The Ohio Sky Survey and Other Radio Surveys

Vistas in Astronomy, Vol. 20, pp. 445-474. Pergamon Press, 1977. Printed in Great Britain.

THE OHIO SKY SURVEY AND OTHER RADIO SURVEYS

John D. Kraus

Ohio State University Radio Observatory Colombus, Ohio

ABSTRACT

Section I indicates the importance of astronomical surveys and comments on some of the earliest radio surveys. Section II describes the Ohio Sky Survey which totals over 19,0OO radio sources at 1415 MHz. Methods of observation, data reduction and analyses of reliability and completeness are considered. Section III discusses the special Ohio sources with centimeter-enhanced (CE) spectra, including ones of the BL Lac type such as OJ287 and ones with high redshlft such as OH471 and OQ172. Section IV relates the Ohio survey to other surveys, including data on other large surveys such as the Cambridge and Parkes surveys. Source counts are considered and statistics presented on the numbers of radio sources, identifications and radio astronomers. The last section, V, treats source nomenclature and the need for a single perpetual epoch.

I. INTRODUCTION

Progress in astronomy is dependent on surveys, optical, radio, X-ray, and infra-red. Thus, the National Geographic Society-Palomar Observatory Sky Survey of the early 1950s has been of enormous importance. This survey with the 1.2 m Schmidt telescope was done to provide the information necessary to properly utilize the larger 5 m (200 inch) telescope but its usefulness has far exceeded anything envisioned originally.

The many radio surveys are also important. They facilitate studies of individual sources and associations and make for efficient use of large instruments. For example, the new Very Large Array (VLA) of the National Radio Astronomy Observatory being constructed at a cost of $80 million on the Plains of San Augustin in New Mexico (Heeschen, 1975) will have such high resolution that to survey the entire sky at this resolution would take centuries. The VLA is not an all-sky survey instrument but a sumvey-dependent instrument.

The early observations of Jansky and Reber established the existence of radio emission from the sky but the broad beams of their telescopes hinted only vaguely at the details of its distribution. Later, with sharper beams, surveys were made at Cambridge, England (Ryle et al., 1950), at Sydney, Australia (Mills, 1952) and in Ohio, U.S.A. (Kraus et al., 1954). These surveys listed 50, 77 and 207 sources, respectively.

Ryle's and Mills' observations were made with interferometers at 81 and IO1 MHz, respectively, while the Ohio survey was conducted with a fan beam at 250 MHz. In the region common to the Mills and Ohio surveys 83% of Mills sources "coincided" with ones in the Ohio. In the region common to the Ryle and Ohio surveys, 50% of Ryle's sources "coincided" with ones in the Ohio survey, but only five sources were common to all three surveysl Some lack

445 446 J.D. Kraus of coicidence could be attributed to the different frequencies used. Another inference is that the surveys had not yet reached a high level of reliability. They were how-- ever, an essential first step. With higher resolution radio telescopes more and better surveys followed.

The first surveys suggested that none of the radio sources corresponded to any optical star (except for the sun). The identification of the strong radio sources Cassiopeia A and Cygnus A with a supernova remnant at II,000 light years and a peculiar galaxy at almost one billion light years underscored the additional conclusion that the radio sky was no carbon copy of the optical sky and that radio sources, even a few of the strongest ones, represented a sample in great depth of the universe.

For example, of the 500 strongest radio sources in the Ohio survey (see section II) nearly all are probably farther than IOOO light years and most are much farther, including 0Q172 with the largest measured redshift (z = 3.53), placing it at more than 12 billion light years (assuming a cosmological interpretation of the redshift). By comparison most of the 500 brightest optical objects are closer than 500 light years. Put another way, the most distant known object (0Q172) is among the 500 strongest radio sources but there are over IOO million optical objects brighter than OQ172. This penetrating, in-depth perspective of radio telescopes makes radio surveys very important in furthering our knowledge of the cosmos.

In this article the term "survey" is used to mean a true, primary or finding survey, that is, one which is conducted without regard to any prior knowledge of what may be present and this article is confined to a discussion of surveys of this type. The re-examinations of particular objects or groups of known objects are, by contrast, secondary surveys. The Ohio sky survey is a true, primary, finding survey and is described in section II. A summary of other large primary surveys and the relation of the Ohio survey to them is given in section III.

II. THE OHIO SKY SURVEY

The first Ohio State University survey, already referred to, was conducted in 1954 at 250 MHz with the 96 helix radio telescope of 330 m 2 aperture shown in Fig. 1 (Kraus, 1953, 1956). In 1956 construction, with student labor, was started on a much larger telescope consisting of a iiO x 21 m standing parabola and a 104 x 31 m tiltable flat reflector of 2200 m 2 aperture as shown in Fig. 2 (Kraus, 1955, 1963, 1966). A personal behind-the-scenes story of the construction of these telescopes and of observations made with them is related in Big Ear (Kraus, 1976).

By the mid 1960s the big telescope was sufficiently complete for a small sky area, which included M31, to be surveyed at 612 and 1415 MHz (Kraus, 1966). A total of 128 sources was found. These were designated with the prefix OA (O for Ohio, A for List A) followed by a number which increased monotonically as a function of right ascension.

Then beginning in March 1965 and continuing for nearly IO years the new telescope was used to survey 94% of the sky area between the limiting declinations of 63°N and 36 ° S at 1415 MHz.* The telescope was designed as a primary survey instrument to search for sources and

*The remaining 6% of sky was largely along the galactic plane and, although surveyed, the data were not reduced because the background gradients were too large to permit data reduction by the digital methods employed. The Ohio Sky Survey 447

its merit for this work has been widely recognized, for example, by Gindilis (1971) who states "the Ohio radio telescope is the most suitable for sky survey purposes". The survey yielded over 19,0OO radio sources at flux densities above about 0.2 janskys. (Jy). It is this survey which will be described in this section and which will be referred to as the "Ohio Survey".

The Ohio Survey was published in seven installments and two supplements. Data for the survey are summarized in Table I. The Ohio Survey is unique in that it covered a larger portion of the entire sky (69% or 8.7 ST) to a greater depth (2270 sources per ST) and was at a higher frequency than any previous deep large-area sky survey. The survey is also unique in that all previously catalogued sources are also tabulated and maps of the areas surveyed are included with the positions of all catalogued sources shown.

Fig. 3 is a master map (epoch 1950.0) for the regions covered. The 6% of sky not included is shaded. The roman numerals indicate the survey installment containing data for each block of sky (i h x 15°). The numeral "2" indicates that the area was included in Supplement 2. All seven installments and Supplement 2 contained both lists and maps. Supplement I listed fourteen sources but with no maps. The positions of the Supplement I sources are shown in Fig. 3 by dots.

The seven installments and two supplements of the Ohio Survey are available as individual reprints and also as a single bound volume. The source list is also available on magnetic tape or in computer printout form with listings in both right ascension and declination order.

The sources in the Ohio Survey are assigned names according to a coordinate numbering system consisting of a two letter prefix followed by three digits. The first letter (0) stands for Ohio and the second letter (B to Z, inclusive, omitting O) indicates the source right ascension in hours (O to 23, inclusive). The first digit indicates the declination zone in increments of IO ° while the last two digits give the right ascension to the nearest one- hundredth of an hour. For example, OD124 represents a source at a right ascension of O2 h 14~4 (±O~3) (D = 2h; .24 x 60 = 14~4) and a declination between IO ° and 20 ° . This designation is both more concise and also more accurate in right ascension than many other coordinate system designations. In an Ohio name the right ascension is given to the nearest ±18 s while in the Parkes system the right ascension is quantized in i m steps. Thus, B1216+18 could mean a source at 12 h 16 m O0 s or one at 12 h 16 m 59 s.

For southern declinations a minus sign is inserted in the Ohio name between the two letter prefix and the digits which follow. An extra digit (with decimal point) may also be added to an Ohio source name to give the right ascnesion in thousandths of an hour. This has been done in those cases where the source density near a given right ascension is sufficiently great. For example, OZ- 58.5 indicates a source with a right ascension of 23~585 or 23 h 35 m o5S±2 s. The matter of source nomenclature is discussed further in section V.

