524-544, May 1988 SURFACE PHOTOMETRY of GALAXIES

Publications of the Astronomical Society of the Pacific 100: 524-544, May 1988

SADANORI OKAMURA Kiso Observatory, Tokyo Astronomical Observatory, University of Tokyo Mitake-mura, Kiso-gun, Nagano-ken, 397-01 Japan Received 1988 January 4

ABSTRACT Surface photometry of galaxies has undergone a great advance recently with the development of fast digital plate-measuring machines, powerful computers to process the huge amount of data from them, and efficient image-processing software. Further, the recent advent of charge-coupled devices (CCDs) has made the technique effective even with relatively small telescopes. Because of their very high sensitivity, especially in the red wavelength region, CCDs have opened a new era of surface photometry. The methodology of surface photometry of galaxies is reviewed and recent results are summarized. Future prospects of the technique in galaxy research are briefly discussed. Key words: surface photometry-galaxies

I. Introduction JK80; Kormendy 1982, hereafter JK82; Capaccioli 1984, Surface photometry is a technique to measure the 1985, 1987; Nieto 1986). Overlap with these reviews has surface-brightness distribution of extended objects such been minimized. as galaxies and Η π regions. It is one of the oldest tech- In what follows the technical aspects are reviewed in niques in modern astronomy. The first attempt at surface Section II, recent results are summarized in Section III, photometry of galaxies dates back to Reynolds (1913). and future prospects are presented in Section IV. Comprehensive historical reviews are given by de Vau- II. Methods, Problems, and Accuracy couleurs (1979; hereafter dV79) and de Vaucouleurs (1987). A. The Night-Sky Light: Intrinsic Limitation on Surface The number of galaxies whose brightness distributions Photometry are mapped in detail has been rapidly growing. An exten- The most serious difficulty in surface photometry of sive bibliographical compilation has been published by galaxies is that we have to measure very faint galaxy Davoust and Pence (1982) and is continually updated signals in the presence of a superimposed strong (Pence and Davoust 1985). Basic catalogs of galaxies in- "background" signal, i.e., the night-sky light. This prob- clude Nilson (1973), de Vaucouleurs, de Vaucouleurs, lem was reviewed by Capaccioli and de Vaucouleurs and Corwin (1976), Sandage and Tammann (1981), (1983; hereafter CdV). The brightness of the moonless sky is typically 22 magnitudes per square arc second in the Lauberts (1982), and Corwin, de Vaucouleurs, and de -2 Vaucouleurs (1985). The data from surface photometry blue band (written as mag arc sec in the Β band or have been combined with those from kinematical obser- simply μΒ, or sometimes B/ss). The peak brightness at vations to yield new results which have had strong influ- the nuclear region of giant galaxies is ^ 17 μΒ, a hundred ences on our understanding of the structure, formation, times as bright as the night sky, while the brightness and evolution of galaxies (e.g., Binney 1982). distributions in the outer regions of galaxies are usually This paper gives an overview of the surface photometry traced to below 26 μβ, i.e., ~ 2% of the night-sky bright- of galaxies. Surface photometry of galaxies is a technique ness (e.g.. King 1978). In extreme cases, investigators and does not define a field of research by itself. Accord- claim to have measured the surface brightness down to ^ ingly, its applications in astronomy are highly diverse. It 0.5% of the night sky (Hegyi and Gerber 1977; Carter and is almost impossible to cover all the related topics, and Dixon 1978; de Vaucouleurs and Capaccioli 1979; Capac- this review is inevitably biased according to the author s cioli, Held, and Nieto 1987). interest. My task in writing this review was greatly eased Major components which contribute to the night-sky by the existence of recent comprehensive reviews on light are the following: closely related topics (dV79; Kormendy 1980fc, hereafter 1. Airglow caused by the collision of atoms and molecules in the upper atmosphere with charged parti- *One in a series of invited review articles currently appearing in these cles and X-rays from the Sun or outer space. Publications. 2. Zodiacal light, which is sunlight scattered by inter-

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planetary dust. 3. Unresolved faint stars and galaxies. These three components have roughly equal contributions to the total night-sky light (e.g., Allen 1973). They vary with either the position in the sky or time, or with both. The airglow fluctuates by a few percent on time scales of a few minutes (Gallagher and Hudson 1976¾). It also increases with latitude and varies with solar activity. In the imaging observations using photographic plates or other imaging digital detectors, short time-scale fluctu- ations and small-scale spatial variations of the airglow are smeared out during a long exposure time. Accordingly, we conventionally represent the distribution of the background night-sky light as a function of position in the sky only as /sfe*/)· The galaxy light, lG{x,y) is obtained as the difference, îg(^Î/) = ic+s{x>y) . (1) between the measured intensity from galaxy+sky, Ic+six>y)> and the local night-sky intensity. It should be noted that Is{x>y) must be interpolated across the face of the galaxy between adjacent "blank-sky" fields. It is rather difficult to define "blank-sky" fields, especially when the Fig. 1-Effect of incorrect sky level on the brightness profile of a model galaxy has a nearby companion or when it is located in a galaxy obeying the r1/4 law (Capaccioli and de Vaucouleurs 1983). cluster. Faint outer regions of galaxies in clusters and small groups often tend to overlap with each other (Kor- cially at low galactic latitudes and at low light levels, may mendy and Bahcall 1974; de Vaucouleurs and Capaccioli show a finer pattern than we usually assume. 1979) or merge into the diffuse intracluster light (Thuan 5. There are faint emission and/or reflection nebulosi- and Kormendy 1977). ties at intermediate galactic latitudes which exhibit fila- In case of two-dimensional photometry, interpolation mentary cirrus-like structure (e.g., Sandage 1976; Arp of Is{x,y) is often performed by fitting an appropriate and Lorre 1976; Cannon 1979). analytic function to the data in the "blank-sky" fields, with 6. A typical point-spread function (PSF) of a star ob- field stars or galaxies, if any, masked out. The fitting served through a telescope extends well beyond the read- function most commonly used is a two-dimensional poly- ily visible image up to r ~ 1?5 (e.g.. King 1971; Kormendy nomial in χ and y (e.g., Jones et al. 1967; Barbon, Benac- 1973; CdV). If there are bright stars near the object chio, and Capaccioli 1976; Ichikawa et al. 1987fc), al- galaxy, the faint outer parts of their PSFs overlap on though other types are also used (e.g., Okamura 1977; h&y)· Strom and Strom 1978a; Sulentic and Lorre 1983; All six components described above introduce statisti- MacGillivray and Stobie 1985). This means that only the cal "noise" in the determination of the local night-sky underlying components that have spatial wavelengths light. This sets the practical faint limit for surface photom- longer than or comparable to the size of the galaxy may be etry of galaxies. On the basis of a comprehensive quantita- reasonably well modeled. Actual unknown local night-sky tive study of these noise sources, CdV concluded that it is light, however, can naturally be different from the inter- difficult, if not impossible, to obtain significant quantita-

polating surface or line. It is this error in the interpolated tive information at brightness levels fainter than μΒ ~ 28 2 -2 sky brightness that drastically affects the brightness dis- mag arc sec" (L· ^ 0.3 Lö pc ). In addition to these tributions in the faint outer part. Figure 1 shows the effect natural components, scattered light within the telescope/ of incorrect sky level on the brightness profile of a model detector system is often a problem which limits the preci- galaxy obeying the r1/4 law. It is seen that almost any sion of surface photometry. profile, one with an extensive envelope or one that is It should be noted that the above limit cannot be tidally truncated, could be derived merely by adjusting lowered significantly even with the Hubble Space Tele- the local sky level (see also Fig. 5(b)). scope because only the airglow component, which con- There are other components affecting Zs(x,î/), although tributes roughly one-third of the total night-sky light, can to a lesser extent than components 1-3 above. They are be eliminated in observations from space. An ultimate summarized below: limit for surface photometry will be set by the statistics of 4. Galactic extinction due to absorption patches, espe- faint sources as discussed by Miller (1963). Previous re-

