Table IV.

TABLE IV

1. OSU Scale (bh) mQr = -57.25 - 2.5 log P at 250 Mc/s

2. Cambridge Scale (70) mcr = -57.5 - 2.5 log P at 81 Mc/s

3. Manchester Scale (28) m ^ = -53.U - 2.5 log P at 158.5 Mc/s where P is the power flux density of a radio source in janskys at a specified frequency.

The OSU scale is due to Professor Kraus (I4J4 ). K is chosen so that the first magnitude source has a flux density of 5 x 1 0 “^ janskys at

250 Mc/s. In the OSU and Cambridge scales, the zero of the magnitude scale corresponds to about the third and fourth most intense radio sources, respectively, while the Manchester scale is chosen so that at

158.5 Mc/s the observed radio and photographic magnitudes of normal extra-galactic nebulae (late-type spirals) are approximately the same.

If the dependence of the flux density of a radio source on the

frequency f is f”n , then n is called the radio spectrum index.

Thus a radio color index Cr between two frequencies f^ and fg may

be defined as:

cr = mr (fi) " “r (f2> " n log

where n is a radio spectrum index.

Spectra may be classified arbitrarily as of 1; types, according to

the value of n:

(I) n > o (II) n < o (III) n = o (IV) Combinations of (1), (2), or (3)

Fig. 11 shows spectra of several typical examples. These spectra

were obtained by combining the results of different observers.

The majority of radio sources so far discovered belongs to the

class I. Radio sources identified as colliding galaxies, such as

Cygnus A, Centaurus A, and NGC 1275, and peculiar galactic nebulosities

such as Cassiopeia A and Puppis A are included in this class.

In class II, we find radio sources identified with thermal radia­ tion due to free-free transition in dense ionized nebulae and H II re­ gions. The Orion Nebula, North American Nebula and Cygnus X are typical examples of this class.

Class III and IV sources are uncommon. There are only few sources in these classes. The spectrum of Taurus A (supernova of lO^it) belongs to class III and the intense discrete source at the galactic nucleus has a spectrum of class IV. 50

Class I

xlO Cassiopeia A Cygnus A 20 20

10 BH

KM BH

HMH

HMH

l 1*0 100 2*00 1000 2*000 2*0 100 2*00 1000 2*000 Mc/s Mc/s

xlQ" xlO- 22*

Virgo A Centaurus A 20 20 * KKM

10 10

•H HMH KMH

HMS EMS 2*0 100 2*00 1000 2*000 2*0 100 2*00 1000 2*000 Mc/s Mc/s

Fig, 11, Spectra of radio sources. Power flux density (jansky) Power flux density (janskys) 1000 1000 100 100 10 10 xlO' xlO - -25 -2 -2 U 00 1 100 OSU ClassIV O 10 UOOO 1000 UOO ClassII Orion nebula Uoo Mc/s Mc/s HMH HMH 1000 Uooo HMS HMS i.1. (cont.) Fig.11. H •H *n> & A tw nd § loq § 0) fn S 0) O C O C h lOOOr 0 1 xlO - Referencesto 11,Fig. r25

100 R(70) H(55) MB(5U) HH(35) HMH(27) HMS(26) B(7) Kf^r KKM(U9) KK(UO) BH(31) BSS(ll) Crabnebula ClassIII O 1000 UOO Mc/s HMH

HMS Uooo 51 Although the accuracy of the spectrum is not high, it is sufficient

to show significant differences in the shape of the spectra for various

sources. It is unlikely that the spectrum index n remains constant

over the entire frequency range.

The adcuracy of the measurements of flux density is estimated to

be better than + 20 °/o to + $0 °/o by some observers. However, these

estimates may possibly be too optimistic in certain cases. The deter­

mination of the flux density for weaker sources and for extended

sources introduces additional difficulty. For a weak source the effect

of confusion with other sources and poor accuracy of reading increases

the error. For an extended source, the accurate correction for the

finite size of the source is difficult. Futhermore, there are no

inter-comparisons of fundamental standards of calibration for absolute power measurements among the radio observations. U.5 Identification of Radio Sources

It is now important to examine the possibility that some radio

sources might be identified with visible objects. The identification x of radio sources with visible objects should provide information on the

origin of the cosmic radio radiation. Furthermore, we may rely upon

well established identifications as a guide in making a radio survey.

The agreement of the position of a radio source and a visible ob­

ject is not in itself sufficient to establish the identification of a

radio source. The measurement of the angular diameter or actual

structure of a radio source is equally important. Various techniques

have been used by radio astronomers to determine more' precise positions

and detailed structure of radio sources. Using the large telescopes

at Mt. Palomar and Wilson, Baade and Minkowski (£, 6) have recently

made intensive searches in the vicinities of the radio sources and

succeeded in definite identifications with visible objects.

At present the radio sources which have been identified fall into

five categories:

(I) Remnants of supernovae

(II) Galactic nebulosities

(III) Thermal emission from H II regions

(IV) Normal extra galactic nebulae

(V) Abnormal extra galactic nebulae

The first three are in our own galaxy.

(I) The Crab Nebula is the remnant of the supernovae of 105U, a

supernovae of type I (3). The identification is now well established for recent data of the position of the radio source place it well with- 5k

in the nebula, while the radio brightness contour is in excellent agree­

ment with the size and orientation of the nebula. The nebula is at a

distance of 1000 psc and the total radio energy emitted by the nebula 32 is about 7 x 10 ergs/sec (5).

There are known three supernovae in our own galaxy, the Crab Nebula

(M 1), T^cho Brahe's nova of 1572 and Kepler's nova of l60l*. All are

supernovae of type I. Though the original light curve of Tycho Brahe's

nova showed it to be a supernova like the Crab Nebula, no visible rem­

nant of this supernova has been found (3). It may be due to the fact

that Tycho Brahe's nova is heavily obscured compared with the Crab

Nebula. The remnant of Kepler's nova has been discovered (k)•

Since the identification of the Taurus source, Haribury Brown and

Hazard (29) observed a weak radio source near the position of Tycho

Brahe's nova of 1572. In view of the uncertainty of the angular size

of the radio source, the identification can be considered as only ten­

tative. Attempts have been made to detect a radio emission from

Tycho Brahe's nova by The Ohio State University radio telescope at 21*2

Mc/s without success. Unfortunately the nova is located at high declin­

ation (+ 65°) at which severe radio interference is encountered by The

O.S.U. radio telescope most of the time.

Kepler's nova of 1601* has not yet been observed by radio. A

radio detection of this nova would be of great value as it would add

further evidence that the remnants of supernovae of type I are radio

sources. A search for radio emission from Kepler's nova has been

attempted with The O.S.U. radio telescope at 21*2 Mc/s. Although a weak radio source was discovered in the vicinity of Kepler's nova, the agreement of the position and angular size with those of visible rem­

nant is not satisfactory. Therefore, the radio detection from Kepler’s

nova is still not certain.

(II) Peculiar galactic emission nebulosities

The nature of the most intense radio source, Cassiopeia A had re­

mained a mystery for a long time. As a result of searches with the 200

inch telescope for a visible object in the position of the Cassiopeia

A, Baade and Minkowski (6) finally discovered a new type of galactic

nebulosity. The nebulosity consists of a network of filaments in vio­

lent motion. The random internal velocities of the filaments are very

high ranging from -1000 Km/sec to +3000 Km/sec. A nebulosity, very

similar in appearance to that in Cassiopeia A, has also been identified

in the radio source Puppis A (6). The velocity dispersion is, however, much smaller than the Cassiopeia source being only about 200 Km/sec.

In addition to these two nebulosities in which gaseous material

appears to be in violent motion, several other galactic nebulosities

have also been found as radio sources during the present O.S.U. radio

survey at 21*2 Mc/s* These are shown in Table V. Although no detailed

optical data on these nebulae are available, they appear to be different from the Cassiopeia A type.