A sample of a few sources from the list of Installment V is presented in Table 2. The first column gives the source name, the next columos the right ascension and declination (epoch 1950.0) and the last two columns the flux density at 1415 MHz in janskys (Jy) and "Remarks". Note that the last digit of the source designation increases monotonically with right ascension. The sources with 75 for the last two digits are at 0.75 h past 6 h or at 06 h 45 m (±18s). In point of fact 0H575 is at 06 h 44 m 58 s and 0H475 is at 06 h 45 m 05 s. co

Table I. Ohio sky survey data (1415 MHz)

Flux Number of Coverage density Sources sources Coordinates (1950.0) limit Area Number of per previously Part RA Dec. (Jy) (st) sources steradian uneatalogued References

Installment I 08 h to 16 h +25 ° to +38 ° 0.37 0.31 258 830 129 Scheer and Kraus, 1967 Installment II 00 h to 16 h +19 ° to +37 ° 0.16 0.64 1200 1870 750 Dixon and Kraus, 1968 Installment III O0 h to 24h 0 ° to +20 ° 0.20 1.22 2325 1910 1195 Fitch et a/.1969 Installment IV O0 h to 24 h 0 ° to -36 ° 0.16 2.36 4550 1930 3354 Ehman et al. 1970 Installment V 00 h to 24 h +40 ° to +63 ° 0.18 1.34 3475 2590 2388 Brundage et al. 1971 Installment VI 00 h to 24 h 0 ° to +41 ° -25 ° to -31 ° 0.18 1.83 5187 2830 3060 Ehman et al. 1974 Installment VII O0 h to 24 h 0 ° to -36 ° 0.18 0.87 2393 2750 1850 Rinsland et al. 1974 Supplement i 14 II Kraus and Andrew, 1971 Supplement 2 05> to 07~ +I ° to -II ° O .25 0.09 218 2526 170 Rinsland et al. 1975 21 h to 24 h

Totals 8.66 19 620 2270 12 907 The Ohio Sky Survey 449

Fig. I. Ohio State University 96-helix radio telescope in 1953 with tiltable flat steel ground plane 49 m long by 7 m wide. This telescope antenna was built by university students and was used in the first Ohio sky survey at 250 MHz. 450 J.D. Kraus

Fig. 2. Ohio State University standing-parabola, tiltable-flat reflector radio telescope in 1970. The parabola dimensions are 110 x 21 m and those of the flat reflector 104 x 30 m with both connected by an aluminum covered ground plane of 13,000 square meters area. A cluster of large horus at the prime focus (in the middle of the ground plane near the base of the flat reflector, at right) collect the incoming radiation and convey it by waveguides to sensitive refrigerated receivers in an underground laboratory directly below. Construction was done by university students working part time. Work started in 1956 and by 1965 the telescope was sufficiently complete to begin the Ohio sky survey at 1415 MHz. x x ~ x ~ ~ ,~ x ~ ,= x x x x ~ x x x ~ ~ ~ x x i i I i i I I I I I I I I I i ~3" 63" /~ ~/i~' ~K/HI.' ' '= • ' ' ~o'~ 0~6 08~ 60"~///,,/~Z+6";/.~ ~OY+6/qj/,// OX+6 OW+6 0V+6 OU+6 OT+6 0S+6 OR+6 00+6 OP+6 ON+6 OM+6 0(-+6 OK+6 OJ+6 Ol+6 OH+6 OG+6 O9+6 OE+6@7~, ~., /. . ~,r~ 60" ~W 0W+5 0V+5 OU+5 OT÷5 0S+5 OR+5 OQ÷5 OP+5 ON+5 OM+5 OL+5 OK+.~ OJ+5 OI+5 OH'kS OG+5 OF+5 OE+5 ~'O0+5 0C÷5 OB+5 OY+5 OX+5/~,~ :,.'v "~"

50* /. ~ /.~ OF+4 /' 50" ~OW+4 ~ ~' /

7'j///ll/~ ~ ,'m'a ~.-,-,-r// V/////./~ I////// ~ 4 O" 40"

OZ+~ OY+3 OX+B 3 OU+3 OT+3 OS+3 OR+3 O0+3 OP+3 ON+3 OM+3 OL+3 OK+3 Od+3 O1+3 OH+3 ~ ,///./~ OE+3 O0+3 0C+3 OB+3 OW+3 "mr I ~t" I "°1" 131~ ~/~//~r "gl 'm ]ZI ~ ~1 ~0" ~rr ~I ~t" ~,~ ~ ~ GALACTIC PLANE I ~r .Gnr "or ~ ~ ~ -oi" 30"

ou. T. o o o OZ+2 OY+2 OX+2 ~O*+F"~ 2 O 2 0S+2 OR+2 OQ+2 OP+2 ON+Z O"+20L+2 OK+2 OJ+2 OI+2 O"+2 /OG+Z, ~/~/~+2/~ OE÷2 O0+2 0C+2 OB+2 ~'v'r//, "~I ~1 "91 "j~ "o1'

OZ+I OY÷I OX+I OW+I OT+I OS+I OR+I OQ+I OP+I ON+I OM+I OL+I OK+I OJ+l OI+l OH+I OG+I OF+l OE+I OB+I OC+l OB+I 8 O m m m Tit ~ HI T~ m Tn~I ~21 rrr~ m~r Tff~r "O1' ~ ~rr/ "o1" ~ ~ 'O1' ~I ~r =" IO" ~ • ,;. . ~//////.~ / IO" Tit m,m wt ~ Tn HI rrt 1Tt ~I m m m tit m rn m OZ÷O OY+O OX+O OW+O OT+O OS+O OR+O O0+0 OP+O ON+0 OM+O OL+O OK+O OJ+O O|+O OH+O OG+O OF+O OE+O OD+0 OC+O OB+O m' m' 111 "oi ~GALACTIC SPUR "oi" -oi. ~'~mt ~ "oT ~t" "~ ~ ~r "oT ~ TIT m Tn" ~ TIT O"

~ ,~Z ~lt OX-O ~Z ~_C OT-O T~ I~ t~ ~Z ~Z 20G_O < OZ- OY-O~ ~(~ .....~11 ~ ]OW;O OV-O ~ . OS-O OR-O 00-0 OP-O ON-O OM-O OL-O OK-O OJ-O O| -0 / OH-O 2 OF-O OF-O OO-O ~Z~ OB-O

o T~ e~7 ~] T~ I~Z T~ -I ~ ~Z 117 ~ 1'~ IV OJ-I "t~ n7 ~Z ~ i'~ o~-, o~-, ox-, 0.-, ov-, ~~ o~-, OR-, 00-, oP-, ON-, .-, 0~-, 0~-, ~. 0.-, 0~-, 0~-, OE-, .-, 0~-, 0~-, •on ~ "o'r[ ¢ntt = ~V/~AGIT TA RIU S COMPLEX ~qT I~ 1~ ~o'n" = "Off ~TI e/,mT OH-' ¢ql = ~2~ F'7"/~ = = -:,o" ;. ]z~//.d ~//)., / m] v. -zo* =

oz-~ o~-~ o o,,-~ ov-~n__._~ TT~-~ oP-~ ON-~ "~"-~ o~-~ o~-~ o ~-~ o.-~ o~-~ o~-~ OE-~ 0~-~ 0C-~ 0~-~ GALACTIC NUCLEUS---~ "m'T~II ~'I,.~H "~l..~II Vl'~n "~rt:~ "~)'O'ff ~ I/~'~TI ~ "/v ~ "r~ 117 Vl '~ 3~ 'o'I ~ ~3~ ~ ~r/~ GALACTIC PLANE -30" o o OF o oz-~ oY-~ ox-~ ow-~ ov-slou-~ os-~ OR-3 0Q-3 OP-3 ON-3 OM-3 OL-:3 0K-3 O OH-3 OG-3 117 OE-3 00-3 OC-3 OB-3

-36"24h 23 h 22h 21 h ;)Oh 19h 18h 17h 16h 15h 14h ~3h G 12h IIh I0 h 09 h 08 h 07 h 06h 05h 04h 03h 02 h 0 Ih uoh

Fig. 3. Master map (1950.O) of the area covered in the seven installments and two supplements of the Ohio survey at 1415 ~z amounting to 94% (or 8.7 steradians) of the sky area between the declinations of 63 ° N. and 36°S. Regions not covered (mostly along the galactic plane) are shaded. Supplement I sources are indicated by dots. This map is useful in directing one to the installment or installments containing a particular Ohio source. L~ 452 J.D. Kraus

The letter symbols in the "Remarks" column indicate whether the source appears to be a point source (p), that is, one which is sufficiently remote from other sources to be a completely resolved and unambiguous entity and one which also shows no apparent beam broadening (source extent less than the half-power beam width of the antenna); a confused souse (c), that is, one which is apparently single but so close to neighboring sources that the position or flux density may be affected; an unresolved source (u), that is, one consisting of two or more closely grouped sources causing increased uncertainty in the position and flux density. Other classifications are designated by additional symbols as described in the texts of the various installments.