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 526 SADANORI OKAMURA views discussing the effect of the night-sky light on sur- is usually called the contrast or simply gamma. The emul- face photometry include Carter (1979), Mihalas and sion approaches saturation in the shoulder, and finally, Binney (1981), CdV, de Vaucouleurs (1984), and Capacci- the maximum density is reached. Further exposure oli (1987). causes reduction of density known as solarization. The characteristic curve is usually defined by measur- B. Photographic Surface Photometry ing the densities of a set of spots or step wedges with The photographic emulsion is the oldest but still com- known relative exposures. They are exposed using cali- monly used imaging detector in astronomy. Input of the bration devices such as tube sensitometers and step- photographic process is the exposure E, the amount of wedge sensitometers (Latham 1969; Schoening 1976; light to which the emulsion is exposed; and the output is Hoag 1978) in the margin of the plate. When no calibra- the blackening of the developed emulsion. Exposure is tion exposures are available, as in the case of old plates, the product of the intensity of light I and the exposure star profiles could be used to define the characteristic time t, that is, Ε = It. The quantitative measure of the curve although the accuracy is relatively poor (Kormendy output is the photographic density D, which is defined 1973; Kormendy and Bahcall 1974; Feitzinger et al. 1983; by, de Vaucouleurs 1984). In a photographic observation, a D = -logio Τ , (2) portion of the sky including an object galaxy, represented by the intensity distribution I (x,t/), is exposed on a photo- where Τ is the transmittance, the fraction of incident light graphic plate producing the corresponding density distri- that is transmitted through the emulsion layer. It should bution D{x,y). Photographic surface photometry is then be noted that density depends on the geometry of the defined as the process by which l{x,y) is determined from measuring machine and, to a lesser extent, on the wave- D{x,y) with the help of the characteristic curve for the length of the measuring light because of scattering by the plate on the assumption that the characteristic curve is grainy emulsion layer. However, absolute scaling of the valid at any position on the plate. density is usually not necessary in photographic photome- It is not the scope of this review to describe detailed try except for particular kinds of experimental investiga- practical procedures to obtain photographic plates of good tion (e.g., Furenlid 1978). quality for use in surface photometry. That subject is The most common form of expression for the input/out- reviewed by Smith and Hoag (1979) and scattered in the put relationship of a photographic process is the plot of literature. Major topics include method of photographic density D versus the logarithm of exposure Ε, which is plate processing in the darkroom (e.g., Abies 1971; Miller often referred to as the Η-D curve or the characteristic 1977), a quantitative characterization of the performance curve. A typical characteristic curve is characterized by of emulsions in terms of signal-to-noise ratio and the four distinct parts as shown in Figure 2, namely, the toe, estimation of the optimal exposure time (Latham 1974, the linear part, the shoulder, and the region of solariza- 1978; Hoag 1978; Furenlid 1978), hypersensitization to tion. The toe represents the fact that there is a threshold increase the sensitivity of emulsions (Sim 1978; Schoen- exposure below which no blackening is produced except ing 1978; Dawe, Coyte, and Metcalfe 1984), and sources for the chemical fog. In the linear part, D is almost of photometric errors due to the photographic processing linearly proportional to log Ε ; and the gradient, (CdV; de Vaucouleurs 1984). 7 = dD/dlog Ε , (3) Three important features of photographic surface photometry are described below: 1. Even for a given type of emulsion, the exact form of the characteristic curve depends on many variables such as (a) the developer composition, (b) the development time, temperature, and degree of agitation, (c) the wavelength distribution of the exposure, (d) the nature of variations in exposure over the image area (e.g., whether due to changes in exposure time or exposure intensity), and (e) the geometry and spectral characteristics of the densitometer used (e.g., Dainty and Shaw 1974). Accord- ingly, calibration exposures should be made, at least in principle, on the same plate and under the same conditions, processed in the same manner, and measured with the same measuring machine, using the same measuring parameters as for the galaxy exposure. Violation of this principle will introduce photometric errors, although the Fig. 2-Schematic representation of the characteristic curve. magnitude of the error depends on the variables con-

cerned and is often difficult to quantify accurately. The JrelM) = Ug + S (*>*/) - hix>y)}IIs{x>y) ? (4) size of spots or steps of a sensitometer is often much larger where Z is the relative intensity. By carefully modeling than that of stars. This may introduce an additional error rel the nonuniform sky background and normalizing l (x,y) due to Eberhard effect in the star-like nuclear region of Q by l {x,y), we can correct, at least to the first-order ap- galaxies (Nieto 1983). s proximation, the intensity distribution of the galaxy for 2. The typical dynamic range of emulsions, i.e., the nonuniformity of actual night-sky brightness and nonuni- linear part, used in astronomy is less than 102 in log Ε, formity of background density due to vignetting and/or which is narrower than the range between the peak photographic processing. brightness of giant galaxies and the night-sky brightness. 5. Relative-intensity distributions obtained from dif- Accordingly, successful surface photometry of bright ferent plates are registered with each other using appro- galaxies would require a series of plates having different priate field stars as fiducial points, and they are combined exposure times. to yield a final relative-intensity distribution. Weights in 3. The photographic emulsion is a nonlinear analog the combination should be functions of intensity such that detector which is not favorable to quantitative image the bright central region from the long-exposure plate analysis such as subtraction of the sky background de- and the faint outer region from the short-exposure plate scribed in Section II.A, unless the recorded image is have lower weights. digitized by some means. Several photographic tech- 6. Finally, the zero point of the relative intensity, i.e., niques are known to be effective in bringing out special the sky brightness, in mag arc sec 2, is determined on features which otherwise cannot easily be made visible μ5ΐ<γ the basis of photoelectric magnitudes. A convenient com- (e.g.. Malin 1978, 1981, 1982; Malin and Carter 1983), pilation of photoelectric magnitudes of galaxies is pub- but they are more qualitative than quantitative. Further, lished by Longo and de Vaucouleurs (1983). In practice, the photographic emulsion is a detector which can be is obtained by the least-squares method by comparing used only once. This prohibits the stable calibration for μ$1<γ the photographic magnitudes defined by nonuniformity in the sensitivity over the detector surface which is commonly done for other digital imaging detec- mjA ) = - 2.5 log { ljx,y } + μ,κ , (5) tors. JA A standard procedure of modern digital photographic with the photoelectric magnitudes, mpe(A), where A is the surface photometry involves the following steps: aperture within which the magnitudes are measured. An 1. The plates are scanned with a microdensitometer example is shown in Figure 3. The run of integrated (e.g., Lasker 1983) and the density distributions D{x,y) of magnitudes as a function of the aperture size is called the the object galaxy and surrounding area are recorded on a magnitude-aperture relation or the growth curve, which magnetic tape in the form of digital arrays. The area is used to estimate the asymptotic total magnitude or the scanned should be sufficiently wide compared with the magnitude at a specific aperture size (e.g., de Vau- extension of the object galaxy to allow for the precise couleurs 1977; Sandage and Visvanathan 1978). In deriv- determination of the local sky level. At the same time, ing μ^γ by this method, care should be taken for contami- calibration wedges or spots are scanned to define the characteristic curve. Preferably, this should be done be- Magnitude fore and after the galaxy scan in order to monitor possible drift of the microdensitometer output. 2. The characteristic curve is defined for each plate by plotting the densities of the wedge steps against their known relative exposures or wedge constants. Several analytic functions that conveniently model the characteristic curve have been proposed (de Vaucouleurs 1968; Tsubaki and Engvold 1975; Boroson 1981; Lehmann and Häupl 1986). 3. The density distributions D{x,y) are converted to (specific) intensity distributions Ig+s^?/) wtih the help of the characteristic curve. 4. The background local sky level is determined by the procedure described in Section ILA and subtracted from Z (x, y ), leaving Z (x,í/), the intensity distri- Aperture (arcsec) G+S g Fig. 3-Zero-point calibration of photographic surface photometry on bution of only the galaxy. This is the most crucial step in the basis of photoelectric magnitudes. The solid curve shows the photo- surface photometry. In practice, lG{x,y) is expressed in graphic magnitude as a function of aperture (growth curve) and the filled units of the local sky intensity as circles are photoelectric magnitudes (Ichikawa 1987).