Fig. 12 shows a sketch of the outline of ICW43 in relation to the position of the radio source (No. 1, Table V). The identification of the radio source with IClil±3 was first suggested by Baldwin and

Dewhirst (8). They placed the radio position at the center of the mass of nebulosity. Our measurements, however, show that the radio Table V

List of Radio Sources Identified with Galactic Nebulosity

R.A. (1950) Dec. (1950) Flux Density Angular h. m. deg. xlO" janskys Width deg. Remarks

1 06 15 + l +23 + 1 200 1.2 lcUi3

2 20 I4.8 + 1 +30 + 1 500 2.5 Network Nebula

3 o k 57 + 1 +U6 + 1 11*0 1.7 Auriga Nebulosity Power flux density janskys 10 l ^ xlO - BD( - *3 1 a (a) The position of the radio source in relation relation in source radio the of position The (a) 50 8 (b) Radio spectrum of Ic445« of spectrum Radio (b) + ■+ 25 8 ) 22 to the outline of IC445» of outline the to 100 BD( 06 8 hl ) 6 200 n 6lm o o6hl4m In OSU ih seso (1950»0) Ascension Right OSU Pig. 12. Mc/s + + BD( 500 8 ) 6 1000 hi HMH(27) 2 m

2000 HMS(26) 57 58 position lies close to the bright arch on the north following side of

the nebulosity (See Fig. 12). This makes it probable that radio emission is stronger near the northern bright arch. The radio spectrum of the

source is also shown in Fig. 12

The ICkh3 is strikingly similar in shape to the Network nebula

NGC 696O and NGC 6962 in Cygnus, and it has been suggested that the latter may also be a radio source (8). Walsh and Hanbury Brown (8I4.) have recently reported a radio source associated with the Network nebula. At The Ohio State University an extended radio source was first detected in the vicinity of the Network nebula during the background radio survey at 2JU2 Mc/s in December, 195U. Unfortunately it was ob­ served only during the day at times at which severe interference from the sun was encountered. The source was in a favorable observing posi­ tion in the spring of 1955. The accurate measurements were then made of the position and the size of this radio source. The results are shown in Table V. A sketch of the Network nebula is presented in Fig.

13. The radio position and angular diameter are also shown. A radio position agrees satisfactorily with that of the probable center of the

Network nebula, and a radio angular diameter is about 2?5 which is com­ parable with the extension of the nebula. The position agrees with that measured by Walsh and Hanbury Brown at 92.5 Mc/s, but the angular diameter is much smaller than their value (3°~ 6°),

It should be also noted that the radio spectrum of the Network nebula seems to be significantly different from that of IC44^.

(see Fig. 1.2 and Fig. 13). The former has a negative spectrum index, suggesting the possibility of thermal radiation, while the latter has 59

osu &

+ 28'

Right Ascension (1950»0)

(a) The position of the radio source in relation to the outline of the Network nebula.

janskya xl0“25 n = -0.5 100

♦rl osu

w 10

100 20)0 5 0 0 1000 Mc/s

(b) Radio spectrum of the Network nebula.

Fig. 13. 60 a positive spectrum index. It may be possible that the mechanisms of radio radiation in the nebulae are different although they look similar optically. A more detailed study of the Network nebula is required be­ fore the matter can be finally settled.

Fig. Ill shows a sketch of the Auriga nebulosity in relation to the position of the radio source No. 3 in Table V. The radio position meas­ ured by Hanbury Brown and Hazard (31) at 1^8.5 Mc/s is also presented for comparison. Our radio position shows an excellent agreement in position with Hanbury Brown and Hazard's source No. 9 (31) which has been identified with the nebulosity in Auriga by Minkowski (33). Our measurements, however, show that the 1?7 width of the radio source is slightly larger than Hanbury Brown's 1?U and is closer to the extension of the nebulosity ( ~ 2°).

+48°

BH(|1)

ostr

0 , 1 05 iom 05hoora o4hpom Right Ascension (19^0*0)

Fig* lit* The position of the radio source in relation to the outline of the Auriga nebulosity. 61

(III) Thermal emission from H II regions.

It has been known that the H II regions are strongly concentrated

in the galactic plane. The optical depth of these regions should be

appreciable so that ite radio emission should be detectable. Scheuer

and Ryle (71) using an Interferometer, have found a bright belt of radio

emission confined to the galactic equator and attributed to the effect of the H II regions. Piddington and Minnett (61;) have detected a large

source, Cygnus X, at 1210 Me and tentatively identified as thermal

emissions from several dense H II regions surrounding "I Cygni.

It seemed desirable to confirm these results by the radio detection of an individual ionized hydrogen nebula. However, Baldwin (7) attempt­ ed to detect the Orion nebula, an exceptionally dense H II region, at

210 Mc/s without success. Recently Haddock, Mayer and Sloanaker (26) working at 9.1; cm, and Hagen, McClain and Hepburn (27) working at 21 cm have succeeded in detecting radio emission from the Orion, Omega,

Trifid nebulae and other individual H II regions.

No work had been accomplished on these measurements at meter wave­ lengths. Several H II regions have been detected for the first time at meter wavelengths in our radio surveys at 2l;2 Mc/s. Table VI contains the list of these radio sources identified with H II regions.

Source No. 1 has been identified with the Rosette Nebula surround­ ing galactic cluster NGC 2214; (38). Minkowski (56) found a fairly symmetrical sharp-edged disk of 80* in diameter extending around NGC

22kh. The photographs of the region taken in red and blue light by

Dr. Rudolph Minkowski with the l;8-inch Schmidt at Mt. Palomar are shown in Fig. 15. A signature of the radio source taken with The Ohio State Table VI

List of Radio Sources Identified with H II Regions

R.A. (1950) Dec. (1950) Flux Density Angular h. m. deg. 10“ .ianskvs Width Remarks deg.

1 0 6 3 0 . 5 + 0 . 5 + 5 + 1 200 1 . 5 Rosette Nebula 2 0 5 1*2 + 3 0 + 3 - - 9 Orion Complex 3 0 8 2i| + 3 -111* + 1* - - 5 Vela Complex k 20 1 9 . 5 + 0 . 5 +1*0 + 2 800 2 H II regions, Cygnus X 20 5 5 3 + 2 - +1*1* + 3 - - North American Nebula F ig .i5 .( a) Record of the transit of the Rosette Nebula taken w ith the OSU radio telescope. (b) The Rosette Nebula photographed in red (above) and blue light (bel ow) w ith 48 inch Schmidt telescope at M t. Palom ar by Dr. Rudolph M inkowski. 6U

radio telescope is also presented. The radio position lies close to

the center of the Rosette nebula and the angular diameter is slightly

larger than the visual diameter of the nebula. These values are com­

pared in Table VII. Fig. 16 shows a sketch of the outline of the

Rosette nebula in relation to the position of the radio source.

The apparent black-body temperature of the nebula is approximately

200°K at 2U2 Mc/s. Assuming the electron temperature of lcA degrees K

in the nebula, the average optical thickness at 2U2 Mc/s over this re­

gion is then 0.02. According to the analysis of Greenstein and Min­

kowski (2U), the optical depth of an ionized gas cloud at frequency of

f megacycles is

= 0.39 EM where 2 -3 EM ~ n “ x Lpsc n © = electron density in cm lpSC = size of clouds in psc.

From this an emission measure of 3000 is obtained for the Rosette

Nebula. Assuming that the nebula is spherical in shape and at distance of llj.00 psc (57), with a corresponding radio linear diameter of 37 psc, this leads to an average electron density of 9 c m “ 3 . 2his value is in good agreement with the recent data of lU cm. given by Minkowski (57) based on the improved measures of the surface brightness by Kron, and supports the hypothesis of thermal emission by free-free transition from the Rosette nebula.

Source No. 2 has an approximate diameter of 9° and occupies the regions corresponding to the Orion aggregate of early-type stars. The 65

Table VII Data on Radio Source and Roaette nebula

R. A. (1950.0) Dec. (1950.0) Angular Diameter

Radio Source 0 6 ^ 0 , ^ ± 10 5°:*: 1° l»5°i 0*5°

Rosette nebula 06^J0»0m 4°54' 1*55° (NGC 2244) (Optical data)

+ 6 '

2 o ,RA0I0 1- / p o s it io n » < + 5C D 7 BD5° 1283 I o \ QLlJ I

(1950.0) + 4 ‘

0 6 h3 4 m 0 6 h3 0 m 0 6 h2 6 m RIGHT ASCENSION

Pig. 16. The position of the radio source (cross) in relation to the outline of the Rosette nebula (dashed lin e). 66 area consists of a complex network of bright and dark markings and sur­ rounded by an extended feeble H o(. emission. The radio brightness is approximately 1|0°K. On the assumption of thermal emission from the

Orion complex, the regions have an average optical thickness of O.OOlj. at 2U2 Mc/s and correspond to an emission measure of 600. This is close to the value of 800 found by Stromgren (75) for an average extended emission region. An attempt has been made to measure the intensity from the Orion nebula. Unfortunately there is confusion due to the presence of the intense radio source Taurus (Ml) which has approximately the same right ascension. The flux from the Orion nebula is not greater than 2 ,x 10“^ janskys.