Table 2. Sample list of sources from Installment V of the Ohio survey.

Celestial cordinates S1415 Source (1950.0) (Jy) Remarks a

0H571 06 h 42 m 53 s +54 ° 46' 0.20 p 0H471 06 42 54 44 52 2.09 p 0H473 06 44 06 47 48 0.19 p OH573 06 44 08 57 52 0.20 p 0H474 06 44 09 42 09 0.33 p,c,4C42.20, VR042.O6.031 0H674 06 44 33 61 46 0.83 p, 4C61.16, 4CP61.16 0H575 06 44 58 59 38 0.34 p,c 0H475 06 45 05 40 34 O.18 p 0H476 06 45 23 41 50 0.38 u,c

The classification symbols may be used alone or in combination. For example, the combination "p,c" denotes a point source (as opposed to an unresolved or extended source) possibly confused by nearby sources. These classifications were arrived at for each source by studying its contours on maps, as shown in Fig. 4.

Additional entries under "Remarks" give the names for a source listed in any previously published surveys. Thus, for 0H474, the names 4C42.20 and VR042.06.031 indicate that this source was previously catalogued in the 4C (4th Cambridge) and VRO (Vermillion River Observatory of the University of Illinois) surveys. The 4CP61.16 name listed under "Remarks" for 0H674 indicates that it was in the 4CP catalog (4th Cambridge Pencil beam supplement).

Sources for which no names are listed under "Remarks" are previously uncatalogued. Many of these do not have large flux densities so this is not surprising. However, the source 0H471 has a flux density of 2.09 Jy and is in no prior list. The region of sky containing OH471 was covered by the 4C survey. If 0H471 were a normal spectrum source it could be expected to have a flux density of nearly I0 Jy at the 4C frequency of 178 MHz. The fact that 0H471, with a flux density of 2.09 Jy at 1415 MHz, was not listed in the 4C survey suggested at once that it had a centimeter-enhanced (CE) spectrum since the lower limit of the 4C survey is 2 Jy.* Therefore, 0H471 was added to the list of Ohio Specials, a group of sources with CE spectra discovered in the Ohio Survey and described in more detail in section III.

* Subsequent measurements indicate that the actual flux density of 0H471 is probably less than 0.I Jy at 178 MHz. 07boo m 06h45 m 06h30 m ,', (1950.0) 06hl5 m 06hooT so. j.lCbo.4, i~, .... I1~ ~ L ~ ~ I1~ U n U 15°" 3,1X1~¥/ "~ae o4e IL~/I\oH 461 ~ V ~W~OH 4~ (,~

49° II^~P, \o. 498~ f~ ...... I ,, //d/ .: ...... ~ : ...... ~ 0 I (ll~J \L .... Ir'~ 0 u .~]11 7 ~1,./, ",~,,.c ..... , f",I/~'~°e~=~ *1 PART 2 I //~o. 4,, .... ~:['~" ..... / ° o.,,,., 3v/.&o:. ?.\ o ~ ,~ °"'"~JI-,~,~,,.,,I

t • i ~ • , ...... J~) t~o,',o,, ~4"k~!o.4,, /:~o.%,°' I ~, /

,,7o I o ~ t ,~ V ..... ~iI\ ~'o.444 ~ v ~n. ~ i ~" A I o.,..~/. / ~/~o. 46, ,~ :°.oo.:~ ..... o. 4,, ~ I o.4,°~ ...... ~ t (1%,%? / "="'" E~I oH 47zl / k J ~ ~l OH 4462 . q / ~ I+ ~ OH 407~\ / V ÷ -- ~ OH 427 4C 4~.11 o

,,,,;o,L " o..,. 0 ~\o.,,, II I~-,o'=,,IV"Z-~ • ~/ I ILII l\ Yl ,-,145o -r- ';5°i~. .... ~4,,.~,o.,...~.~^~o..6, OH4,J~//I . o. 4=,~11 I(~ o.4%L.J~t 0

i,:'."o o "" ,,o ~1" ...... h I ~ ~ I o ~ J~l o. 4o,~, .,,,0 ¢::: ,< ,.,¢ ~,.~ o.,,6.~ ~ . o o.4,,iy~ v~ I- 0°" 44, ...... ~o.4~4 u o~ ~/~ I~ ~ •"~ ,,{7 , o. 46, o. 4,4"/U ,] n "v ...... • • ~/~I U (~ o. ,5/._~__14~'°

I "~ I OH ~66 P T I U OH 4e~ ~ V ~#~ ~, ^~J[~ WO~II/'x ~ ",llllUl ~' o, 4~6 U/I ~1"3~ ,c "'"~oH 404 \~

41 ° i~,-~ -- ~ 4oo~.~i .oH 47o ~ - /v/ ~,~'~'~ ",~o ~ o~ ~ ,,c ~,,,~ \~ -~ J=' I Pr'~m I o. 4,,,a ~ o.,,,,n ~ ~ ...... I//J ...... ~. I ~ ~%r:;, /\°" ~.~1 J //s.zE OH 460 ~ 40~04 U VRO 40.0001 OH 412 V rl~ i-,~ ...... "[ /I//ll \ .... 7 VRO 40 OS OS ~'~JI~ NR~O ?.~o,o I~ o.su . ~ ^ =! J 40ol ,~/\L~/ /_,,~//,~\~ I //I/ I( ^1 1-141~ o*zo~ 4o..~ j., y.o.~., .... /I..~::~...J40 o 07ho0 m 06h45 m 06h50 m = (1950.0) 06hl5 m 1415 MHz 06nO0 m

Fig. 4. Map for the OH4 block of sky. This map is typical of the 250 maps in the Ohio survey. Names of Ohio sources and all previously catalogued sources are indicated. For a point source the contour interval corresponds to about 0.i Jy and the contours are a representation of the antenna pattern. One of the strongest sources on the map is 0H471 (left of center) which was identified with a neutral 18 m QSO. The object was the first measured with a redshift over 3 (z = 3.40). 454 J.D. Kraus

Measurements of OH471 at frequencies higher than 1415 MHz confirmed its unusual spectrum and with a more accurate position it was identified by Gearhart et aZ. (1972) with a neutral 18 m quasar found later by Carswell and Strittmatter (1973) to have a redshift of 3.40, the first object measured with a redshlft greater than 3.

The maps were produced by a computer driven plotter, then reinked and lettered by a draftsman. A typical map is shown in Fig. 4. Like the other maps of the survey it covers a lh right ascension x I~ declination block of sky. The OH4 block shown in Fig, 4 covers 07 h - 08 h right ascension and 40 ° - 50 ° N. declination. The contour interval corresponds to about 0.I Jy for a point source so the flux density can be estimated by counting the number of contours. For an unconfused point source the contours are a map of the telescope antenna pattern It is interesting that OH471 (z = 3.40) is one of the three or four strongest sources in the entire OH4 block.

The measured positions of all Ohio and previously catalogued sources are shown by a cross (+). Crosses with no name correspond to Ohio Survey sources whose measured flux densities fell below the survey limit (usually 0.2 Jy) but above about 0.15 Jy. There are several thousand sources in this category. The zig-zag line divides zones surveyed at different epochs sepa- rated by several months. Although positions can be interpolated from the maps they are not as accurate as those given in the table. The last step in the production of a map was the insertion of the positions (small dots) and names of all previously catalogued sources. This was done after all Ohio sources had been plotted and labelled so as to avoid any possible bias

The maps are a unique feature of the Ohio Survey providing at a glance data on all objects catalogued up to the time it was prepared. With the maps source associations and relation- ships can be seen which are difficult or impossible to visualize from tabular data. For example, Dr. Ronald J. Allen of the Westerbork, Netherlands, radio observatory wrote me in 1969,

'We have found the Ohio catalog and maps very helpful in estimating the effects of nearby sources on the radio synthesis picture of a galaxy".