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 528 SADANORI OKAMURA nation of field stars in the diaphragm of photoelectric 1974; Burkhead 1978; de Vaucouleurs and Nieto 1978). observation and for possible saturation in the nuclear Sky background is interpolated from the readings on both region of the galaxy. It is noted that the integrated mea- sides of the galaxy. surements are off not by a certain magnitude but by a In order to measure extremely low brightness levels, certain luminosity when the nuclear region is saturated. the beam-switching technique pioneered by Gallágher The trend of residuals, Δ?η = mn{r — mnp versus A, is a and collaborators is employed (Gallagher and Hudson useful diagnostic tool to check the accuracy of calibration 1976a,fo; Price and Grasdalen 1983; Skrutskie, Shure, and sky subtraction. and Beckwith 1985). In this technique, the field of the Descriptions of more detailed practical procedures can photometer is switched between blank sky fields and a be found in the literature (e.g., Strom and Strom 1978ß; region of interest in a galaxy at a regular interval, typically Kormendy 1977α, 1980a; Burstein 1979; Boroson 1981; 1-10 Hz, using a chopping secondary mirror of the tele- Watanabe, Kodaira, and Okamura 1982; MacGillivray scope or specially-designed photometers (e.g., Hegyi and and Stobie 1985; Ichikawa et al. 1987¿; Walterbos and Gerber 1977). The reference blank-sky fields are taken on Kennicutt 1987). Digital data-reduction procedures described above were pioneered by Jones et al. (1967) and both sides of the region to be measured, situated well are conventionally called the "numerical mapping tech- outside the extension of the galaxy. Errors due to varia- nique". This terminology, however, might be rather ob- tions of the airglow component of the night sky and the solete since it was born when a major fraction of surface scattered light can be minimized by this differential photometry relied on analog techniques. method, and the lowest order of the gradient of the sky It should be noted in passing that electronography has background can be canceled by the linear interpolation of been successfully used in surface photometry although it the readings for the two reference sky positions. Photom- is not as popular as ordinary photography (e.g., Thomsen etry obtained in this way using a pulse-counting photome- and Frandsen 1983). Electronographs give better resolu- ter is shown to be essentially limited by photon statistics tion, better linearity, and wider dynamic range than ordi- (Gallagher and Wirth 1980). nary photographs (e.g., McMullan 1980; Picat 1985). D. Surface Photometry Using CCDs: A New Era C. Conventional Photoelectric Photometry The photographic emulsion has been a successful de- Photomultipliers (Lallemand 1962; Johnson 1962), tector in optical astronomy because it is simple to use and which produce a signal current proportional to the amount it is supported by a very wide commercial market. How- of light received, have been used for photometry of ex- ever, it does suffer from a low detective quantum effi- tended objects as well as that of stars since the 1940s ciency of a few percent, nonlinearity, narrow dynamic (dV79). In recent applications photomultipliers have of- range, and various instabilities, such as adjacency effects ten been operated in a pulse-counting mode, i. e., a mode (e.g.. Dainty and Shaw 1974), inherent to the photo- in which every photon is counted that arrives at the graphic process. photocathode and produces a photoelectron. Linear-imaging detectors with a higher quantum effi- In practical observations an aperture is introduced at ciency and a wider dynamic range, preferably digital the focal plane of the telescope in front of the photomulti- ones, which would bring surface photometry to fainter plier to define the field of measurement. The telescope is light levels and to higher accuracies than those attained pointed to a region of interest in a galaxy and then to the by photographic photometry had long been sought. Much area of blank sky for reference. By subtracting the reading effort was made in the 1970s to develop television-type for the sky from that for the galaxy, we can obtain the sensors for use in astronomy (e.g.. Strittmatter 1973; signal produced only by the galaxy. The signal is con- Lawrance 1973; Miller, Robinson, and Wampler 1979; verted to surface brightness by observing in the same Benedict 1980; Gallagher and Hunter 1981). However, manner and with the same equipment several standard all these television-type sensors became rather obsolete stars with known magnitudes. with the advent of two-dimensional, charge-coupled It is extremely time consuming to map the two-dimen- devices (GGDs). Recent reviews of CGDs in astronomy sional brightness distribution of a galaxy by this method are given by Djorgovski (1984), Fort (1985), and Mackay because the observation should be made point by point. (1986). Useful papers on the optimal use of GGDs are Only a few galaxies were measured in this way (e.g., de found in Baluteau and DOdorico (1986). GGDs have now Vaucouleurs 1958, 1959fc ; Miller and Prendergast 1962, become one of the most commonly used detectors for 1968; de Vaucouleurs and Abies 1968; Strom et al. 1977, ground-based astronomy, and present standard pho- 1978). A conventional and less-tedious technique to ob- toelectric surface photometry is done with GGDs. tain brightness profiles of galaxies is the drift scan, in GGDs are "within a small factor of being perfect detec- which the telescope is drifted across the face of a galaxy at tors" (Mackay 1986), having a quantum efficiency of ~ a constant speed, normally in E-W or N-S directions 80% at the peak, a linearity accurate to ~ 10 3, a dynamic (Burkhead and Burgess 1973; Burkhead and Kalinowski range in excess of 103, and a spectral response between