Source No. 3 has an angular diameter of at least 5°* The deter­ mination of its declination and angular extent is difficult because it lies at the limit of the field of view of the antenna. The radio source

Puppis A (6) which has been identified with a peculiar galactic nebu­ losity lies within this area. However, the angular extent of the radio source is much larger than that of the nebulosity associated with the

Puppis A and therefore does not seem to be connected with the same nebulosity. Baade and Minkowski (6) found that this area is rich in large diffuse nebulosities. Recently Gum (25) has reported the exist­ ence of a large H II emission region in this area. It is likely that these large nebulosities may be responsible for the observed radio emission of large angular extent. A more detailed survey of this region will be required before the matter can be settled. 67

Source No. U (Cygnus X) is located in the region near Y Cygni.

Identification of Cygnus X with thermal radiation from the bright nebulosities surrounding Y Cygni has been suggested by Piddington and Minnett (6I4).

The position of source No. 5 agrees closely with that of the North

American nebula. Since this radio source lies on a steep gradient of the general background radiation, it is difficult to determine its power flux density and angular extent. 68

(IV) Normal extra galactic nebulae.

Reber (66) first attempted to detect a radio emission from the

Great Nebula in Andromeda ( M 31) without success. The identification of some faint radio sources with near-by extra - galactic nebulae was first suggested by Ryle, Smith and Elsinore (70). However, it remained for Hanbury Brown and Hazard (28) to remove any doubt that the Andromeda nebula ( M 31) is a radio source, by showing the radio isophotes for the region around M31. Seven individual nebulae have been detected by

Hanbury Brown and Hazard since then and they have shown that there is remarkably close agreement between the radio and the photographic mag­ nitude except for M31 and NGC 891. All the extra-galactic nebulae de­ tected so far are spirals of types Sb and Sc which are a mixture of stars of Baade's population I and II.

It is interesting that so far no radio emission has been detected from elliptical nebulae, in which stars of Baade's population II pre­ dominate. The measurement of the radio emission from elliptical nebula will be of great significance, since it should be possible to determine in which stellar population the radio emission originates. Elliptical nebulae in general are relatively faint. The brightest nebulae are:

NGC Predicted radio intensity at 21*2 Mc/s "pg 205 8.89 ll..25 x 10" janskys 221 9.06 3.63 x 10“^ janskys 311^ 9.8 I.83 x 10“2° janskys

Assuming that the ratio of radio intensity to light intensity for spiral nebulae found by Hanbury Brown and Hazard (28) also applies to the ellip­ tical nebulae, and that the spectrum index of the nebulae is unity, the 69

radio intensity to be expected from the elliptical nebulae at 2l;2 Mc/s

has been computed for the above nebulae.

Attempts to detect radio emission from these nebula by The Ohio

State University radio telescope at 2U2 Mc/s have been made without suc­

cess. NGC 205 and NGC 221, the companions of the Andromeda Nebula, are

unfortunately so close to the Andromeda nebula that they cannot be re­

solved by our radio telescope. NGC 3115 is at a favorable observing

position in the sky. However, no radio source was found in the vicinity

of NGC 3115* The minimum detectable signal of the radio telescope was

about 10 x 10” janskys. The present measurement shows that the ratio

of radio intensity to light intensity for the elliptical nebulae cannot be greater by more than 2 magnitudes than that found for late-type spir­

als, for if it were greater, radio emission from the elliptical nebula

NGC 3115 should be detected. The problem of which stellar population is responsible for radio radiation is still undetermined.

(V) Abnormal extra-galactic nebulae.

The photographs of the Cygnus A region taken with the 200-inch tele­ scope by Baade and Minkowski (6) show that the radio position coincides with one of the members of a fairly rich cluster of galaxies. It has two nuclei and appears to be two late-type spiral galaxies in face-on collision. Only three examples of colliding galaxies (Cygnus A, NGC

1275, NGC 5128) are identified as radio sources at present and these differ in the type of galaxy involved and in the geometry of the colli­ sion. There are two other abnormal extra-galactic nebulae which are identified as radio sources, that is, NGC ijJ+86 and NGC 1316. 70

NGC 127^ is one of the members of the cluster of nebulae in Perseus.

Baade and Minkowski (5) have suggested that the radio source is associ­

ated with NGC 1275 and not with the cluster of nebulae as a whole. The

cluster has a visible diameter of about 2°, while that of NGC 1275 is

only 0.71. Measurements at 21x2 Mc/s of NGC 1275 with Ihe O.S.U. radio

telescope shows that there is no broadening of the antenna pattern by the

Perseus cluster. This indicates that the radio emission is associated

with NGC 1275, since if it were associated with the whole cluster the

antenna pattern would show appreciable broadening effect. This result

confirmed a similar result reported by Baldwin and Elsmore (9).

A radio source (No. U, Table III) corresponding to NGC 1316 shows

asymmetry as observed at 21+2 Mc/s with The Ohio State University radio

telescope. This may indicate that the radio s tructure of NGC 1316 is

complex. Bolton, Westfold, Stanley and Slee (l5)s using an azimuth-

interferometer, measured a position which differs by 2 minutes in right

ascension. The present finding of asymmetry of radio emission at 2i+2

Mc/s may account for this discrepancy.

It has been shown that a variety of visible astronomical objects can be strong radio emitters. However, it should be remembered that all of these objects, except normal galactic nebulae and H II regions, are very peculiar and rare astronomically. No stars, except the sun, has been observed as a radio source. Attempts to identify individual radio sources with those of the nearest stars, stars brighter than magnitude

U, were unsuccessful. It appears that the intensity of radio sources is probably not related simply to the intensity of the light which they emit. In fact, one of the strongest radio sources Cygnus A appears to 71 be a very faint optical object of 18th magnitude. An intense radio source such as Cygnus A would be still easily detectable even at several times the distance. However, at such a distance it would be too faint to be observable optically even by the largest telescope.

There are some cases where an extremely accurate radio position has not led to the identification of a source with any visible object.

These facts seem to suggest that space contains a large number of stellar objects ’’radio stars" which are not visible or at least are extremely faint visually. 72

CHAPTER V

COSMIC RADIO BACKGROUND RADIATION

^.1 Introduction

This chapter describes the results of recent surveys made at a

frequency of about 250 Mc/s with Tie Ohio State University radio tele­

scope .

Radio surveys of the cosmic radio background radiation over the

sky have been made by a number of observers covering the frequency range

18.3 Kc/s to I4.8O Mc/s. Table VIII contains a list of the major publish­

ed surveys over various parts of the sky. Most of these surveys were

made with antennas of low resolving power, and were only able to dis­

close certain general features of the distribution of the cosmic radio

background radiation. A difficulty in the accurate measurement of the

distribution of the background radiation arises from the finite width

of an antenna beam used to intercept the back,ground radiation. The

observed distribution is smoother than the true distribution; the poorer

the resolving power of an antenna, the greater the smoothing. There­

fore, some of the detailed structure of the background radiation is

irretrievably lost.

The radio survey of the sky with, an antenna of high resolving power

is of considerable interest and importance. Two such attempts have been recently published by Hanbury Brown and Hazard (30) at 158.5 Mc/s in the regions of Cygnus, Cassiopeia and Perseus, and by McGee and

Bolton (5U) at UOO Mc/s in the region of the galactic center. Both

these maps were made with 2° x 2° beams but they cover only relatively

small regions of the sky. 73

Table VIII

Major Surveys of Galactic Background Radiation

Frequency Beam Width Survey Observers (Mc/s) (dog.)

1 Shain and Higgins (72) 18.3 17 x 17

2 Hey, Parsons and Phillips (36) 61* 13 x m

3 Bolton and Westfold (12) 10 0 17 X 17

h Brown and Hazard (30) 158.3 2 x 2

3 Reber (66) 160 12 x 12

6 Allen and Gum (2) 200 25 x 25

7 Kraus and Ko (i|7) 250 1.2 x 8

8 Ko and Kraus (i|0) 21+2 1.3 x 8

9 McGee and Bolton ($k) lt00 2 x 2

10 Reber (6 7 ) J4.80 k x it Recent radio surveys made with the high resolving beam of the Ohio

State University radio telescope have revealed much detailed structure of the background radiation. The radio maps cover most of the regions of the sky observable from Columbus. They also contain many regions of particular interest such as the regions of the galactic center, the galactic anti-center and the spiral arms. Thus our survey is the most complete one that is presently available. 75

5.2 Construction of a Radio Map of the Sky

The method of observation has been described in Chapter III. The

results of the observations consist of .sets of profiles showing equiv­

alent antenna temperature plotted against right ascension. ■ Each set

represents the observations made at a certain declination. Sets are ob­

tained at intervals of four degrees in declination between declinations

of -I4.O0 and + 70°. These sets of profiles were normalized to the same

standard receiver sensitivity. The effect of the side lobes were then

corrected. The bumps on the profiles due to the presence of intense

localized radio sources were also removed so as to leave only the

general background radiation. Such a profile then shows the variation

of an antenna temperature due to background radiateon as a function of

right ascension at a certain declination. Each profile is a curve

which has an arbitrary zero (or minimum level) of the antenna temper­

ature.