Other wor~ers have found the maps valuable for selecting regions free from sources (to the limit of existing surveys) for studies of the density of very weak sources. Gorshkov and Popov (1975) have found the Ohio survey to be the most suitable one for statistical analyses while Bridle and Fomalong (1974) state, "the O.S.U. (Ohio) survey is uniquely valuable as a finding survey."

The antenna beam size at the survey frequency of 1415 MHz was IO min of arc in right ascension by 40 min of arc in declination for the first four installments and the first two-thirds of Installment V. For the last third of Installment V and the remainlng installments the beam size was 8 mln of arc in right ascension by 40 min of arc in declination. The change was occasioned by the installation during the autumn of 1969 of two extra bays on the tiltable flat reflector, increasing the horizontal east-west dimension of the telescope from 79 to 104 m.

The standard (rms) position errors are about 3s of time in right ascension by 3 min of arc in declination for sources stronger than i Jy. The flux density errors are for most of the survey ±O.15 Jy or ±25% whichever is larger. The Ohio Sky Survey 455

Although 1415 MHz was the principal frequency used for the entire Ohio Survey described herein, simultaneous observations with the same telescope were conducted at 612 and 2650 MHz during most of the survey. These measurements at frequencies above and below 1415 MHz were useful in providing spectral data on the survey sources and for discovering ones with unusual spectra as discussed in section III.

At 612 MHz the telescope was resolution (or confusion) limited while at 2650 MHz it was sensitivity (or noise) limited. At 1415 MHz the telescope condition was intermediate with this frequency close to the optimum for detecting and resolving the maximum number of sources (Kraus, 1966a).

Observations were conducted by the drift scan technique whereby the declination was set and the sky scanned in right ascension as the earth rotated. Usually one declination was scanned for at least 2 days providing redundancy and also higher sensitivity (by averaging records).

Data reduction for the survey was done by a program developed largely by Robert S. Dixon (Dixon and Kraus, 1968). The principal steps of this program were:

(I) Drift removal. A running standard deviation criterion along a drift scan was used to remove all radio sources. After filling the resulting gaps with straight lines an integration over IO beamwidths was carried out resulting in a very smooth curve which was then subtracted from the original scan yielding a drift-free record whose zero level was the average of the noise and, therefore, an unambiguous reference level for flux density measurements.

(2) Averaging. All scans processed as in (i) for a given declination setting were averaged. Usually two or more scans were available for such averaging at each declination.

(3) Source extraction. A procedure was used to remove all components of the drift scan which could have been caused by time-invariant radio source structure. The technique is based on the fact that, in the absence of noise, the observed scan is the convolution of the actual sky distribution with the antenna pattern.

The result of this three-step process is an output scan showing only source structure and no noise. Sample scans illustrating the steps are shown in Fig. 5.

During the survey, studies were also made of the reliability and completeness of the survey (Dixon and Kraus, 1968). It is desirable that any survey be highly reliable (probability high that it contain no spurious sources) and also be essentially complete (probability high that it include all sources in the area surveyed above a given flux density limit).

Four quantities were defined as follows:

(I) The incremental reliability, h(S) is the number of sources catalogued in the flux density increment S to S + AS divided into the number of these which are real with true flux densities in the increment S - e to S + AS ÷ e, where e is the survey flux density error times m, where m is an arbitrary integer. Typically, m might be taken equal to 3. Thus, (b) ...... ~__

~ Iom---t IF,,. i

(d)- ~~l ~ ~

Fig. 5. Sample scans showing steps in the data processing: (a) unprocessed scan, (b) scan after drift removal, (c) average of two scans as in (b), and (d) final processed scan after source extraction. The Ohio Sky Survey 457

S + AS + e f nr(S) dS S - e AN (S) 5, r S + AS AN (S) f n(S) dS S where n(S) = number of sources catalogued per unit increment of S nr(S ) = number of real sources catalogued per unit increment of S AN(S) = number of sources catalogued in the flux density increment S to S + AS ANr(S ) = number of the A~(S) sources catalogued which are real and whose true flux densities are in the increment S - e to S + AS + e

(2) The total reliability, Rt(S) , is the number of sources catalogued with flux densities equal to or exceeding S divided into the number of these which are real with true flux densities equal to or exceeding S - e. Thus,

dS nr(S) N (S) o- £ r R t (s) = = N(s) f n (s) ds S where N(S) = number of sources catalogued at flux densities greater than S ~(S) = number of sources N(S) that are real with flux densities equal to or greater than S - c

(3) The incremental completeness, ~(S), is the actual number of real sources (in the area of the catalogue) with true flux densities in the increment S to S + AS divided into the number of these same sources catalogued in the increment S - e to S + AS + e. Thus,

~~S+AS+E f nr(S ) dS S - e A~~

(4) The total completeness, Ct(S), is the actual number of real sources (in the area of the catalogue) with true flux densities equal to or exceeding S divided into the number of these same sources catalogued with flux densities equal to or exceeding S - c. Thus, 458 J.D. Kraus

~~oo f nr(S dS (s) S - £ r ct(s) = oo fl (s) a f na(S) dS S where N a (S) = actual number of real sources with flux densities equal to or greater than S~~

~~All of the four quantities Ri(S) , Rt(S) , ~(S), and Ct(S) may have values of unity or less.~~

In these definitions of completeness the resolution of the telescope is an implicit factor. Thus, it is understood that completeness is achieved with a given telescope if all the sources which the telescope can resolve are catalogued. It follows that with a higher resolution telescope more sources might be found in a given region. Therefore, the "actual number of real sources" given in the definitions means mere explicitly the actual number of real sources which can be resolved by the telescope.

Exact knowledge of the quantities in these definitions is usually not available. However, by means of a comparison survey (at the same frequency and assumed to be 100% reliable and complete, both incremental and total at all flux densities involved) approximate values of reliability and completeness may be obtained. Additional relations were developed for such approximate reliabilities and completenesses and also for the cases where the comparison survey was not 1OO% reliable and complete with examples given (Fitch, et al., 1969) to which reference may be made.

Additional studies of completeness (and also of position and flux density errors) were conducted by Ramakrishna as part of Installment V by means of a Monte Carlo method (Ehman et al., 1974). These studies indicated that at a flux density level of 0.5 Jy for the sky areas analyzed in Installment V, the incremental completeness is 88% (±14%) and the total completeness 96% (±13%). Referring to column 7 of Table I (sources sr -I) it is seen that Installment V and later installments have a higher source density than the earlier installments (II, III, and IV) and, therefore, presumably are more complete at the flux density limit.

Our Monte Carlo method provides no information regarding reliability. This requires a comparison with another survey of equal or higher resolution at the same frequency and of higher reliability and completeness to a lower flux density level. No such survey exists. However, source sample and sky sample tests made with the 91-m Green Bank telescope as described in Installment II yielded an estimated incremental and total reliability at 0.5 Jy of 98% (±3%). Also measurements by Stull (1973) with the 307-m Arecibo telescope at 430 MHz of a sample of weak Ohio sources implied a total reliability of more than 99%. Stull's measurements are described in more detail in section III.

~~III. THE OHIO SPECIALS~~

It was noted in the first installment of the Ohio survey that a considerable number of relatively strong sources (flux densities of I Jy or more at 1415 MHz) were not catalogued in any previous surveys, all of which were at lower frequencies. This suggested that these sources had flat or otherwise unusual spectra. Measurements of the OSU records obtained The Ohio Sky Survey 459

~~simultaneously at 612 and 2650 MHz confirmed this for many of the sources.~~

To obtain data on these special sources at still higher frequencies a cooperative program was begun with Dr. Bryan H. Andrew of the National Research Council of Canada to measure these specials with the 46-m telescope at the Algonquin Radio Observatory. In this way flux densities were obtained at 3.2, 6.5, 10.7, 13.5, and 22.2 MHz for several hundred specials. The positions obtained during these measurements were also accurate enough in many cases to permit optical identifications. Some additional measurements at 31 and 85 GHz were made by E. H. Conklin with the ll-m Kitt Peak telescope of the National Radio Astronomy Obser- vatory.