4000 Â-11,000 A. The most serious noise in CCDs is the observations; however, see Baum et al. 1986); (4) flat readout noise introduced by the on-chip amplifier, which fielding by an appropriate flat field; (5) cleaning for cos- ranges from ~ 10 to ~ 100 electrons per pixel depending mic-ray events; and (5) sky subtraction. on the type and manufacturer of the chip. Hence, suc- Several comments on CCD data reduction are given cessful exposures should be such that photoelectrons ac- below. Attention should be paid not to deteriorate the cumulated in a pixel are much larger in number than the object frame by simply subtracting a bias/dark frame of readout noise. CCDs are also good cosmic-ray detectors. poor signal-to-noise ratio. A constant corresponding to In modern thinned CCDs, cosmic-ray events are seen the mean count per pixel is often subtracted instead of the mostly as high-signal single-pixel peaks, although seen actually observed bias/dark frame. When bias/dark sometimes are short streaks a few pixels in length due to frames show significant features (see Fig. 4(a)), use of a grazing incidence events. It is rather easy to discriminate template frame is recommended (Meurs 1986). In flat these events from stars or galaxies. fielding, it is useful to distinguish between high spatial Modern CCDs show intrinsic nonuniformity of a few frequency (pixel-to-pixel) and low spatial frequency percent rms in the sensitivity. But this nonuniformity can (global) sensitivity variations. A practical method of ob- be corrected, because of the excellent linearity, by divid- taining a flat field adopted by our group is the following ing the raw data by a uniformly illuminated image (a flat (see also Djorgovski 1984; Tyson et al. 1986). A lamp flat field) in the process known as flat fielding. However, flat frame of good signal-to-noise ratio is smoothed by a me- fielding is not an easy task in practice because of the dian filter of, e.g., 3 by 3 pixels. The smoothed frame is following major effects: subtracted from the original and the resulting frame is normalized by dividing it by the smoothed frame to yield 1. The nonuniformity in sensitivity is color dependent. the high-frequency flat field. Many short exposure Thus, the flat field should, in fact, be defined by a uniform "blank-sky" frames taken near the object galaxy are used light source that has the same energy distribution as the to produce the low-frequency flat field which allows re- object galaxy. This is, of course, impossible. Often the moval of both global sensitivity variation and fringing. inside of the telescope dome illuminated by an incandes- The blank sky frames should be taken shifting the posi- cent lamp, the twilight sky, or the blank night sky is used tions slightly (several times the seeing size) between ex- to produce a flat field. Among these the blank night sky is posures. The number of necessary blank-sky frames de- the best because of the spectral matching and identical pends on the accuracy required and varies typically light path to the galaxy, but it takes a long time to obtain a between 4 and 20. After masking obvious stars and galax- night-sky frame of a high signal level. ies in each blank-sky frame, we produce a mean frame 2. F ringing due to multipath interference effects in the with masked pixels rejected. An appropriate algorithm is CCD itself introduces a pattern that is not corrected for also incorporated to reject pixels affected by unmasked by flat fielding when spectral matching is poor between faint objects when many frames are available. The mean object and flat-field frames. frame is normalized and slightly smoothed with care 3. There are "deferred-charge columns" which show taken not to smear out the fringing pattern. The resulting up as nonlinearity in their responses at low charge levels frame is multiplied by the high-frequency flat field to (Baum, Thomsen, and Kreidl 1981). produce the final flat field. Sky subtraction should also be Mackay (1986) concluded that flat fielding is at best a done very carefully because many more faint objects are compromise and in no way a substitute for intrinsic device recorded by a CCD than a photographic plate and defini- uniformity. However, it is possible to reduce residual tion of "blank sky" is very difficult. An example showing nonuniformities to 0.3%-0.5% rms with careful flat field- our data-reduction procedure is given in Figure 4. More ing. Accuracy better than this may be obtained by special detailed descriptions of CCD data reductions are found in techniques including drift scanning (Wright and Mackay the literature (e.g.. Carter et al. 1983; Kent 1984; Djor- 1981; Boroson and Thompson 1987) and the subtraction of govski 1984; Fort 1985; Schild, Tresch-Fienberg, and an empty field obtained under conditions identical to Huchra 1985; Lauer 1985α; Baum et al. 1986; Grosb0l those for the galaxy (Baum, Thomsen, and Morgan 1986). 1986; Meurs 1986; Pedersen 1986). Because of the complicated factors noted above, One essential disadvantage of CCDs is the small size (^ "standard" reduction procedures for CCD data appear to 2 cm) compared with the large photographic plates (^ 50 vary in subtleties among different CCD camera systems cm). With the conventional focal-plane scale of large tele- or groups of people. They normally consist of (1) correc- scopes, presently-available CCDs can cover a field of tion for deferred charge effects suggested by Baum et al. several arc min2, which is insufficient for surface photom- (1981); (2) subtraction of the bias frame, i.e., zero expo- etry of apparently large nearby galaxies (Capaccioli 1987). sure frame; (3) subtraction of the dark frame (dark noise of Because of this limitation, recent surface photometry of modern CCDs cooled to below ~ —110° C is a few elec- galaxies appears to be divided into the following four trons pixel1 hour"1, which is negligible in most imaging categories:

Fig. 4—Examples showing the procedures of CCD data reduction. The device is RCA SID503EX with 1024 by 640 pixels operated in the 2 by 2 pixels binning mode, (a) Highly-smoothed dark frame (mean of three 10-minute exposures). The global feature is seen although the amplitude is small (0.5 AD units rms). (b) Lamp flat frames showing clear fringe patterns (V, R, and 1 bands from left to right), (c) A blank-sky (SA57) frame in the 7 band (left, raw data, right: bright objects masked), (d) Low-frequency flat field composed from 13 blank-sky frames (left) and high-frequency flat field created from 5 lamp flat frames (right). Both are in the I band.

1. CCD surface photometry using a large telescope or Kruit, and Allen 1986; van der Kruit 1987; Walterbos and at the slow focal plane of a small-sized telescope of either Kennicutt 1987) and du Pont plates (Binggeli, Sandage, apparently small galaxies, including QSOs, or bright cen- and Tarenghi 1984; Wakamatsu and Hamabe 1984; tral regions of relatively nearby galaxies (e.g., Hoessel Ichikawa, Wakamatsu, and Okamura 1986) have been 1980; Young et al. 1980; Hickson et al. 1982; Carter et al. successfully used. 1983; Schneider, Gunn, and Hoessel 1983; Gehren et al. 3. CCD surface photometry of apparently large galax- 1984; Price 1985; Kormendy 1985α,Lauer 1985α; Fort ies with small telescopes at the cost of resolution (e.g., et al. 1986; Jedrzejewski 1987; Daly, Phillips, and Disney Schild et al 1985; Kent 1987α). Availability of a large 1987). amount of observing time on relatively small telescopes 2. Photographic photometry of apparently large bright has made it possible to collect data for large numbers of galaxies or galaxies in clusters (e.g., Strom and Strom galaxies (Kent 1984, 1986, 1987a,fc; Djorgovski 1985; 1978a,fc,c ; Morbey and Morris 1983; Romanishin, Strom, Lauer 1985α). and Strom 1983; Carignan 1985; Schombert 1986). Large 4. A hybrid method in which luminosity distribution in wide-field plates such as Schmidt plates (e.g., van der the central region is obtained at high resolution with Kruit and Searle 1981; Watanabe 1983; Wevers, van der CCDs while that in the faint outer region is measured at

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System SURFACE PHOTOMETRY OF GALAXIES 531 low resolution using deep Schmidt plates. This method firmed carefully because sources of dominant errors are has been receiving increasing interest since sky subtrac- quite different between the bright nuclear region and tion for large galaxies can be done most accurately on faint outer region (cf. Nieto 1986). The discrepancies in deep Schmidt plates (e.g., Capaccioli et al. 1987). the bright nuclear regions are most likely attributable to Intensive efforts have been made in all of these cate- errors in photographic photometry. However, failure to gories to accumulate information of good quality on properly correct data for the seeing profile is also a signifi- brightness distributions of galaxies. In particular, CCDs cant source of error in both photographic and CCD pho- on small telescopes will open a new era of surface photom- tometries (e.g., Schweizer 1979, 1981; Thomsen and etry of galaxies by enabling large photometric surveys of Baum 1987). On the other hand, the discrepancies in the relatively bright nearby galaxies, as pointed out by Djor- faint outer region are probably due to composite effects of govski (1987). poor sky subtraction and improper allowance for scat- As we have seen, surface photometry of galaxies today tered light, from which both photographic and CCD is almost entirely based on digital techniques. Two com- photometries suffer. ments would be appropriate in this context. First, the Table I summarizes the typical errors found in the importance of efficient image data-processing systems literature. In addition to these systematic errors, a zero- should be emphasized. Second, we need a longsighted point difference of ~ 0.1 mag is quite common (e.g., strategy for data compilation and preservation, since de- Burstein 1979; Watanabe 1983; Djorgovski 1985). These tails of digital data can be easily lost from our memory. values can be taken as a guideline to show the typical Without these, we will be only piling the room with accuracy attainable with a current standard method of magnetic tapes or optical disks and most of the digital data surface photometry although the magnitude of actual error is different case by case. may not be transferred properly to successive genera- tions. F. Presentation of Surface Photometric Data The final result of modern digital surface photometry is E. Accuracy of Surface Photometry the calibrated surface brightness distribution, S{x,y) in The history of surface photometry of galaxies is the units of mag arc sec-2, in the form of a digital data array. It history of the struggle for better accuracies (cf. dV79; consists of a huge amount of numbers, which do not allow Capaccioli 1985). In 1961 a Working Group on Galaxy any easy physical interpretation by themselves. The most Photometry and Spectrophotometry was formed by IAU natural way to "visualize" the data array is to present it in a Commission 28. The group selected several galaxies grey-scale map or a false-color map using an image-dis- (NGC 3115, NGC 3379, NGC 4486, and NGC 4594) as play system. In the false-color map, pixels of different brightness-distribution standards aiming (1) to provide intensities are assigned different colors to make subtle internal absolute calibration of wide-field photographs, changes in intensity easily visible. Color variation across (2) to check the reliability of new photometric instru- ments and data-reduction procedures, and (3) to permit the face of a galaxy can also be made visible by making the same sort of maps of the color-ratio image, precise tests of empirical formulae or theoretical models