In order to construct a radio map, it is necessary to know the rel­

ative temperature of the minimum levels of the profiles. To this end,

the measurements were made in the vicinity of R.A. llh, the minimum

antenna temperature being near this region. The antenna is kept at each declination for a period of about five minutes, and moved"at four de­

gree steps in declination until the sky was covered from about ~1|0° to

+ 70°.

These results were then combined to produce a system of radio isopliotes of the sky temperature on an equatorial map (1950.0). Fig. 17 shows the radio map of the winter sky observed at 2lj.2 Mc/s. It sug­ gests how the sky would look like if our eyes were sensitive to radio + + 70 Fig. 17. Radio map of the winter sky made with The Ohio The skywith of made thewinter 17. Radio map Fig. • “ g 5 O z State University radio telescope.radio State University o o o K 5 © © © ©

76 in 77

waves instead of to light.

The observations were made during November 19 Sh through March

1?55 « A number of the intense radio sources detected in the course of

the survey are also indicated on the map. Solid dots represent local­

ized sources while open circles represent extended sources with angular

diameters of 1° or more. The size of each open circle is equal to the radio size of the source. The magnitude scale was chosen by Professor

Kraus (I4I4). The first magnitude corresponds to an intensity of 5 x

2) 10" janskys. (1 jansky = 1 watt per square meter per cycle per sec­

ond) . The equivalent black-bcdy temperature interval of the isophotes is about 5°K. The temperature of the coldest part of the sky (about

80° K) must be added to these values to give the absolute sky temper­ ature .

Fig. 18 shows a radio map of the summer sky observed at 250 Mc/s.

The observations x^ere made during the spring of 195U. In constructing this map, it was assumed that the sky temperature along a line at R.A. llh |iacj same absolute value. Each profile was therefore arbitrary adjusted to the sane zero level for all declinations along a line near R.A. II*1 30m , the minimum being near this line.

Scores of such profiles 'taken at four degree steps in declinations were combined to produce the map. Thus, the map may be subject to a slight distortion which would appear chiefly as a small variation of the sky temperature in declination.

The contribution of the radiation from intense radio sources was not removed from the profiles of the map of the summer sky. Hence, intense sources, such as Cassiopeia A and Cygnus A have the elliptical iim 79

shape of the antenna pattern. A minor lobe of the antenna pattern also

appears as an ellipse at the preceding sides of the intense localized

sources Cassiopeia A and Cygnus A. The power flux densities from radio

sources Virgo A and Centaurus A are however, not intense enough for the

minor lobes to appear on the map. The elliptical contours for Cassiopeia

A and Cygnus A resull from the effect of finite resolving power of the

antenna (1.2° between half-power point in right ascension and 8° in dec­

lination) and represent a direct measure of the antenna pattern of

the Ohio State University radio telescope. The elongated shapes and

the accompanying minor lobes are analogous to the defects in an image

seen with an optical telescope due to the aberration of the lens. Since

the map is a Mercator projection, a localized source near the celestial

equator such as Virgo A appears as a more slender ellipse than does a

radio source at higher declination such as Cassiopeia A. The broaden­

ing of the ellipse is proportional to sec £ where & is the declin­

ation of a radio source.

The radio isophotos of Fig. 18 have been drawn at equal intervals

of about 15°k . Also, since the minimum sky temperature is finite, not

zero as assumed, all contour-levels on the map should be increased by this minimum sky temperature (about 80°K) in order to give the absolute

sky temperature. 80

5.3 Discussion of the Distribution of Cosmic Radio Background Radiation

The results in Figs. 17 and 18 show the detailed structure of the distribution of the cosmic radio background radiation observed with The

Ohio State University radio telescope.

Of particular interest are:

(1) The distribution of cosmic radio background radiation shows

a high concentration in the plane of our galaxy. The plane defined

by the radio isophotes lies very close to the galactic plane de­

rived from visual observations.

(2) As a function of galactic longitude, the distribution increases

towards the galactic center and shows a general decrease towards

the anti-center. However, the minimum or col occurs at about

galactic longitude 193° instead of II480 which is the direction of

the galactic anti-center. \ o (3) The coldest parts of the sky extend from about Dec. -10 to

+ I4O and along the meridian line near R.A. 13*1 30m . The sky

temperature in this region is about 80°K.

(10 The plane of the radio isophotes shows a significant bias to

the south of the galactic equator at both galactic center and anti-

center regions. The amount of bias varies with the galactic long­

itude and reaches several degrees at certain places. This may in­

dicate that the plane of symmetry of the radio galaxy is biased to

the south of the galactic plane and, in addition, the pole of the

radio galaxy is tilted from the galactic pole derived from visual

observations. The width of the radio Milky Way also changes with

galactic longitude. 81

(5) There is a low intensity extended source near R.A. 13^

covering at least from Dec. -15° to + 15°. This elongated low

intensity source coincides with the plane of our local super­

galaxy described by de Vancouleurs (21) and probably is due to

radio radiation emanating from it. A detailed analysis made by

Professor Kraus (U2) of this region has established a close corre­

lation of the radio isophotes and the distribution of the Shapley-

Ames galaxies.

(6) The highest elevation on the maps, designated Sagittarius A,

stands out as a very sharp spike superposed on the general back­

ground radiation. It indicates the existence of a very intense

localized source. The equational coordinates (1950.0) of the

source as determined from the map are:

RA : l?n b2m U8S + 15s;

Dec.: -28° 50' + 3 0 '

The corresponding galactic coordinates (Ohlsson) are:

1 = 327°.8 ;

b = -1°.1|.

The flux density from this source is estimated at about 25 x 10”^ janskys. The angular extent of the source appears to be about

1°.5 at 120 cm. This region is heavily obscured by dust from op­ tical observations. Even observations in infra-red light have been unable to penetrate the dark obscuring clouds. In view of the fact that radio waves can penetrate these clouds with little absorption, and the position of the radio source agrees well with 82

the expected position of the galactic center, the radio source may

in fact be the galactic nucleus. A more detailed discussion on

the radio position of the galactic nucleus has been given elsewhere

0 *8).

(7) The region containing the constellation of Cygnus is quite

complex. This region contains at least several radio sources

(Cygnus A, Cygnus B, Cygnus X and Worth itaerican Nebula). Further­

more, there is a broad maximum of background radiation on which

these radio sources are superimposed. This broad maximum presum­

ably represents the radiation from the spiral arm in the galaxy.

(8) The most intense radio source in the sky is Cassiopeia A

(R.A. 23h 2lm ; Dec. + 58°).Cygnus A (R.A. 19h £8m j Dec. + i+0°.5)

is the next strongest source. In the winter sky, Taurus A (R.A.

cjh 3lm .£j Dec. + 22°.5) is the most intense radio source. The

radio map of the winter sky contains a large number of intense

localized and extended radio sources. One of the striking features

is the existence of a large number of sources having angular dia­ meters of 1° or more. Both intense localized and extended sources

show a marked concentration along the galactic plane. We consider

that they represent a definite class of objects in the galaxy. CHAPTER VI

A RADIO MODEL OF THE GALAXY

6.1 Introduction

One of the earliest attempts to explain the galactic radiation was made by Whipple and Greenstein (86) in 1937, who suggested that galactic radio radiation may be due to thermal emission from the dust particles in interstellar space. However, it was shown that the tem­ perature of the particles was too low to account for the measured radia­ tion intensity. In 19li0 Reber (69) suggested a mechanism in terms of free-free transitions in interstellar matter, which become the first theory to give a reasonable quantitative explanation to the galactic radio radiation. A quantum-mechanical treatment of this problem by

Kenyey and Keenan (3U) showed that it was in good agreement with the observational value given by Reber. Subsequently the problem has been treated ih more detail by several authors (van de Hulst (81), Towns

(78), Unsold (79)). Towns has shown that although the intensity can be explained at the higher frequencies, the results cannot be reconciled with Henyey and Keenan's formula at wavelengths longer than 5 meters un­ to less electron temperatures of at least lCr K are assumed.

With the discovery of increasing numbers of localized radio sources, these discrepancies are rapidly leading to the conclusion that the interstellar process does not represent a significant part of the galactic radio radiation. Various hypotheses have been advanced.

Pawsey, Payne-Scott and McCready (62) suggested intergrated emission from individual optical stars by a process similar to that operating on the sun at the times of high sunspot activity. Greenstein, Henyey and

Keenan (23), however, pointed out that if this were the case the aggre­ 8U

gate intensities of radio ra.diation from the stars 'would be far too

small to account for the measured intensity of galactic radio radiation

by factor of about 10^ to 10"^.