~~These results have been published in five spectral lists and four optical identification lists to date as follows:~~

~~Spectral Lists~~

~~I. Kraus and Andrew (1970) and Andrew and Kraus (1970) 71 sources 2. Wills et al., (1971) 30 " 3. Conklin et al., (1972) 32 " 4. Andrew et al., (1973) 37 " 5. Gearhart et al., (1976b) 104 "~~

~~Optical Identification Lists I. Thompson et a/.,(1968) 17 sources 2. Radivich and Kraus (1971) 50 " 3. Gearhart et al., (1972) 47 " 4. Gearhart et al., (1976a) 35 "~~

Prior to the above, preliminary flux density data were published on about twenty Ohio specials by Kraus and Scheer (1967) and Kraus et al., (1968). Additional identifications of Ohio specials have been published by Andrew et al., (1971) and by Edwards et al., (1975).

The source 0Q208 was one of the first specials to be found. It has one of the most steeply peaked spectra on any source with a maximum at 5.3 GHz. Thompson et al. (1968) identified it with a compact galaxy which Roark (1969) and Schmidt (1969) found to have the characteristics of a Seyfert galaxy, and a redshift of 0.08. From accurate positions, Ryle and Pooley (1969) demonstrated that the radio emission was from the galaxy and not from a nearby stellar object.

Other special sources have flat spectra like 01478 or OK-232, increasing like OY091, inverted like OJ320, or complex like ON471 and OQ172. The spectra of these and other sources are presented in Fig. 6a.

~~The spectra of radio sources can be conveniently classified by means of two spectral indices for the frequencies 408 and 1415, and 1415 and 65OO MHz, with one index defined as~~

~~log ($408/ S1415)~~

~~= log (1415/408)~~

~~and the other as 460 J.D. Kraus~~

~~log (S1415/$6500) ~2 log (65OO/1415)~~

Plotting the index ~I as abscissa and the index ~2 as ordinate a diagram is obtained which is analogous to the UBV color-color diagram used in optical work. Such radio "color-color" diagrams have been presented, for example, by Kraus and Gearhart (1975) and by Pacht (1976).

~~I \ I I t I I~~

~~0B565 0S387~~

~~/ OTOei~~

~~/ 0V-236 Peaked 0H335 variable bOM129~ / 0H471~~

0W316 0[478 0H471 • Normal F-,_~ POW316 z" 3.4 00172 0X-192 0d320 variable z.3.5 ~ %- r \ OK270 / ~lOC //OY091 / / 0J287 / "~.2.9 variable 0(2 x mc 0K-232 • /~OZ-IB?OS~B7, ~)N303 ON3.25) ---'-- ---"'~'OZ-I87 '"~/°~'~' I -r ..... + ) ~_, /\ 0K037 / oTos, • \ lox-,s~ / J '~""0K270 VR042.22.01 OY401 / +o.42~ \T / o~'~o variable 0N303 / +o~os, 0~4~8 /\ 0N325 \ SL LC;÷.o' o joc,s ON42B 0MI29.4 NGC3627

00208 KEY ' OY358 GC7331 • Qso •~ SL Lo¢ object - • 0Q208 \ • Spiral 1415 MHz galaxy 1415 MHz Increasin9 Inverled • Other I I I I J i i j I I I I I ; It I I I I I 0.11 I GHz I10 I00 0.1 t GHz I0 I00 (a) (b)

Fig. 6. (a) Spectra of some special Ohio sources with CE (centimeter-enhanced or centimeter- excess) spectra including several large redshift QSOs (OQ172, OH471, OK270), a few BL Lac objects (OJ287, OYO91, OX-192) and, for contrast, two spiral galaxies with normal spectra (OM129.4 and OY358). (b) Graph showing same sources as in (a) plotted on a "two-color" diagram based on two spectral indices ~] ,(frequeney interval 408 to 1415 MHz) and ~2 (frequency interval 1415 to 6500 MHzT. Pacht (1976) has shown that the BL Lac objects and spiral galaxies cluster in two distinct regions. These are indicated by contours which enclose 80% of the objects in Pacht's analysis. The large redshift QSOs are CE sources but exhibit more dispersion. This type of diagram is valuable for classifying radio sources according to their spectra and predicting their optical properties.

The sources with the spectra shown in Fig. 6a are plotted on the radio "color-color" diagram of Fig. 6b. On this diagram sources with nor~Z (N) spectra may be arbitrarily defined as those for which both indices are greater than 0.5 and sources with centimeter-enhanced (CE) spectra as those with either or both indices equal to or less than 0.5.

~~Most radio sources have normal (N) spectra placing them in the upper right part of the diagram (Fig. 6b). Most of the Ohio special sources have CE spectra which can place them~~

~~anywhere else in the diagram. The Ohio Sky Survey 461~~

From a study of 179 quasars for which redshifts had been measured, Kraus and Gearhart (1975) found that there was a tendency for high redshifts to be more corm~on among sources with CE spectra. Further studies by Pacht (1976) on a large number of quasars (311) have confirmed this. His sample includes all of the quasars for which radio spectra and redshifts had been measured. Furthermore, Pacht has found that all of the known BL Lacertae objects (twenty- six) and all of the spiral galaxies for which radio spectra and redshifts had been measured (seventy-five) cluster into two distinct regions of the diagram as illustrated in Fig. 6b. The contours include 80% of the objects of each class. Pacht is continuing his study to include other classes of objects and it is apparent that this type of diagram may be very useful for classifying radio sources and for predicting optical properties o~ radio sources from their radio spectra.

~~Referring to Fig. 6b, 0J287 and OX-192 are BL Lac objects, OY358 is a spiral galaxy, and OH471 and OQ172 are quasars with the highest measured redshlfts, 3.40 and 3.53, respectively.~~

Soon after the first identification list for Ohio special sources was published by Thompson et el. (1968), Dr. Alex G. Smith with a group of his students at the University of Florida began to monitor the seventeen objects in the list with the 76-cm Rosemary Hill telescope. Two and one-half years of photographic monitoring of these objects resulted in the discovery of short~term variability in eight of the seventeen and possible changes in three of the remaining ones (Folsam et al., 1971). Noteworthy among their results was their discovery that OJ287 showed large (I~5) and rapid (few day) variations. Radio variability was also established by Brandie and Stull (1971). Subsequent coordinated observations by twenty- three astronomers at eight observatories showed that 0J287 varied at optical, infra-red and radio wavelengths on a scale of days (Epstein et al., 1972).

~~0J287 is now classed as a BL Lac object. It has a featureless optical spectrum so its redshift has not been determined. It is believed to be a young, compact, evolving, highly energetic galaxy.~~

~~Observations by Stull (1973) of 259 weak Ohio sources (down to 0.4 Jy at 1415 MHz) with the 307-m Arecibo telescope at 430 ~lz resulted in the following conclusions:~~

~~(I) The Ohio Survey is important as a finding list for objects with peculiar spectra. He estimated that one-thlrd of the sources in the survey had CE spectra.~~

(2) "The Ohio Survey appears reliable since only three sources were not found and these may have low-frequency cut offs", making them undetectable with the Areclbo telescope, Thus, the total reliability implied is greater than 99% for this sample.

(3) The Ohio Survey "is at present the only survey at a frequency above iOOO ~z which extends to a low flux density level and covers most of the sky. Since it is reliable it forms a good finding list for objects with peculiar spectra".

~~Jauncey (1975) wrote that "the Ohio Survey has been important in uncovering many unusual radio sources".~~

~~Margaret Burbldge photographed spectra of many Ohio specials and in 1972 while she was Director of the Royal Greenwich Observatory wrote me,~~

~~JPVA 20"4--D 462 J.D. Kraus~~

"It is interesting that the Ohio (special) radio sources have proved to be such intriguing optical objects and that identifications from this source list have such a high probability of being correct."