(dV79). Necessity for such standard galaxies with small Ci2(x,t/) = S^y) - S2{x,y) apparent sizes for CCD observations is emphasized by = const. - 2.5 log , Djorgovski (1985). where l{x,y) is the relative intensity distribution and the In spite of intensive efforts for better accuracy, too subscripts 1 and 2 discriminate the color bands large systematic differences were often found between (Schweizer 1976; Talbot, Jensen, and Dufour 1979; Schild measurements of the same galaxy by different observ- et al. 1985; Ichikawa et al. 1987α). A pseudocolor map is ers—in particular, those in early studies as shown, for most convenient to display the color-ratio image, where example, by Carter and Dixon (1978) and Burstein (1979). the actual color variation in the galaxy is imitated. An- With the advance in modern hardware and data-reduc- other, more conventional, way is to plot a map of isophotal tion techniques, consistency between different observations has become significantly better. However, consid- TABLE I erable discrepancies are still present, especially in the Typical Errors in Magnitudes in Surface bright nuclear region, where the luminosity gradient is Photometry of Galaxies steep, and in the faint outer region of a galaxy (e.g., Davis et al. 1985; Djorgovski 1985; Vigroux and Nieto 1985; p.g. CCD (region: surface brightness*) Capàccioli et al. 1987). ^0.1 ^0.05 (nuclear region: Djorgovski (1985) claims that consistency was much <0.1 (intermediate region: 18<μ<25) better among CCD photometry than among photographic photometry or between CCD and photographic ^0.1 ^0.1 (faint outer region: 25<μ<27) photometries. However, this statement should be con- *Units are mag arcsec"2 in the Β band

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 532 SADANORI OKAMURA contours, which are often simply called isophotes. Exam- components. Two points should be noted. First, the ples of these various methods of presentation are shown in equivalent profile is affected by the change of geometry Figure 5. due to the inclination. Second, the equivalent profile is Two-dimensional data are often reduced to a one-di- computed in modern digital surface photometry by mensional brightness profile, Z(r), in quantitative analy- counting pixels whose surface brightnesses are above the ses of the structure of galaxies. A variety of methods exist threshold. A straightforward application of this algorithm to extract the brightness profile from the two-dimensional to fainter levels would produce a spurious extensive enve- brightness distribution, and so many kinds of profiles lope due to noise (cf. Blackman 1979). have been used in the literature that there is some confu- 4. The ellipse-fit profile is obtained by fitting the sion in terminology. It is obvious that there is no best isophotes of a galaxy with a set of concentric ellipses, profile for all purposes. One should be careful in choosing which are often found to be satisfactory approximations the best profile according to the purpose of the study and for elliptical galaxies and bulges of disk galaxies (Barbon et the morphological characteristics of the galaxy. Widely- al. 1976; Barbon, Capaccioli, and Rampazzo 1982; Kent used profiles are the following: 1983; Lauer 1985a?c; Carter 1987; Jedrzejewski 1987). By 1. The major/minor-axis profile is the profile along the doing this fit, isophote ellipticities and position angles of major/minor axis of a galaxy. Directions of the major axis the major axis are derived as functions of the semimajor are usually defined by the isophotes in the faint outer axis in addition to the brightness profile. These two addi- region. However, many elliptical galaxies show the twist tional "profiles", ellipticity and position-angle variations, of the major axis (e.g.. Leach 1981; Michard 1985) and are quite important in the study of geometry of early-type spiral galaxies often have subcomponents such as bars and galaxies (e.g., Mizuno and Hamajima 1987; Jedrzejewski, lenses whose major axes are not coincident with those of Davies, and Illingworth 1987). the outer regions (Kormendy 1979). There is some evi- 5. The generalized radial profile was introduced by dence suggesting the presence of subcomponents even in Watanabe et al. (1982). It is derived by integrating the elliptical galaxies (e.g., Michard 1984; Nieto and Vidal two-dimensional brightness distribution I{x,y) onto the 1984α). Accordingly, it is often difficult to define an un- major axis and reconstructing the face-on radial profile by ambiguous direction of the major axis. In order to trace solving an integral equation on the assumption that the the profile well into the faint outer region, the sampling galaxy is optically thin and axially symmetric. This profile aperture in this area should be increased to compensate is potentially advantageous for comparing characteristics for the lower signal-to-noise ratio. A mean profile often of galaxies having different inclinations on a uniform ba- refers to the major/minor-axis profile averaged over both sis. However, internal absorption and nonaxisymmetry of halves with respect to the galactic center (e.g., de Vau- real galaxies produce spurious features in the profile. The coüleurs 1975). generalized radial profile is expected to have great suc- 2. The azimuthally-averaged profile is the profile ob- cess when applied in the future to near-infrared surface tained by averaging the brightness distribution Z(r, Θ) photometry. over θ on an ellipse with a semimajor axis r and an axial Examples of various profiles are shown in Figure 6. ratio e. The axial ratio is usually defined by the isophotes Discussion of various profiles is also given in Ichikawa in the faint outer region. This profile is effective, espe- (1987) and an analysis of different profiles of a model cially for spiral galaxies, in smearing out nonaxisymmetric elliptical galaxy is found in Nieto (1982). irregularities caused by spiral structure or in examining disk-arm contrast (e.g., Schweizer 1976; van der Kruit G. Empirical Fitting Functions and Profile Decomposition 1987). For a quantitative study of global structure of galaxies, 3. The equivalent profile was introduced by de Vau- it is essential to characterize the profile on the basis of a couleurs (1948). Let S be the area included in an isophote few parameters. This is most conveniently done by fitting of a surface brightness level L If the isophotes consist of simple analytic functions to the profiles. It is known that several "islands", S includes the area of all such islands. elliptical galaxies and bulges of disk galaxies have similar The equivalent radius, r*, is defined as the radius of a profiles. Several fitting functions have been proposed to circle which has the same area as S, that is, model the profile (cf. de Vaucouleurs 1958α; Oemler 1976; Kormendy 1977c). Among the most commonly r* = (SAtt)172 . (7) used is the "r1/4 law" proposed by de Vaucouleurs (1948). A plot of the surface brightness as a function of the equiva- This is written as lent radius is the equivalent profile. The equivalent profile has an advantage in that we can obtain a very smooth ^χ--3·33^)"-1} · <8> profile even for galaxies whose isophotes have considerable irregularities. The disadvantage is that it tends to where re is the effective radius that contains half of the prevent the discovery of subtle features due to weak total luminosity and Ie is the effective surface brightness

NGC 5194/5195 (M51) .

2' .

. s. (a)

(d)

Fig. 5-Examples (M 51) of visual presentations of digital data arrays in surface photometry of galaxies, (a) Reproduction of the plate in the Β band, (b) Display of the area scanned by a microdensitometer showing the nonuniform background (top, 1 band). Black rectangles show the areas excluded in the background fitting. Four displays at the bottom show residuals in linear scale when sky background is modeled by zeroth-, first-, second-, and third-order polynomials, respectively (from left to right). The display range is —5% to +5% in relative intensity, (c) Grey-scale map of the Β surface brightness distribution (linear scale for —0.2 < ITe\ < 0.2, and logarithmic scale for 0.2 < lTe\ < 5.0). (d) Grey-scale map of (B - /) image displayed in magnitude scale. Bluer and redder regions in the galaxy are assigned darker and lighter, respectively. The display range is 2 mag, the zero point being arbitrary. Note the blue spiral arms in NGC 5194 and very red features in NGC 5195. (e) Isophotes in the Β band. The faintest isophote is 25 mag arc sec-2 and the isophote interval is 1.0 mag.