The observation of radio discrete sources suggests the existence

of a second mechanism. Furthermore, the lack of coincidence of radio

sources with visible stellar objects tends to support the theory that most of the galactic radio radiation can be attributed to the aggregate

emission from a distribution of a new type of star (Unsold (80), Ryle (68).

Westerbout and Oort (85) compared the radiation to be expected from objects distributed like common stars, (i.e. like ordinary G- and

K-t.ype dwarfs) with the radio isophotes observed by the Bolton and

Westfold at 100 Kc/s and found a reasonable agreement. They further postulated the existence of an isotropic component of radiation having an intensity of about 600°K. However, their model shows poor agreement at either higher or lower frequencies when comparisons were made with the observational data at 72 Mc/s measured by Hughes, 6U Mc/s measured by Hey, Parsons, and Phillips and at 160 Mc/s measured by Reber.

Bolton and Westfold (II4) have also made an analysis of the distri­ bution of sources of galactic noise in the galactic plane from the data of their own 100 Mc/s survey. They subtract 1000°K from the brightness temperature which is assumed to be due to "extranuclear" sources. These authors extended Von Zeipel's classical method of determining the dis­ tribution of stars in globular clusters to the case where the rays are not parallel, and the source function in the interior regions of the galaxy is found by solving Abel's integral equation. It shows that 85

there is a relatively high concentration about the center, five or six

times as dense as in the neighborhood of the sun and the steepest

gradient occurs from 0.2 to 0.5 in terms of the sun's distance. Away

from the galactic plane, the noise contours do not conform to a single

spheroidal source distribution.

Another analysis made by Wyatt (87), utilizing the Bolton and West-

fold's 100 Mc/s data (12) shows that the density function in the galactic

plane turns out to be approximately Gaussian in character. The model

has a fairly flat nucleus, 5*U + 1.2 (p.e.) times the local density near

the sun, and the sharpest decay in the regions of 2-k Kps from the cen­

ter, and a nearly linear decrease in the vicinity of the sun. This re­

sult agrees with that found by Bolton and Westfold (1U) for the inner

part of the galaxy. Wyatt also finds the spherical excess of U50°K

which is discovered by Westerhout and Oort (85), and an asymmetric com­

ponent in the anticenter direction. The asymmetric component may well

be due to the H II clouds in the Morgan-Osterbroek-Sharpless (59) spiral

arm which extends beyond the sun and exterior to it about 300 psc dis­

tant.

All these analyses have utilized the result of the Bolton-Westfold

100 Mc/s survey made with a 17° beam width, although correction was

made of the effect of antenna smoothing and the presumed true radio iso­ photes were deduced by successive approximation. However, the correction

of antenna smoothing effect may result in large errors when the actual dis­

tribution is considerably narrower than the antennabeam and the distri­

bution is unknown in both longitude and latitude. Therefore, it is

possible that the isophotes used by these authors do not represent 86

the true distribution.

Analyzing the spectrum of the radiation from chosen regions of the

galaxy, Piddington (63) has shown that 'the galactic radiation at radio

frequencies probably originates partly in hot ionized interstellar gas

and partly in stellar atmospheres. The ionized gas provides most of the

radiation at the higher frequencies and absorption at the lower. How­

ever , it is impossible to reconcile the shape of the radio isophotes at

higher frequencies with the optical evidence for the distribution of

ionized gas.

Recently, Hanbury Brown and Hazard (33) have constructed a radio

model of the galaxy which represents the results of radio surveys in

the range from 18.3 Mc/s to 1200 Mc/s. Their model consists of a

system of localized sources., a disc of ionized gas and an isotropic

component whose origin is probably extra-galactic. The space distri­

bution of the localized sources in the model differs significantly from

that given by Bolton and Westfold (1U)> and by Wyatt (87). The model

has a much greater concentration of sources towards the center of the

galaxy.

These galactic analyses at radio frequencies are based on the ob­

servations made with radio telescopes. The significant difference be­

tween the model of Hanbury Brown, and Hazard (33)> and that of Bolton

and Westfold (lit) arises on the choice of the radio data on which the

analysis is based. The former utilized the results of Reber1s (67)

I4.8O Mc/s with a J40 x i|° beam, while the latter used the results of

Bolton and Westfold's (12) 100 Mc/s observations made with a 17° x 17° beam. 87

Previous attempts to construct a radio model of the galaxy uti­

lized the radio data obtained by a reduction of observations taken with the broad antenna beam. If the antenna beam is sharp enough com­ pared -with the structure of the radio brightness distribution, then

the observed distribution m i l approximate closely to the true distri­ bution. Radio astronomers have attempted to restore the true distri­ bution which gave rise to their observational result, on the assumption

that the restoration obtained must be a close representation of the actual distribution. The reduction was carried out by applying the shape of the antenna beam to the assumed true distribution and compar­ ing it with the observational result. Recently Bracewell and Roberts

(16) have proved the invalidity of such a restoration by showing that for a practical antenna there is always an infinite number of different restorations all. of which give the sane observed distribution.

Since in the process of observing with an antenna, some of the de­ tail is irretrievably lost, correction for the blurring which arises from the finite beamwidth is in any case only partial. Thus observa­ tions with sharp antenna beams have become of increasing importance.

Such attempts have been made by Kraus and Ko (U7) in the case of one- dimension and McGee and Bolton (Sh}} and Hanbury Brown and Hazard (31) in two-dimensional observations. The former survey covers large areas of the sky while the latter is limited to relatively small regions of the sky.

The present analysis utilized the data obtained by The Ohio State

University radio telescope. This survey is the most complete one that is presently available. 88

6.2 Galactic Structure

The high concentration of the cosmic radio background radiation towards the galactic plane and galactic center has been clearly shown in radio maps (Figs. 17 and 18). these results leave little doubt that the major part of the background radiation originates within our own galaxy.

In this section the distribution of background radiation obtained by our radio surveys at 25>0 Mc/s and 2l|2 Mc/s is interpreted in terms of galactic structure. It is assumed that the observed radio intensity in any direction is a direct measure of the spatial extent of radiating matter in that direction.

Longitudinal and transverse profiles through the galactic nucleus as obtained from our radio maps are presented in Fig. 19. Fig. 19(a) shows the variation of radio brightness in the plane of the galaxy plotted as a function of galactic longitude. Fig. 19(b) shows the radio brightness at right angles to the galactic plane plotted as a function of galactic latitude at a constant galactic longitude of

327°.8 . The regions between galactic longitude 230° and 310° are be­ low the horizon and outside the view of our radio telescope.

In Fig. 20 the profile of Fig. 19(a) is shown plotted in polar co­ ordinates. All values between galactic longitude 230° and 310°, which are shown as broken lines, are extrapolated and follow the general trend of the observed value. The radio isophotes made by Bolton and Westfold at 100 Mc/s were used to guide the extrapolation.

Fig. 20 shows a main maximum in the direction of the galactic center (1 = 327°.8) and a secondary maximum towards the constellation ©1000

800

w 6 0 0

•H 2 0 0

5 6 O 3 2 0 2 8 0 2 4 0 2 0 0 1 6 0 1 2 0 eo 4 0 0 Galactic longitude 1 (degrees)

6 0 0

.3 2 0 0

-10 0 + 1 0 + 2 0 + 3 0 + 4-0 -t-50 + 6 0 + 7 0 + 8 0 + 9 0 Galactic latitude b (degrees)

Fig. 19* Profiles of the radio brightness temperature in and perpendicular to the galactic plane through the galactic center. 90

1 8 0 c 90

Sun Spiral arm

270c

1 Galactic center

Fig. 20. Polar variation of radio brightness temperature in the galactic plane. 91

of Cygnus (1 = 1|8°). If the variation of sky temperature can be inter­ preted as a measure of spatial extent as seen from the sun, the pro­

file shown in Fig. 20 indicates that the position of the sun is within

a spiral arm or between the arm and the nucleus. In the direction of

secondary maximum toward Cygnus, we may be looking tangentially along

an inner spiral arm.