~~In her article with Peter Strittmatter (1972) on QSOs, it is stated that of various lists "the most productive is the Ohio list of (special) radio sources."~~

The identified Ohio specials can be divided into three groups: galaxies about 40%~stellar objects, mostly quasars, about 50%, and blank fields about 10%. A 'blank field" means that no optical object is found at or near the position on either the red or blue Palomar Sky Survey prints (limit ~20m). *

Dr. Alex G. Smith of Florida has continued his observations of Ohio specials and extended them to study some of the blank fields by means of long exposures with special photographic techniques so as to go fainter than the Palomar magnitude. His study got off to a specta- cular start when he found that in the field for the first source he photographed (OEIIO) there was a prominent quasar of about 18m where no object existed on the Palomar survey prints. Subsequent mon&toring of OEIIO established that it was a violent optical variable

~~of the BL Lac class exhibiting variations of more than 2 TM in the course of a year (Leacock et al., 1976).~~

Studies at the Steward Observatory of the University of Arizona, the Algonquin Radio Observatory, Canada, and OSU indicate that the special source 0X-192 is another BL Lac object of unusual optical and radio characteristics (Craine et al., 1976). An examination

of archive plate collections reveals that it has undergone variations of at least 7 TM giving it the distinction of having the largest known range of optical variability for objects of this class. It has also emitted a well-defined radio pulse lasting about 1 year.

Craine has continued to sift the Ohio specials in search of BL Lac objects and has succeeded in finding about twenty-five new candidates which more than doubles the number of known BL Lac objects (Craine et al., 1976a).

~~Some of the BL Lac objects among the Ohio Specials are OJ287, ON231, 0N235, OYO91, OEIIO, OXO29, and OX-192.~~

Carswell and Strittmatter (1973) found that the Ohio special source 0H471 has a redshift of 3.40, the first object measured with a redshift greater than 3. Some months later Wampler et al. (1973) reported a redshift of 3.53 for another special source OQ172. These two sources still hold the record for the highest redshifts. During 1973 and 1974 cooperative observations were conducted by twenty-three astronomers at thirteen observatories to obtain radio spectral data on OH471 and OQ172 on frequencies between 16.7 MHz and 85.3 GHz, a frequency range of more than 5000 to 1 (Gearhart et al., 1974).

~~A recent listing of the objects with the highest redshifts is given in Table 3. Of the seventeen objects listed, thirteen were discovered in the Ohio survey. All of the remaining~~

* In a recent study of 660 Ohio sources with the Texas interferometer (Douglas et al., 1973) at 380 l~z, Ghigo (1976) identified 25% as QSOs (quasars), 25% as galaxies and 50% as blank fields. His blank field fraction is higher because he included weaker Ohio sources with flux densities down to 0.3 Jy at 1415 MHz. The inference is that at low enough flux densities most of the fields will be blank. The Ohio Sky Survey 463

~~four are also in the Ohio survey although three were found first in the 4C (Cambridge) survey and one in an optical search. Of the seventeen sources only two are known to have normal (N) spectra.~~

~~Table 3. List of objects with highest known redshlfts (epoch 1975.9)~~

~~Radio Other Name Redshift spectrum* Name~~

0Q172 3.53 CE 0H471 3.40 CE Q0938+I18 3.2 CE OK162 4CII.28 2.99 N OJl51, PKSO830+II, MO830+I15 OK270 2.91 CE 4C05.34 2.88 CE OJOO8 OLIO8.1 2.71 CE 4C25.21 2.69 N 01250 OZ-187 2.68 CE 0Y094.1 2.66 CE PKS2256+O17 01199 2.66 ? MO758+120 0CO62 2.55 CE? OC383 2.43 CE OXI61 2.43 CE OL492 2.39 CE OJ234 2.37 CE 0FO97 2.37 CE

* CE: centimeter-enhanced N: normal

In a letter of November 1975, Margaret Burbidge and Harding Smith of the University of California, San Diego, commented, "It is abundantly clear that the place to look for high redshift QSOs is among the Ohio CE sources~"

The studies referred to above are only a few of the many that have been or are being done on sources from the Ohio Survey. It is likey that Ohio special sources will prove to be re- warding objects for many future studies.

~~IV. RELATION TO OTHER SURVEYS~~

If a radio source is found in two different independent primary surveys conducted with different types of telescopes, the source is established with more certainty. Further, if the surveys are at different frequencies a spectral index can be obtained. And if a source is found in one survey but not in a second one a limiting value for the spectral index can be inferred. Thus, a 2 Jy source found in the Ohio survey but not in the 4C survey implied a flat or CE spectrum.

~~When a radio source has been found in several surveys its reality becomes more firmly established and a body of spectral information may be available. Thus, when we write~~

4C25.46 = PKS 1430+25 = OK250 we know that this source has been found in three surveys, Cambridge, Parkes and Ohio, with three different type telescopes at three frequencies (178, 408 and 1415 MHz) and that flux densities are available at these three frequencies and possibly also at 1410 and 2650 MHz from the Parkes survey. The equality of the three source names is a compact way of con- veying considerable information. 464 J.D. Kraus

In this section the Ohio survey is discussed in relation to other radio surveys. Although there have been over iOO radio surveys to date there have been only a few primary surveys covering more than one-half of the sky (6.3 sr) and with over IO00 sources. These are listed in Table 4.* Of these surveys the Ohio survey has the largest number of sources (19,500), covers the largest sky area (8.7 sr), is the deepest (2240 sources sr-~), is the only one with essentially complete maps and the only one listing all previously catalogued sources in both tables and maps.

Data for the Ohio Survey are given in Table 1 and Fig. 3. Data for the Cambridge and Sydney surveys are summarized in Table 5, for the Parkes 408 MHz survey in Table 6 and the Parkes 2700 MHz survey in Table 7. Various portions of the Parkes 2700 ~fl~z survey have been conducted to limits between 0.5 - 0.06 Jy with overlapping of the parts so the total number of sources (~5000) is less than the sum of the parts.

The Cambridge surveys of Tables 4 and 5 are at the lowest frequencies (38 and 178 MHz) and one of them (WKB) goes to the north celestial pole. The Parkes surveys are at intermediate and higher frequencies (408 and 2700 ~fl~z) and both extend to the south celestial pole. The Ohio survey at 1415 ~fl{z overlaps about 90% of the area covered by the Cambridge surveys and nearly 70% of the area in the Parkes surveys.

The Bologna (408 MHz) (Colla et al., 1970) and Molonglo (408 MHz) (Davies et al., 1973) surveys have over iOOO sources but as yet cover less than half the sky. The Molonglo survey is being done with a one-mile Mills cross telescope. In Texas (Douglas et al., 1973), a finding survey is beginning at 380 MHz with a 3 x 3 km interferometer of hundreds of helical antennas. It is anticipated that ultimately the coverage of these surveys will extend to more than half the sky with lists of perhaps 50,000 sources to limits of iOO to 200 mJy. The Texas survey is expected to have positions accurate to about 1 arcsec. Very deep surveys to levels of a few millijanskys of small selected sky areas are being conducted at Cambridge (5C survey, Pooley, 1969), Westerbork (Katgert et al., 1973) and elsewhere.

Ideally, surveys can provide source count information of cosmological significance. Total counts of sources per steradian at 1415 MHz from Installment V of the Ohio Survey (Kraus, 1972) are presented for three different flux densities for a sample of about 3000 sources in Table 8. Counts are also given from the analyses of Fomalont et al. (1974) for a smaller sample of about 500 sources. The Ohio counts have been corrected for completeness as de- duced from a Monte Carlo analysis (Ehman et al., 1974). It is seen that there is no significant difference between the two sets of counts.