V ( mag arcsec"' 1— • |NGC 4535 NGC4535 I 7- . SAB(s)c --t.·'

Wri:-Ϊ

\lMrry ■

major axis

27 3 4 r( r") (arcmin) Fig. 6-Various brightness profiles of the spiral galaxy NGC 4535 (Takase, Kodaira, and Okamura 1984). Dot-dashed curve: mean profile along the major axis; broken curve: mean profile along the minor axis; solid curve: azimuthally averaged profile; solid curve with circles: equivalent profile. at that radius. On the other hand, disks of late-type spiral then obtained empirically as the approximations of the galaxies are known to show profiles described well by the observed profiles. In many recent studies, however, le "exponential law" of the form and re are determined by fitting equations (8) and (10) to the observed profiles (e.g., Kormendy 1977fo c; Kent /(r) = I exp (—ar) ? 0 (9) 1985; Kodaira, Watanabe, and Okamura 1986). One or should be careful to distinguish between re and Ie obtained in the two different ways. Most disk galaxies are composite systems where bulges log^= -0.729( - 1 (10) Or )· and disks coexist. In order to extract the characteristic where I0 is the surface brightness at the center of the parameters of the two fundamental components compos- -1 galaxy, a is the β-folding scale length, and re and le are ing a galaxy, the observed profile should be decomposed the same as for equation (8) (Freeman 1970; de Vau- into the contributions from the two components. The first couleurs and Freeman 1972). These "laws" do not have systematic study of this decomposition procedure was any firm physical basis (cf. King 1978; Freeman 1970). made by Kormendy (1977fc). He proposed a method Observed profiles of almost all galaxies show, more or known as iterative fitting together with the standard non- less, departures from these laws whichever kind of profile linear least-squares method in which is concerned. Nevertheless, these empirical fitting functions are useful as the first-order approximation to derive ;(r)./„dex(-3.33{(i)"'-l)) global characteristics of galaxies. + Zo)Dexp(-r/r0)D) (11) One point deserves a comment. Conventionally, re and Ie are determined by the integration of the two-dimen- is fitted to the observed profile to find the characteristic sional brightness distribution to a limiting surface bright- parameters for the bulge and the disk. The subscripts ness followed by the extrapolation to the zero intensity Β and D denote the bulge and disk, respectively. In level (cf. de Vaucouleurs 1977). Equations (8)-(10) are practice, fitting is made for μ(Γ) =-2.5 log I (r) instead of

I (r), and the brightness-scale parameter is also expressed μβ(ν) = 2.94 log re + 19.48 . (12) in units of magnitude as μ = —2.5 log I . Figure 7 shows 0 0 JK82 showed that cD galaxies in both rich and poor an example of profile decomposition. Detailed technical clusters fell on the brightward extrapolation of the discussions on this profile decomposition, which is now a μ ,^ r relation for ellipticals. This suggested that the standard technique, are scattered in the literature (e.g., β e main bodies of cD galaxies are photometrically indistin- Schombert and Bothun 1987). A slightly different ap- guishable from ordinary ellipticals except for the fact that proach is proposed by Kodaira et al. (1986). on the average they are more luminous (see also Thomsen ΠΙ. Recent Results and Frandsen 1983). However, recent extensive surface photometry of brightest cluster members (BCMs) by Successful studies of structure, formation, and evolu- Schombert (1986, 1987) appears to show that the μ6,1<^ re tion of galaxies require information from surface photom- relation for BCMs slightly deviates from that for normal etry alongside kinematic measurements since brightness ellipticals, although he points out that "ellipticals are not distributions are a useful measure of mass distribution r1/4 as a class of objects'. He further finds that the shape of despite yet-unknown contributions of dark halos. Com- the brightness profile of BCMs is a well-correlated prehensive reviews in this field are already given by JK80 smooth function of luminosity. A dynamical difference and JK82, to which very little can be added. This section between BCMs including cDs and normal ellipticals was then shows, on the basis of new results obtained mainly also found by Malumuth and Kirshner (1985). They after 1982, the wide variety of problems surface photome- showed that BCMs are on the average 1.2 mag brighter try can tackle. than predicted from their velocity dispersions and the L ^ 4 A. Early-Type Galaxies and Brightest Cluster Members σ relation (e.g., Kormendy and Illingworth 1983) for ellipticals. These new results may be in contradiction to Including cD Galaxies 1/4 the current theories of BCM formation through simple By fitting the r law to the profile of 19 elliptical homologous mergers (e.g., Ostriker and Hausman 1977; galaxies, Kormendy (1977c) found a tight correlation be- Hausman and Ostriker 1978; Malumuth and Richstone tween the characteristic parameters μ in the Β band and β 1984). Multiple nuclei found in BCMs were considered as r in kpc (JK80). The latest version of the relation in the V e direct evidence for mergers (Hoessel 1980; Schneider et band (Hamabe and Kormendy 1987) is given by al. 1983; Hoessel, Borne, and Schneider 1985). However, recent measurements of redshifts for a considerable number of multiple-nuclei galaxies (Tonry 1985; Smith et al. 1985) have shown that the distribution of the velocity differences between multiple nuclei and the central galaxy has an rms width of ~ 800 km s_1. This large velocity suggests that only a small fraction of the nuclei is moving on bound orbits slowly enough to be captured (Tonry 1984, 1985). Studies of the nature of BCMs have been made by Lugger (1984), Lilly, McLean, and Longair (1984), Hoes- sel and Schneider (1985), Lilly and Prestage (1987), and others, their aim being to use BCMs as standard candles or to investigate active-galactic-nucleus (AGN) phenom- ena in relation to host galaxies and cluster environments. Michard (1985) presented detailed surface photometry of 36 elliptical and SO galaxies and found that brightness profiles of ellipticals show systematic deviations from r1/4 laws. Such deviations are conventionally interpreted as a result of tidal interaction (JK82). However, Michard found that deviations are similar for galaxies of similar luminosities suggesting the inherent nature of the deviations. Whether this is the same phenomenon as that for BCMs found by Schombert (1986) is still an open question. Fig. 7-An example of profile decomposition (Kent 1985). Crosses: ob- Intrinsic shape of elliptical galaxies has been a subject served profile; short-dashed line: bulge component; long-dashed line: disk component; solid line: sum of bulge and disk. The two curves are of surface photometry as well as kinematical observations. the major-axis (upper) and the minor-axis (lower) profiles with the latter Are they oblate, prolate, or triaxial (JK82)? A few elliptical being lowered by 5 mag. galaxies are suggested to have close-to-prolate figures