This result is similar to that'obtained by Bolton and Westfold (13)

and consistent with the present picture of the spiral galaxy derived by optical observations. 92

6.3 The Center of the Galaxy

During the past year a number of relatively accurate observations

have been made with high resolving power antennas in the region of the

center of the galaxy. The earlier observations of this region were made with such a low resolving power that the detailed structure was not revealed. However, when observed with a narrow- beam antenna (see

Fig. 18) the sharp maximum at RA I?‘^1i4.2mlt8s* Dec. -28°50'(1950.0)

appears as a remarkably strong discrete source. This is illustrated in Fig. 19 which shows profiles in galactic longitude and latitude

through this discrete source as measured with The Ohio State University radio telescope at a wavelength of 120 cm. The source, Saggitarius A, lies at the highest elevation on the map (Fig. 18) and very dose to

the accepted direction of the optical galactic nucleus which is heavily obscured by dust clouds. The significance of the position may be con­ siderable. Its close correspondence with the expected positions of the galactic nucleus derived by optical data suggests the possibility that it may, in fact, be the center of the galaxy. If this is so, it must represent the effect of the total sum of the radiation from an extreme­ ly high concentration of localized sources in this region. There is left another possibility that it may due to an unusually intense source, such as Cassiopeia A type, which happens to be in the direction of the galactic nucleus. Recently Davies and Williams (20) have argued that the discrete radio source, Saggitarius A, may be identified with a dense H II region which lies at about 2.5 kps from the sun and in the direction of the galactic center. The determination of which is correct is of considerable importance in the development of a radio model of 93

the galaxy.

The radio source near the galactic nucleus was first discovered,

using a pencil-beam antenna, by Piddington and Minnett (61;) at 1210

Mc/s. At this frequency the general background radiation is too weak

to be observable. Similar observations with improved narrow beams have been

recently made by Haddock, Hagen and others at the Naval Research Labor­

atory (26, 27), Mcgee and Bolton (5U)j van de Hulst, Muller and Oort

(82). The wavelengths extend from 3cm to 30cm. These observations

probably refer to the same object.

A determination of the accurate position of the nucleus is a matter

of considerable interest. The positions obtained by five different

groups of observers have been averaged to give a most probable position

for the nucleus (IqB). The most probable position of the galactic

nucleus is

RA 17n l;2ra 1;0S

Dec. -28°5>0' (1950.0).

In galactic coordinates, the position is

327.°79 + 0?10 (longitude)

- l.°39 + 0?10 (latitude)

The measurement of the power flux density for the Sagittarius A

source presents more difficulty and uncertainity than the determination of its position. The correction for the finite size of the source is difficult and gives additional error. The spectrum is shown in Fig. 21,

The source extent observed at different wavelengths is also presented.

There is a large scatter in the data, and the result cannot be re- Angular diameter Pcwer flux density 0 0 0 1 100 1 r ° 2 10 anskys ° xlO-2^ Fig,21, Radio spectrum andangular diameterof Sagittarius A, 100 100

0 0 2 200 OSU 05 U

1*00 MB(5U) U00 *

Mc/s Mc/s 000 1 1000 HMH(27)

0 0 0 2 00 uooo 2000

9h garded as conclusive in view of the many uncertainities involved. We consider that present knowledge suggests the presence of an intense localized source of non-thermal nature with hydrogen clouds in front of the source. The interpretation is complicated by insufficient data on this region and further investigation of the fine structure of this region is desirable. 96

6.1; Construction, of a Radio Model of the Galaxy

In section 2 of this chapter, the results of our radio surveys made with the sharp beam of The Ohio State University radio telescope at the frequency of 2^0 Mc/s were interpreted in terms of galactic structure. Assuming the radio brightness in any direction is a measure of the extent of the galaxy in that direction, the analysis indicates that the sun lies in or near a spiral arm of the galaxy.

In the present section, the same observational data are analyzed in more detail to construct a radio model of the galaxy. It is known that there are at least three distinct components of the observed radio radiation: (i) localized radio sources in the galaxy, (ii) ionized interstellar gas, (iii) an isotropic component.

The effects of interstellar matter on the galactic radio radiation will be first discussed. These effects will be of two-fold: absorption and radiation.

The knowledge of the physical conditions in interstellar space has advanced rapidly during the past ten years. It now appears likely that the interstellar gas is concentrated in the galactic plane. About 10/ of the interstellar clouds are ionized (H II regions) where the electron temperature is about 10,000°K. and the electron and hydrogen densities are about 5 to 10 per cur*. The remainder of the space is neutral hydro­ gen (H I regions) where the kinetic temperature is probably about 100°K with hydrogen density of 1 cm . It can be shown that the H II regions have the greatest effect on the radio radiation at frequencies around

25>0 Mc/s. In all other interstellar regions either the temperature or the density is too low for appreciable radio frequency radiation. 9 7

The effect of the H II regions on the galactic radio radiation will

now be considered. The value of the absorption coefficient for free-

free transitions within an ionized gas is given by Smerd and Westfold

(73) as

K (f) = he6 Al(2) Ne Ni Z2 3 /2? (mek Te)3/2 c f2yU f (6.1)

where Ne = electron density

Ni = ion density

Z = atomic number

JJ'f = refractive index

Te = kinetic temperature

and

where d is the mean distance between electron-ion pairs. Following 2 Westerbout and Oort (85), Z Ni is taken to be 1.1, as nearly all ions are

2 Ne protons and Z Ni will be very close to unity. The value of the refrac- N tive index JX^ was taken unity. With Ne * Ni = 8, A^(2) becomes 31.

Changes in Z, T on d will produce only a small variation in A]_(2). The

absorption coefficient can now be written as

K(f) = 0.17 Te"3//2 f_2Ne2 (6.3)

The optical depth for a frequency f will be

= y K(f) ds

= 0.1? Te"3//2 f"2 ^ Ne2 ds. (6.10 iNe2ds is the "emission measure" of the H II regions. Following the analysis of Westerhout and Oort (Bp), we assume that

a line of sight in the galactic plane crosses 5 to 10 clouds per kps and

about 10/ of the interstellar clouds are ionized. Thus, there is be­

tween vr and 1 ionized cloud per kps. We will adopt 0.8 ionized clouds 2 20 per kps. Stromgren (75) has reported the value of j Ne ds = 12x10 for an average ionized cloud from the observation of the intensity of

H p . Therefore, the optical depth per one ionized cloud with an electron temperature of Te = 10,000°K is - 0.0031). at 250 Mc/s. This corresponds to X = 0.0027 per kps. Thus, the radio brightness temper­ ature due to the interstellar gas is 27°K per kps at 250 Mc/s. Taking the extent of the galaxy from the sun in the direction of the galactic center as 22 kps, the total optical depth at 250 Mc/s is 0.06. Thus, absorption at 250 Mc/s is negligible even in the direction of the largest galactic extent. The only significant effect of the gas will be its contribution to the background radiation.

It may be seen that the observed radio brightness is far in excess of what can be expected from the radiation of the interstellar gas.

Therefore, it seems reasonable to assume that most of the galactic back­ ground radiation is due to the aggregate emission from a distribution of a new type of object or radio source. These radio sources are typified by those discussed in Chapter k-

In constructing a radio model of the galaxy, one needs to introduce several assumptions for the convenience of the analysis. First we assume that the ionized gas clouds have a uniform distribution over a cylindri­ cal region extending 10 kps from the galactic center and having thickness of 200 psc. This model is similar to those used by Westerhout and 99

Oort (8£) and Hanbury Broivn and Hazard (32) except the small difference

in the value of the radius of the cylinder. A radius of 10 kps is used

in our model instead of their 1 2 .5> kps, because, of the recent improved

value 8.2 kps (83) of the distance of the sun from the galactic center.

Second^ we assume that the galaxy is axially symmetric about its polar

axis of the rotation, and the sun is in the galactic plane at a distance

of 8.2 kps from the center of the galaxy.

The results of the observations of the cosmic radio background

radiation (see Chapter $) will be used to construct a radio model of the

galaxy'. The profiles of the observed radio brightness temperature through

the galactic center in and perpendicular to the galactic plane have been

shown in Fig. 19. It has been shown above that the absorption of gas at

250 Mc/s is negligible even in the direction of the largest galactic

extent and the only significant effect of the gas will be its own radi­

ation. The variation of the radio brightness temperature with galactic longitude and latitude due to the radiation from the model of ionized gas clouds is first computed. These calculated values are then used to correct the profiles of Fig. 19 so as to obtain only the radio brightness temperature arising from a distribution of radio sources and isotropic component. These corrected galactic longitude and latitude profiles of the radio brightness temperature will be used to deduce a distribution of radio sources as a function of distance from the galactic center.

We shall now consider the contribution of the localized sources in the galaxy to the background radiation. Ihe method used is similar to

that used by Vfyatt (87). 100

Let

r) = the radio source density function; number of radio

sources per cubic parsec at a distance r psc from

the galactic center in the galactic plane.

L = the average radio luminosity of the source, w/cps

I = the radio brightness of the background radiation

observed at the earth; (w/m /cps/steradian)

SI = beam area of the radio telescope

S = distance of the source from the sun

rQ = distance of the sun from the galactic center, 8.2 kpc.

l r = modified galactic longitude, the angle between line

of sight and the galactic center.