~~In Fig. 7 the incremental counts at 1415 ~{z relative to a differential Euclidean model number per unit flux density per steradian or~~

~~-2.5 no(S1415) = 300 S1415~~

* A complete listing of radio surveys is given in the "Master List of Radio Sources" by Dixon (1970) which is continually updated on magnetic tape and available by writing to the OSU R~di6 Observatory, Columbus, Ohio 43210. Update number 41 with 75,000 separate entries (about 30,000 different sources) is currently available. (The Cambridge 2C survey (Shake- shaft et al., 1955) had 1936 sources in over half the sky but is not included in Table 4 because the 3C survey (Edge et al., 1959) which superceded it was down to 471 sources and the 3C revised survey (Bennett, 1962) succeeding it was down to 328 sources. Data on the 3rd Cambridge revised survey is given in Table 5. This survey is of high reliability and, in spite of the small number of sources, is perhaps the most famous radio survey of all.) Table 4. Primary surveys covering more than half of the sky with over IOOO sources

~~Total Previously Frequency Number of Lowest flux area catalogued Survey Beam (MHz) sources density (Jy) (st) Maps courees listed Dates~~

Sydney (MBH) Pencil (cross) 86 2270 5.0 7.0 No No 1958-1961 Cambridge (4C) Interferometer 178 4843 2.0 6.8 No No 1965-1967 Cambridge (WKB) Pencil (T) 38 1069 14.0 6.5 Yes (63%) No 1966 Parkes Pencil 408 1600 1.4 7.5 No Yes 1964-1966 Parkes Pencil 2700 5000 0.06 7.8 No Yes 1971-1976 Ohio Fan 1415 19500 0.2 8.7 Yes Yes 1966-1975

~~D" Table 5. Cambridge and Sydney surveys o m" o~~

~~Frequency Number of Lowest flux Survey (MHz) sources density (Jy) Region References~~

~4 Cambridge 3C (rev) 178 328 9 -5 ° to +90 ° Bennett, 1962 4C Part I 178 1219 2 20 ° to 40 ° Pilkington and Scott, 1965 4C Part 2 178 3624 2 17 ° to +20 ° Gower et al., 1967 40 ° to 80 ° WKB 38 1069 14 -i0 ° to +90 ° .Williams et al. 1966

~~Sydney (MSH) Part I 86 1159 5.3 +I0 ° to -20 ° Mills et al., 1958 Part 2 86 892 5.0 -20 ° to -50 ° Mills,et al., 1960 Part 3 86 219 8.0 -50 ° to -80 ° Mills et al., 1961~~

~~4> O~ tn 466 J.D. Kraus~~

Table 6. Parkes 408 MHz survey*

~~Number of Lowest flux Part sources density (Jy) Region* References~~

~~i 297 2.0 -20 ° to -60 ° Bolton et el°, 1964 2** 138 1.4 -60 ° to -90 ° Price and Milne, 1965 3 564 2.0 O ° to 20 ° Day et al., 1966 4 628 2.0 O ° to -20 ° Shin~ins et el., 1966~~

Note: A later survey by Shimmins and Day (1968) covered 20 ° to 27 ° at 635 MHz with 397 sources to 0.8 Jy. * also gives flux densities at 1410 and 2700 MHz for many sources found at 408 MHz. **The region -75 ° to -90 ° was also surveyed at 1410 MHz.

~~Table 7. Parkes 2700 ~z survey~~

~~Number of Lowest flux Part sources density (Jy) Region References~~

I 500 O.18 +4 ° to -4 ° Wall, et el., 1971 2 300 O.06 +4 ° to -4 ° Wall, et el., 1971 Partial 3 618 O.O7 -33 ° to -75 ° Shimmins, 1971 Partial 4 454 0.06 -75 ° to -90 ° Shimmins and Bolton, 1972 5 885 O.iO -35 ° to -45 ° Bolton and Shimmins, 1973 6 939 0.07 -30 ° to -35 ° Shin~mins and Bolton, 1974 7 348 0.30 -40 to -30 ° Bolton, et el., 1975 8 515 0.22 -65 ° to -75 ° Bolton and Butler, 1975 9 166 0.5 -~5 ° to -65 ° Wall et el., 1975 Partial IO 181 0.5 +4 ° to +25 ° Shimmins, et el., 1975 ii 819 0.22 -4 ° to -30 ° Wall et el., 1976

~~Table 8. Source counts at 1415 MHz from the Ohio survey and from Fomalont, Bridle and Davis (FBD)~~

~~Flux density Number of sources per steradian~~

~~(Jy) FBD Ohio~~

~~1.O 165 ± 25 (325)* 207 ± 30 (278)* 0.5 610 ± IOO (572) 650 ± IOO (869) 0.2 3500 ± 700 (24) 2700 ± 500 (31OO)~~

* Number of sources in sample. The Ohio Sky Survey 467

~~1415 MHz~~

~~I-- Z O U _J I.- hiZ =E IJJ (DE Z I'1 > N~~

~~.J W ¢Y~~

~~I I I I I IIII I I I I I I I|1 i I i I I I I I 0.1 ).01 0.1 (jy) I I0 S~~

Fig. 7. Incremental counts of radio sources relative to the differential number of sources (for a Euclidean model) per unit flux density per steradian as given by -2.5 no(S1415) = 300 S1415 as a function of flux density S in janskys (Jy) at 1415 MHz for the Ohio survey (Installment V) and a Westerbork survey (W) and from Fomalont, Bridle and Davis (FBD).

are given for Installment V sources of the Ohio Survey (Table 9) and also for the counts of Fomalont et al., (1974) and counts measured at Westerbork (Katgert et al, 1973) as corrected by Fomalont et al. There is no significant disagreement between the three sets of counts. There is also no departure of the Ohio counts from those expected for a Euclidean model although there is a downward tendency in the counts at large flux densities. This deficiency of strong sources is also apparent in the counts of Fomalont et al. A deficiency of weak sources is also shown by the Westerbork counts.

When Jansky (1933) recognized radio emission from the center of the galaxy he had in effect detected the first radio source (Sagittarius A) at the center of the galaxy. A decade later when Reber (1944) found maxima in Cassiopeia and Cygnus he had located two more although at the time the idea of discreet radio sources had not yet evolved. Following World War II the observations of Hey et al. (1946), Bolton and Stanley (1948) and Ryle and Smith (1948) led to the concept of radio sources of small angular extent. With the construction of a number of radio telescopes in England, Australia, Holland and the U.S. the number of radio sources increased gradually until the late 195Os and early 196Os when the Sydney, Parkes and Cambridge surveys resulted in a rapid increase in the number of known radio sources reaching into the thousands. During the late 196Os the Ohio Survey raised the total number into the tens of thousands. In 1970 the Ohio Survey had the largest number of catalogued sources, comparable to the total of all other surveys combined. With the Bologna, Molonglo and Texas surveys now in progress the total number is climbing toward the IOO,OOO mark which may be reached by the early 198Os. 468 J.D. Kraus

These trends in numbers are reflected by the curve in Fig. 8. Other curves show the approximate numbers of identifications of radio sources with optical objects and the estimated number of radio astronomers (full time professionals) as a function of date.