(Barbon et al. 1984; Davies and Birkinshaw 1986). In his Finally, it should be noted that two recent large sur- recent review, Davies (1987) summarizes the current veys of elliptical galaxies have led to the discovery of an status as follows: "The data suggest that all ellipticals are improved correlation between luminosity, velocity dis- triaxial with most being close to oblate spheroidal and a persion, and surface brightness, which can be used as a small fraction being close to prolate spheroidal." The better distance estimator than the conventional L ση presence of weak stellar disks in ellipticals has been given relation (Djorgovski and Davis 1987; Burstein et al. 1987; much attention by several researchers in relation to Dressler et αί. 1987). the rotational properties and classification of ellipticals Β. Spiral Galaxies (Capaccioli, Held, and Rampazzo 1984; Michard 1985; Spiral structure has been studied by multicolor surface Lauer 1985c; Capaccioli 1987; Davies 1987; Carter 1987; photometry to decompose the disk brightness distribu- Jedrzejewski et al. 1987). tion into young and underlying old stellar populations, a Surface photometry has been used to map dark nebulae technique pioneered by Dixon, Ford, and Robertson in early-type galaxies and to investigate their quantitative (1972) and extended by Schweizer (1976) and Talbot et al. characteristics (Gallagher and Hunter 1981; Lauer 1985c; (1979). Brightness and color variations obtained by them Sparks et al. 1985; Gallagher 1986). On the basis of a for prominent grand-design spirals were found to be in complete sample of southern early-type galaxies, Sadler good agreement with those predicted by the density- and Gerhard (1985a) have estimated the fraction of galax- wave theory (e.g., Toomre 1977). However, Elmegreen ies with dust to be ~ 40% for nearby ellipticals and (1981) and Elmegreen and Elmegreen (1982) recognized somewhat higher for SOs. They found no strong correla- the variety of spiral structure (see also JK82). On the tion between the presence of dust and that of shells, in basis of the first systematic quantitative study of spiral apparent disagreement with the idea that the presence of structure involving the near-infrared 1 band images, dust in ellipticals is a signature of past mergers. Although Elmegreen and Elmegreen (1984) concluded that arms in color maps are a basic tool to detect reddened features flocculent spirals owe their structure not to density waves due to dust absorption (e.g., Sadler and Gerhard 1985&), in an underlying old stellar population but to star forma- a variety of image-processing techniques such as digital tion. Kennicut and Edgar (1986) studied three gas-poor, unsharp masking are found to be more effective in bring- smooth-armed spirals to obtain accurate quantitative ing out weak dust features superposed on the strong measurements of the properties of the underlying broad radial-intensity gradient (e.g., Schweizer and Ford 1985; density waves. Gallagher 1986; Djorgovski and Ebne ter 1987). Multicolor surface photometry has also been used to The metallicity gradient in galaxies poses a strong con- estimate stellar populations in different parts of a galaxy straint on theories of galaxy formation (e.g., Strom et al. (e.g., Jensen, Talbot, and Dufour 1981; Schild et al. 1985; 1977; Carlberg 1984α,έ>). It has been measured for early- Ichikawa et al. 1987α). Barred galaxies and ringed galax- type galaxies by either wide-band color photometry (e.g., ies have also been favorite targets, the aim of the studies Strom et al. 1976, 1978; Strom and Strom 1977) or nar- being the comparison with stellar dynamical models (e.g., row-band photometry in which bandpasses are chosen to Benedict 1982; Blackman 1983; Elmegreen and Elme- select strong absorption-line features (Faber 1977). Gal- green 1985; Pence and de Vaucouleurs 1985; Duval and lagher, Faber, and Burstein (1980) measured (ß — V) Monnet 1985; Buta 1986, 1987a,fc). color profiles for two normal ellipticals and NGC 6166, a Color gradients and brightness distributions in bulges multiple-nucleus cD galaxy. They found that the cD and disks provide important constraints on theories of shows a larger color gradient than the ellipticals. The galaxy formation. Color gradients of bulges are most con- color gradient in NGC 6166 was confirmed by detailed veniently measured in edge-on systems. Van der Kruit CCD surface photometry by Lachièze-Rey, Vigroux, and and Searle (1981, 1982) found outward bluing in the Souviron (1985), who concluded that the presence of a bulges of NGC 891 and NGC 7814, which they inter- color gradient covering the whole galaxy is hardly com- preted as due to a decrease in the mean heavy-element patible with a merger origin for the galaxy. Baum et al. abundance with increasing radius. They also found that (1986) and Thomsen and Baum (1987) have measured the the isochromes are closely similar in shape to the iso- magnesium gradient for two giant ellipticals in the Coma photes. Color gradients of underlying old disk compo- cluster using a CCD camera and a set of medium-band- nents have been measured in a number of face-on spirals width filters. All the studies found a significant color (e.g., Shostak and van der Kruit 1984; Wevers et al. 1986; gradient for all galaxies observed. Measurements of accu- Walterbos and Kennicutt 1987; Kent 1986, 1987a). All rate metallicity gradients for large samples of ellipticals the studies found no significant color gradients with a few and BCMs, including cDs, would provide an important exceptions in accordance with earlier results (e.g., clue to the problem of their formation (cf. Wirth 1981; Schweizer 1976; Okamura 1978). This is consistent with Boroson et al. 1983&; Davis et al. 1985; Cohen 1986; the finding by Elmegreen and Elmegreen (1984) that Boroson and Thompson 1987). there is no systematic difference in the scale lengths of

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System SURFACE PHOTOMETRY OF GALAXIES 537 disks between Β and I bands. Absence of a significant Ichikawa et al (1986) suggested a structural disconti- color gradient implies the approximate constancy of the nuity in a mass sequence of spheroidal stellar systems mass-to-luminosity ratio within the old disk of a galaxy. consisting of giant ellipticals, dEs, and Galactic globular However, a dynamically determined mass-to-luminosity clusters on the basis of the diameter versus surface- ratio for the whole galaxy does vary significantly with brightness diagram (DSBD) introduced by Kodaira, radius (Bosma and van der Kruit 1979; Burstein et al. Okamura, and Watanabe (1983). This feature prompted 1982). Yoshii and Arimoto (1987) to develop a model that ex- plains the apparent discontinuity as the result of mass loss C. Dwarf Galaxies due to supernova-driven galactic wind. The discontinuity One of the highlights in recent galaxy research is the has been discussed on several occasions using apparently recognition of dwarf galaxies outside the Local Group—in different but essentially similar diagrams to DSBDs (e.g., particular, in the Virgo cluster (Reaves 1983)—which was Saito 1979; Binggeli et al. 1984; Kormendy 1985fc; Nieto followed by comprehensive studies of their morphology, and Prugniel 1987ö,fc). It is surprising that the correla- spatial distribution, photometric properties, and lumi- tions of global parameters on DSBDs look so similar to nosity function by Sandage and collaborators based on those of core parameters on various parameter planes large-scale photographic plates taken with the du Pont shown in Kormendy (1985¾). All these photometric re- telescope at the Las Campanas Observatory (Binggeli et sults on Virgo dEs have been confirmed recently by al. 1984; Binggeli, Sandage, and Tammann 1985; Sandage Ichikawa (1987) on the basis of a much larger sample. and Binggeli 1984; Sandage, Binggeli, and Tammann Figure 8 shows DSBD for giant ellipticals and SOs, dEs in 1985). Intensive efforts have been made to understand the Virgo cluster and dEs in the Local Group, and Galac- the nature of this group of objects which overwhelms tic globular clusters. normal or giant galaxies in number both in the Virgo D. Characteristic Parameters of Spheroids (Bulges) region (Caldwell 1983; Bothun et al. 1985, 1986 and and Disks references therein) and in the Local Group and general After JK80 and JK82, several extensive studies of field (e.g.. Hunter and Gallagher 1985 and references profile decompositions have been published. In most therein). Studies of dwarf galaxies have been extended studies (Boroson et al. 1983α; Meisels and Ostriker 1984) recently to the Fornax cluster by Caldwell (1987) and Caldwell and Bothun (1987). Surface photometry of some 70 dwarf ellipticals (dEs) in the Virgo cluster revealed that almost all dEs have brightness profiles consistent with exponential laws (Ichikawa et al. 1986; Binggeli et al. 1984; Caldwell 1983). Local Group dEs have long been known to have exponential profiles but this was conventionally fitted by King models (King 1966) and interpreted as the result of tidal stripping of otherwise more-extended envelopes (cf. Hodge 1971; Faber and Lin 1983). The prevalence of exponential profiles in dEs, however, strongly suggests that this feature is inherent to dEs. On the basis of the statistical analysis of the distribution of apparent flattenings, Caldwell (1983) and Ichikawa et al. (1986) suggested that the three-dimensional shape of dEs is thicker than that of disks in spirals in spite of their exponential profiles. Feitzinger and Galinski (1986) suggested the triaxial shape for dEs. A survey of the isophote twist in dEs would be helpful to the study of their intrinsic shapes. Binggeli et al. (1985) identified a class of compact dEs which they call "M32-type". This class is probably the same as "classical dEs" recognized by Wirth and Gal- lagher (1984) as the true low-luminosity extension of the giant elliptical family. Surface photometry of four such Fig. 8-Diameter versus mean surface brightness diagram (Ichikawa compact ellipticals by Nieto and Prugniel {1987a,b) 1987) for giant elliptical galaxies (circles), SO galaxies (triangles), and 1/4 dwarf elliptical galaxies (pluses) in the Virgo cluster and dwarf elliptical showed that their profiles are closer to an r law than an galaxies in the Local Group (diamonds) and Galactic globular clusters exponential law and that they exhibit a trend for the outer (asterisks). The parameters refer to the values at 26 mag arc sec 2 in the isophotes to be more circular than the inner ones. Β band.