From Fig. 22, we have

S = rc cos 1* + (6.5)

dS r dr (6.6) £ r2 - rJ5 sin^ 1 ’_]/*

Then the radio brightness of the background ground radiation in the direction of the modified galactic longitude l 1 ( =1 - 1 ) is

kJLteL- S2 sin0 d

It has been shown in Eq. (2.1;), Chapter 2 that the radio brightness can be expressed by the equivalent black-body temperature, i.e. 101

galactic center

Fig. 22. 102.

Thus, Eq. (6.7) becomes 00

T (1') = / ( r ) dS (6.8)

V By symmetry, T(l') = T(-l'), and T(ir+1') » T(TT-1‘)*

The sky temperature in the region 0 ■$ 1' ^ j can be rearranged as

T(+l') =2LJL f J P (r) 8 Tr k LJ iLcesr

2.y„cosZ' _ X L f[ J I fP Mm ds + ! f (r)------^ (6 .9 ) S it k L J f? „2 n,.. [r2 -r02 Sin l]'

and 60 00

T(U+1') - P(r) dS = Z i L f P(r) r ■*. „ (6.10) BiTk I J 8irk / J *' [ r 2 - r 02 S i n

Equation (6.10) shows that T(n + I 1) is equal to the second term in

Eq. (6.9). We may thus consider T(tt + I 1) as the contribution from the extranuclear sources. Then the first term of Eq. (6.9) represents the total radiation originating from the sources inside a circle of radius rQ about the galactic center. We shall call it the internuclear source.

The contribution from this component is, DTpC °st'

T( + 1') - T(tt + 1») = * L f P (r) dS (6.11) Q tt J J 0

This procedure (Eq. 6.11) eliminates the effect of an isotropic component 103 and the radiation arising from the radio sources outside the internuclear region.

Our problem now is to solve for the density function of radio sources J3 (r) from a knowledge of T( + 1') - T(tt+ 1*) which is de­ termined from the observations.

The interior radio galaxy is supposed to be built up of n concentric uniformly dense spheroids. In the galactic plane, we will then have n concentric rings of radii and densities as follows:

Radii (in r ) n " ^ n - 2, ...... 2, 1 n n n n n

Source density Jn-1, j*n - 2, ...... f 1,

Sky temperature T(l‘n _ -,_), T(l«n _ 2), .....^(l-^), where l 1 _ m represents the modified galactic longitude in the direction tangential to the (n - m)th ring, (see Fig. 23).

Let us first make a general consideration. The sky temperature

T(l*n _ m ) consists of the contributions of the radiation from radio sources in the (n - m + l)th, (n - m + 2)th, and nth rings.

Let us denote by Tn _ m p(l'n _ m ) contribution from the sources in the (n - m + p)th ring. The density of the source in this ring is f n - m + p.

Then, referring to Eq. (6.11) and Fig. 23, m ' 2 ^ ♦ p d ' n - m ) (6-12) P - 1 104

n_thring n-1 n-m

l‘=0 S

Fig. 25 10$

SR and dS Ln - m + p (1'» - SB

AZ L BA /»- m + p (6.13) UTTk where BA = £ ™ E0 2 - AOTr>-2 ] 2-r>? = f ( n - m + P rD) " (n - m rD) ] L n n J

= r r (n ~ m + p ^2 “ (n - m )21 ° L n n J

Therefore, Z t C11 ^ ^ r0 P n - m + p- in - m + p'- n - m' r - ^ - J Inr k

i . 2 2 ~I 2 (n - m + p ) - (n - m ) (6.1U) n n J

We shall now turn to consider T(l* _ , the sky temperature to be expected in the direction tangential to (n - l)th ring. Since the line of sight does not intersect the remaining (n - 1) rings, only the nth ring will contribute the radiation. Putting m = 1, and p = 1 in

Eq. (6.11).), we arrive at

T(l« ) = f L r° P n T 1 “ (n - 1)2 ] (6.15) n - 1 1+ ir k J L n J

Let us next consider T(l' 2)) temperature in the direction tangential to the (n - 2)th ring. Sources contained in the two rings, 106

n-th and (n - l)th, are now responsible for the radiation. 2^

then consists of two parts, one from the sources in the n-th ring, and

the other in the (n - l)th ring. The first component can be obtained

from Eq. (6 .II4.), by substituting m = 2, p = 2. We have then

j. . z A Lrp 1 - (n - 2)‘ (6.16) Tn - 2) ■ M r k n

The second component is obtained similarly by putting m = 2, p = 1, and is K (n-1)2 - (n - 2)21 T n - 1 (6 .17) 'n - K l ' n - 2) - f n n

Therefore, we have from Eq. (6.12) 2

T(1,n - a5 ’ P ? 1 T" - 2 * P (1’n - 2>

■2. > L rp (n-1)2 (n-2)2 UTT k /"-I n n

■>? L rc 1 - (n - 2) *+ In-f £(n - l)2 - (n - 2)2j^(6.l8) Inr k n

Hence, from Eq. (6 .l£), L J=’n may be computed if T(l* _ -]_) is known from the observation. Substituting this result in Eq. (6.18), we then obtain

or fn-l f n - 1 f n

Tills procedure is followed for the remaining rings and /n-2, f n 107

may be found.

The density function P(r) is then obtained from

m f M . J ( E r0) = 2 f a (6.19) n-1

Using Eq. (6.11) and the profiles of the radio brightness temperature

which has been ocrrected for the effects of ionized gas clouds, the

variation of the radio brightness temperature arising from a distribution

of localized radio sources in the internuclear region can be found. This

is shown in Fig. 2U. The profiles shown in Fig. 2k represent the varia­

tion of the radio brightness temperature due to internuclear radio

sources as a function of modified galactic longitude 1' and latitude b'.

Following the method described above (Eqs. (6.12) through (6.19)),

a distribution of the relative space density of the radio sources in the

galactic plane is then obtained. Fig. 25 (a) shows the variation of the

relative space density of the sources in the galactic plane with distance

from the galactic center. The profiles derived by others (30, 87) are

also included in the figure for comparison. These will be discussed

later. The distribution normal to the galactic plane through the galactic

center is found by assuming that equidensity surfaces are spheroidal and by adjusting the ratio of their axes to give a latitude distribution which agrees with the observed temperature distribution in latitude. The result is shown in Fig. 25 (b).

The profiles in Fig. 25 indicate an outstanding feature in the region 108

g 1000

1 & 800 § •p m 600 a •P § 4oo u rO o $ 200

20 40 60 80 (degrees) (a) 1»

°K

1000

20 40 6 0 80 (degrees) (b) b 1 -=>

Fig. 24. The variation of the radio brightness temperature due to internuclear radio sources as function of modified galactic longitude l 1 (a) and latitude b 1 (b). 109

1.0

•H Wyatt(87) V |0.4 3 0.2

1 2 4 6 7 8 9 Distance from the galactic center (kps)

(a) The variation of the relative space-density of radio sources in the galactic plane as a function of distance from the galactic center.

V 0.6

0.4 •H -P rH 0 . 2

0.1 0.2 0.5 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Distance from the galactic plane (kps)

(b) The variation of the relative space-density of radio sources along a line through the galactic center normal to the galactic plane as a function of distance from the galactic plahe'. Fig. 25 . 110

of the galactic center. The majority of the sources within the galaxy

appears to be densely concentrated in a spherical volume at the center.

The volume has a diameter of about 200 psc. Outside this central nucleus

the density of the sources falls off very rapidly in latitude, but fairly

slowly in longitude.

The density distribution outside of the galactic plane appears to be very complicated. A simple spheroidal distribution about the center has been chosen to fit the observed data without success. Attempts to use a simple model of disk are equally unsuccessful. There seems to be

an irregular excess radiation due to local structure of the galaxy which may destroy the symmetry. The failure to derive the distribution of the

sources outside the galactic plane has also been pointed by Bolton and

Westfold (lU), and Wyatt (87 ).

An examination of the observed distribution of the radio brightness temperature arising from extranuclear regions indicates the existence of a constant residual temperature of about 80°K over the sky. The origin of this temperature may be extra-galactic. The existence of such iso­ tropic component was first noticed by Westerhoutand Oort (85) in their analysis of 100 Mc/s data.

There are also some local excesses of radio brightness temperature.

They may represent the local structures of the galaxy. The spiral struo- ture would presumably destroy the symmetry in the extranuclear regions.

The big rise at about 1 = 1|0° in Fig. 19 is probably due to a spiral arm in the galaxy.