At the end of World War II there was only one active radio astronomer, Grote Reber, although he was neither professional nor full time. However, after the war the number rose rapidly. There was a continued growth during the 1950s and 1960s but with the decrease in financial support in the 1970s the number has levelled off. It is interesting to note that during the 1950s there were more radio astronomers than radio sources, a situation similar to that which now exists in X-ray astronomy. The dotted extensions in Fig. 8 are estimated projections for the next few years. t00 000

~~~oI°~°~~

~~10 000~~

~~,~tO m "e'e°' _..- ...... -%-. 1000~~

~~100~~

~~,,,///o ,-" . 10~~

~~J~nsky W.W.lli~ ~ 0.S.U. tele~cope ....~~

~~i I I 1 1." 1940 1950 1960 1970 19~0 1930 YEAR~~

~~Fig. 8. Number of radio sources, number of identified radio sources and number of radio astronomers versus date.~~

~~Table 9. Incremental counts at 1415 MHz from the Ohio survey relative to a Euclidean model number per unit flux density~~

Lower level of Observed number Correction Corrected Number flux density relative to factor for relative to increment (Jy) model* completeness model*

~~3.0 Jy 0.71 1.00 O.71 (.08)** 2.0 1.24 I.OO 1.24 (.20) 1.0 1.26 i.O1 1.27 (.22) 0.5 1.21 1.07 1.29 (.27) 0.2 0.88 1.69 1.49 (.33)~~

*Model number per unit flux density per steradian is given by -2.5 no(S1415) = 3~ S1415 **Standard error The Ohio Sky Survey 469

~~V. SOURCE NAMES~~

The most important function of a name is to identify an object unambiguously. The name needs to direct one to a particular survey or catalogue and to an object tabulated therein with information if possible regarding its classification, position, strength, extent and other characteristics. The simplest possible name has one letter followed by one or more digits as, for example, MI for object number I in Messier's list of nebulous objects published almost 200 years ago.

How much information should be included in a name? For example, should the name give the object position to arc second accuracy? This would require at least fifteen characters which would not only be unwieldy but impractical because different groups measuring with different instruments might disagree at this accuracy.

Or should a name merely facilitate the listing of the object in right ascension order? In an hour-minute system this can be done with four digits as in a Parkes name although in the Ohio system the right ascension is given more accurately with only three characters. Brevity is an asset. Thus, a short designation, such as 0Q172, is easy to remember, more convenient to use and saves space (equal to one word per use as compared to RSS1442+I01).

Historically, astronomical objects have been named in many ways. Some stars have names of their own like Arcturus or Betelgeuse, others the name of a constellation with letters or numbers such as ~ Centauri or 61 Cygni, still others a catalogue number like Kruger 60, HD 23762, TON-SO376, or SAO 173297. Nebulas or galaxies are commonly identified by catalogue number as, for example, M31, NGC 1275, MRK 235, 4ZWI40, PK266-01.I, Shapley O345-5122.8 or MCG+OO-59-OI7. PK (Perek and Kohoutek) 266-O1.1 is a galactic coordinate system name while Shapley O345-5122.8 is an equatorial coordinate system name (epoch 2000) introduced by Shapley in 1935. MCG+O0-59-OI7 is from Vorontsov-Velyaminov's Morphological Catalogue of Galaxies which lists objects by Palomar Sky Survey zone and plate number.

~~Some radio sources have constellation names like Cassiopeia A and Cygnus A but most have catalogue names such as 3C273, 4C37.43, OJ287, VR042.22.01, PKSO715-25 and BI0715-24.~~

A name should always include a prefix. It is the arrow which points to a particular survey or list where more information can be obtained. An anonymous number, even though it implies a position, is meaningless. Consider, for example, the anonymous number 1234+567. A search of the literature might reveal many stars, galaxies, and radio sources with positions close to that implied with no way of knowing which object is meant. Unambiguous identification is not possible with an anonymous number. However, the name 4C37.43 immediately conveys the information that it is in the 4th Cambridge catalogue and, even without consulting the catalogue, that the source has a flux density of 2 Jy or more at 178 MHz and that its spectrum is likely to be of a normal type. Or the name OEIIO conveys the information that it is in the Ohio survey with a flux density of more than 0.2 Jy at 1415 MHz, a right ascension of 03 h, O6~ (E = 3h,.IO x 60 = o6m), a declination between IO ° and 20 ° N, and possibly a CE spectrum.

As a third example the name PKSO715-25 indicates at once that the source is in the Parkes catalogue and has a flux density of at least 3 Jy at 408 MHz with the approximate right ascension and declination indicated. The name BIO715-24 conveys the information that it is is in the ist Bologna catalogue and that its flux density is I Jy or more at 408 MHz. 470 J.D. Kraus

~~When the same object has been listed in two surveys one can write PKS0715-25 = BIO715-24 This is an actual example of the same source with different Parkes-type names.~~

Occasionally one finds in the literature that an already-named radio source has been uni- laterally renamed with an anonymous designation such as 1538+14 for 4C14.60. This practice can lead to much confusion and should be discouraged because

(a) One cannot be certain that the same number 1538+14 has not already been assigned by someone else to an entirely different object (optical, X-ray, infra-red, or radio) in the same one minute by one degree area of the sky.

~~(b) One cannot be certain that someone who did not see the article in which 1538+14 was assigned to 4C14.60 may not in the future assign 1538+14 to an entirely different object.~~

(c) Another group, based on its measurements, might assign the number 1538+15 or with additional digits for finer resolution in declination, 1538+149 or 1538+150, so there will then be the names 1538+14, 1538+15, 1538+149 and 1538+150 in the literature all for one and the same source, yet none convey the information that it is a radio source with a flux density of 2 Jy or more at 178 MHz as does the designation 4C14.60.

It is thus possible to end up with many different numbers referring to the same source and any one of these numbers also'referring to several different objects. It is for such reasons that Dr. Robert S. Dixon of the O.S.U. Radio Observatory has refrained from doing any renaming whatsoever in compiling his "Master List of Radio Sources" with 75,000 entries (Dixon, 1970) and his "Master List of Non-Stellar Optical Objects" with 200,000 entries (Dixon and Sonneborn, 1976). All entries in both catalogues retain prefixes to direct one to the original lists or catalogues and to unambiguously identify each. Further, Dixon has omitted from his final catalogues the running serial numbers used in their compilation.

Consider the following situation. An object listed as a single radio source in one catalogue corresponds to two sources in another catalogue with each of these corresponding to one or more optical objects none of which are at the exact positions of any of the radio sources. The unilateral, arbitrary creation of another number for any of these already-named objects only makes a complex situation more so.

An anonymous number may be useful as an approximate R,A./Dec. indicator* alongside the actual name, as in a table, but the anonymous number should never be used alone. Commission 28 of the I.A.U. recommended (1975) that a coordinate number system be adopted for all future astronomical catalogues and if the number of objects is of the order of iOO,OOO or more that a Parkes-type 8-digit name be used but it did not recommend that already-named objects be retroactively renamed nor that anonymous numbers be used.

In some coordinate systems the matter of epoch is crucial. For example, all equatorial coordinate systems are tied to a particular epoch and any change in epoch tends to destroy their usefulness. Thus, to retain the full value of any equatorial coordinate system name requires the universal adoption of a single perpetual epoch.

*To emphasize that the R.A./Dec. indicator is not a name some authors write the indicator with a slash (/) thus: 2215/-16. The Ohio Sky Survey 471

Epoch 1950.0 was the last major epoch before the widespread availability of fast programmable electronic computers (circa 1965) which have eliminated the need for further epoch changes since they can precess positions for long or short time intervals with equal ease. Accor- dingly, 1950.O might be chosen as the perpetual epoch. Another contender n%ight be 2000.0.*

Although the Ohio system is an equatorial coordinate one, the last two digits of the number give the right ascension in hundredths of an hour so there is not a one-to-one numerical correspondence with right ascension in minutes. This and the fact that the declination is specified in IO ° steps means that a change in epoch from 1950.O to say 2000.0 would not be as disturbing as it would be for a Parkes type name.

~~A galactic numbering system in the new coordinates (system II) is useful for galactic objects and will be approprlate until such time as the galactic coordinates are further revised.~~

A simple serial numbering system is entirely immune to epoch changes or coordinate modifi- cations. Cataloguing objects so numbered is facilitated if the numbers increase monotonically with right ascension.

The Sm/thsonian Astrophysical Observatory (SAO) catalogue of stars is a serial number system of six digits which is adequate for a million objects. I use the SAO stars continually and find this numbering system extremely satisfactory. Clearly, there is no best numbering system. With the wide variety and vast numbers of astronomical objects, future catalogue makers should consider all the options before adopting a particular system.

~~CONCLUSIONS~~

The Ohio sky survey is discussed in the context of radio surveys past, present, and future. The vital role of surveys is pointed out as is also the need for them at many frequencies. At the time of this writing the Ohio sky survey at 1415 MHz has the largest number of sources (19,5OO), covers the largest area of sky (8.7 sr), is the deepest (2240 sources sr-l), is the only one with essentially complete maps, and the only one listing all previously catalogued sources in both tables and maps.

One of the most important contributions of the Ohio survey has been the discovery of many sources with centimeter-enhanced spectra, which have been found to possess very high redshifts (OH471 and 0Q172), rapid, erratic, variability (OJ287), or are related to peculiar galaxies (OQ208).

Source counts show no significant departure from that expected for a uniform Euclidean model, although there is a downward tendency in the counts at large flux densities. The number of radio sources is now trending upward toward iO0,0OO from one radio source in 1933, while the number of radio astronomers is levelling off at a thousand or two from one radio astronomer in 1946.

~~Acknowledgement: The OSU Radio Observatory is administered by the Electrical Engineering Department in cooperation with the Astronomy Department of the Ohio State University.~~

* Some astrometrists have advised me that although a fixed or perpetual epoch would be satisfactory for most purposes, it is not satisfactory if the highest possible accuracy is required because no analytical expressions for the precession are available, only series developments good for a limited period of time. 472 J.D. Kraus

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