It is of interest to see if the source distribution derived in the model are consistent with the sources which have actually been detected Ill by our observations. Eq. (6.15) enables us to calculate a volume

emissivity L J3 n near the sun where L is a mean radio luminosity of

sources and j3n an average space densitjr of sources near the sun. It Q is found that the volume emissivity near the sun is about 8 x 10 3 w/cps/psc . On the other hand, in section h-3, we have analyzed the

distribution of the Class I radio sources detected in the regions of the

anti-galactic center. We have considered that these sources are near

the sun, and a mean radio luminosity of 1.2 x 10^ w/cps and mean space 8 3 density of Ij. x 10 sources/psc are obtained. This corresponds to a 8 3 volume emissivity of 5 x 10 w/cps/psc , which is in good agreement with

that obtained from the model. Therefore, it may be tentatively assumed

that the Class I radio sources actually detected in our radio surveys

constitute the radio sources In our model.

The space-distribution of the sources derived above differs signif­ icantly from those given by Bolton and Westfold (llj.), Wyatt (87), and

Hanbury Brown and Hazard (32). The profiles derived by these authors are also shown in Fig. 25 for comparison. Our model shows a considerably greater concentration of radio sources in the galactic center than those obtained by the others. Oort and Van Woerkom (61) have derived the dis­ tribution of mass in the galaxy using a dynamical theory and the observed distribution and velocities of the common stars. The density of the mass in the galactic center is found to be about five times as dense as that near the sun. Furthermore, their model is found to fit the results of the radio observations made at 100 Mc/s (85, 87). Therefore, it has been believed that the radio sources are distributed in a more or less similar manner to the common stars. However, our present model indicates that this seems to be incorrect and the density of the sources in the galactic center is much greater than that near the sun. The 100 Mc/s radio data used by Westerhout and Oort (85) were obtained by a reduction of radio surveys made by a 17° antenna beam (12). It is therefore, possible that the isophotes used by them do not represent the true dis­ tribution of the radio radiation. 113

6.5 Comparison with the Andromeda Nebula

It has been shown that most of the cosmic radio radiation received

on the earth has its origin in the radio sources. These radio sources

are widespread throughout the galaxy. Whatever the mechanism of radio

sources may be, this result suggests that extra-galactic nebulae similar

to our own galaxy are also radio emitters. Attempts have been made in

the past by several radio astronomers to detect radio emissioxu from ex­

ternal galaxies. Reber (65) attempted to detect a radio emission from

the Great Nebulae in Andromeda (M 31) at 160 Mc/s without success. The

identification of several faint radio sources with near-by external

galaxies (M31, M5l, M101, M33) was first suggested by Ryle, Smith and

Elsmore (70). However, it remained for Han bury Brown and Hazard (28) to make a definite identification of a radio source with M31. They have shown that the size and shape of the radio source coincide with those of the Andromeda Nebula. Recently Professor Kraus (k3) has measured the distribution of radio brightness across this nebula at 250 Mc/s using the sharp beam of The Ohio State University radio telescope, ob- pj taining a total radio intensity of about 10~ janskys from the nebula.

The Andromeda nebula is a spiral galaxy (Sb-type) similar to the galaxy in which we are located. It is the only spiral visible to the naked eye, and one of the nearest galaxies. It is thought to be larger than our own galaxy. It is of interest to compare the results of our radio observations of M31 with our radio model of the galaxy.

The total radiation from the localized radio sources in the model of the galaxy is calculated by integrating the space distribution shown in Fig. 25. It is assumed that the axially symmetrical source distri- Ill* 12 bution is spheroidal with an axial radio of 0.2 . Hie value of 2 x 10 8 *3 L n is then obtained. Substituting L n = 8 x 10 w/cps/psc derived

above, the total radiation from all the sources in the galaxy is about 21 1.6 x 10 w/cps at 250 Mc/s. Hie total emission from a disc of ionized 20 gas has been calculated to be about 1.2 x 10 w/cps at 250 Mc/s. Since

the absorption due to gas at 25>0 Me/s has been shown to be negligible, 21 the total radiation from the model of the galaxy is about 1.7 x 10

w/cps/ at 250 Mc/s. About 93 per cent of the radiation arises in local­

ized sources and only 7 per cent is due to ionized gas. If our galaxy

is removed to the distance of the Andromeda, say 500 kps, the intensity ^pli O to be expected on the earth would be 0.57 x 10“ w/cps/m . This agrees

reasonably with the value of 10“^ janskys for M31 measured by Kraus (U3)»

It is interesting to see how our radio model of the galaxy would

look at a large distance with our radio telescope. A distance of 100

kps is arbitrarily chosen as at this distance the model will subtend

an angle of about 9° which is comparable to that of M31 measured by

Kraus (1*3)» In this analysis the antenna pattern of The Ohio State

University radio telescope is then superposed on the model of the galaxy.

Hie received intensity is computed graphically at successive positions

across the model. The result is shown in Fig. 26. The antenna pattern for a point source is also shown for a comparison. Since the antenna o pattern for a point source is 1.2 wide between half-power points in o right ascension and 8 wide in declination, the beam width is sufficient to cover a large portion of the galaxy when the antenna is at the center of the galaxy. The total radiation arising frcm the nucleus of the galaxy is less than 1 per cent of the total radiation from the whole \

our galaxy Andromeda nebula antenna pattern

Fig, 26, Distribution of radio brightness across our galaxy removed to a distance of 100 kps. 116

galaxy. Therefore, the nucleus of the galaxy does not stand out as a

sharp spike on the radio brightness profile (Fig. 26). The distribution

of the radio brightness across the Andromeda Nebula measured by Kraus

(U3) is shown by dashed line in Fig. 26 for comparison and it also shows

a similar profile.

In order to resolve the galactic nucleus by radio observation, the resolving power of a radio telescope has to be comparable to the angular

extent of the galactic nucleus. In the case of our galaxy removed to a

distance of 100 kps, it would require a beam width of the order of one

tenth of a degree. 117

CHAPTER VII

SUMMARY

An investigation of the cosmic radio radiation at 2^0 Mc/s and

2I4.2 Me/s using The Ohio State University radio telescope is described.

A radio map of the sky is prepared showing the distribution of cosmic radio radiation. The high resolving power of Ihe Ohio State

University radio telescope reveals much detailed structure of the back­ ground radiation. Ihe distribution of the radio brightness shows a high concentration in the galactic plane and towards the galactic center. Ihe distribution also shows a significant bias to the south of the galactic plane both in the regions of the galactic center and anti-center. The radio map covers most of the sky observable from

Latitude north k 0 ° and is the most complete map that is presently available.

On the basis of the results of the radio surveys of the sky, a radio model of the galaxy is constructed. The model consists of three components: a disk of ionized gas clouds, an axially symmetric dis­ tribution of localized radio sources which are highly concentrated in the galactic nucleus, and an isotropic component whose origin may be extragalactic. Ihere are, in addition, some irregular excesses in the distribution of the radio brightness which may be associated with the spiral structure of the galaxy. The total radiation from the galaxy is 21 approximately 1.7 x 10 w/cps, and the power emitted by the sources in O o a cubic par sec near the sun is found to be about 8 x 10 w/cps/psc at

2f?0 Mc/s. 118

The spacial distribution of the radio sources in the model differs

significantly from those previously obtained by others. Our model

shows a considerably greater concentration of radio sources in the

galactic nucleus than those obtained by others. Furthermore, our model

indicates that the radio sources do not have the same distribution and

space density as common stars.

A number of radio sources have also been detected during our radio

surveys of the sky. Their distribution, spectra and identification

with visual objects are discussed. A striking feature is the large

number of extended sources lying near the galactic plane and presumably

associated with our galaxy. It is suggested that the radio sources in

the model correspond to those which have actually been detected by our

radio telescope.

The radio emission from individual ionized hydrogen nebulae, such as the Rosette nebula, Worth American nebula and Orion nebula, is for the first time detected at the meter wave lengths. The result of the analysis for the Rosette nebula is consistent with the hypothesis of thermal emission by free-free transitions from the nebula. 119

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AUTOBIOGRAPHY

I, Hsien-Ching Ko, was born in Tainan, Formosa, April 28, 1928.

I received my secondary school education in the public schools of the city of Tainan, Formosa. My undergraduate training was obtained at the National Taiwan University, from which I received the degree

Bachelor of Science in Electrical Engineering in 1951. In 1952, I came to the United States, beginning my graduate work at The Ohio State

University. From The Ohio State University, I received the degree

Master of Science in Electrical Engineering in 1953* "While completing the requirements for the degree Doctor of Philosophy, I acted in the capacity of research assistant to Professor John D. Kraus of the Depart­ ment of Electrical Engineering from 1952 to 1955* My research activi­ ties have been in the field of r§dio astronomy.