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 538 SADANORI OKAMURA observed profiles are decomposed on the basis of a con- 5. Bulge-to-disk ratio does vary systematically along ventional composite model consisting of a r1/4-law the morphological sequence but with a large scatter. spheroid and an exponential disk as described in Section Simien and de Vaucouleurs (1986) claimed that the domi- II. G. In some studies dealing with spirals only, profiles nant source of the large scatter is the decomposition are fitted by exponential laws (Grosb0l 1985; van der error, but Kodaira et al. (1986) suggested a large cosmic Kruit 1987). It is well known (e.g., JK82) that nonexpo- dispersion. It has been shown that other intrinsic proper- nential disks are very common and that many spheroids ties of galaxies have quite large dispersion within a given 1/4 are not described very well by r laws. Nevertheless, Hubble type (e.g., Rubin et al. 1985; Okamura, Kodaira, characteristic parameters derived from profile decompo- and Watanabe 1984). sitions are very important to obtain a statistical overview on the systematic properties of the most fundamental E. Dark Halos components constituting galaxies. There are lines of evidence supporting the existence of It is somewhat surprising that results of all these recent massive unseen halos around spiral galaxies (Faber and studies are in broad qualitative agreement. In particular, Gallagher 1979). The strongest one appears to be the flat parameter correlations obtained from three extensive rotation curve (Rubin et al. 1980, 1982, 1985; Burstein studies of similar approach (Kent 1985; Simien and de and Rubin 1985; Bosma 1981). The nature of the dark halo Vaucouleurs 1986; Kodaira et al. 1986) exhibit the same is to date totally unknown. It may be composed of non- behavior despite numerous differences in samples, obser- baryonic strange particles (e.g., Hegyi and Olive 1986) or vational material, and data-reduction procedures. Results possibly baryonic matter (Bahcall and Casertano 1985). of the recent studies and their implications may be sum- Optical searches for dark halos are reviewed by JK80 and marized as follows: JK82. 1. Spheroid parameters cover a very wide range while Surface photometry has been used to investigate the disk parameters are confined within a narrow range. This nature of dark halos in disk galaxies in combination with regularity seen in disk parameters suggests that disks are dynamical data. The usual method is to map the total formed through a process which is not very much affected gravitational potential as far out as possible by measuring by spheroid properties and environmental factors. It the rotation curve and subtracting the contribution of disk should be noted, however, that the samples contain few potential, and bulge potential, if necessary, which are very late-type spirals (Sm,Im) and dwarf irregular galaxies computed from the measured brightness distribution on which may have a considerable range in disk parameters the assumption of constant mass-to-luminosity ratio. (Hunter and Gallagher 1985; Bothun et al. 1986). Then one can find a mass model of the dark halo which 2. Bulges of disk galaxies do not exactly follow the best describes the residual potential. An isothermal μ6,^ re relation for ellipticals. They show larger disper- sphere is often employed as the mass model of the dark sion around the mean relation than ellipticals. This im- halo and an infinitely thin disk is assumed for the disk plies that, unlike ellipticals, bulges of disk galaxies cannot component. The observed radial-luminosity distribution be regarded as a nearly one-parameter family. in the disk is either modeled by an exponential (van 3. The answer to a long-lasting debate, regarding Albada et al. 1985) or converted directly to potential using whether or not the great majority of galactic disks have a a numerical method (e.g., Kalnajs 1983; Carignan and constant face-on central surface brightness around the Freeman 1985). An example of the analysis is shown in Freeman (1970) magic number, μΒ = 21.65 ± 0.28 mag Figure 9. arc sec 2, appears to be yes, at least for bright disk galax- Bahcall and Casertano (1985) summarized the halo ies. The dispersion is, however, much larger than Free- and disk parameters of eight late-type spiral galaxies man originally stated. Van der Kruit (1987) quotes the with detailed mass models that reproduce high-quality value of 22.7 ± 0.9 mag arc sec-2 in the J band on the basis rotation curves. They found a remarkable regularity of of a complete sample. He discusses this approximate SKh/SKd ~ 1 within the optical radius in spite of the fact constancy in relation to a model of galaxy formation. that 2)ΐΗ and S)ÎD separately vary by a factor of 100 among 4. Another problem often focused on in the analysis of the sample galaxies, where S)ÎH and S)ÎD are masses of dark characteristic parameters is whether lenticular galaxies halo and the disk, respectively. This "disk-halo conspir- (SOs) form a "gas-poor" sequence parallel to the gas-rich acy" (Kent 1987a) was taken as evidence that favors the spirals or whether SOs were born as transitional objects baryonic dark matter. between ellipticals and spirals (e.g., van den Bergh 1976; Kent (1986) made a similar analysis for 37 Sb and Sc Gisler 1979). Results of the recent studies appear to be in galaxies with optical rotation curves using a bulge-disk- favor of the latter idea. However, the possibility that SOs halo three-component model. However, definite conclu- are a heterogeneous class of objects is pointed out by sions could not be drawn even on the existence of dark Dressier and Sandage (1983) on the basis of kinematical halos due to the insufficient coverage of optical-rotation data. curves and ambiguity in the assumed mass-to-luminosity

nal absorption and galactic extinction become much smaller. Second, the contribution from young stars de- creases in the longer wavelengths and practically vanishes beyond ~ 1.5 μιη. Third, there is little contamination from dust or gaseous emission at/(1.25 μιη), Η (1.6 μιη), and Κ (2.2 μιη) bands. Thus, we can measure in these bands the distribution of luminous mass that is least affected by young blue stars. Photographic I-band (0.7-0.9 μιη) surface photometry has been presented for dozens of galaxies (e.g., Boroson et al. 1983α ; Elmegreen and Elmegreen 1985; Baumgart and Peterson 1986; and references in Section III.B). However, surface photometry beyond 1 μιη is very scarce in the literature. The first high-resolution infrared mapping of a galaxy was made by Hackwell and Schweizer (1983) for the spiral galaxy NGC 1566. Their map in the Η Fig. 9-Investigation of the dark halo (Carignan and Freeman 1985). Circles are rotational velocities obtained from H I observations. The band showed a prominent bar that is hardly visible in the long-dashed curve shows velocity due to the disk component computed optical image. A few infrared maps for other galaxies have from surface photometry on the assumption of a constant mass-to-lumi- been published only recently (Prieto et al. 1985; Martinez nosity ratio. The short-dashed curve is the velocity due to the dark halo Roger, Phillips, and Sanchez Magro 1986; Adamson, component (isothermal sphere), and the solid curve shows the sum of the two. Adams, and Warwick 1987). Homogeneous surface photometry of large samples of galaxies beyond 1 μιη would open a new field in galactic ratio. Kent (1987a) has extended the analysis to 16 galax- research where morphological classification is reviewed ies with more extended H I rotation curves. He finds and new analyses are made for many "old" quantities such evidence for dark halos in all the galaxies this time, but as the characteristic parameters of spheroids and disks. the relative contributions of stellar and dark matter can- Such studies will be made feasible by the advent of not be well determined. Further, he finds no halo-disk two-dimensional infrared arrays, e.g., infrared CCDs conspiracy in direct contradiction with Bahcall and Caser- (Mackay 1986). In particular, a detailed study of the cali- tano (1985). The estimated mass-to-luminosity ratio of the brating galaxies for the infrared Tully-Fisher relation disks in Kent's (1987a) sample (mostly Sb-Sc) varies by a (Aaronson, Huchra, and Mould 1979; Aaronson, Mould, factor of 20 (2K/L ~ 0.4-8). This appears to be too large R and Huchra 1980) and of galaxies with high-quality rota- a variation considering the similarity of disk colors in tion curves could well yield results that would strongly Sb-Sc galaxies, although a part of the large variation may influence our present understanding of the structure of be due to internal absorption and distance error. Clearly, galaxies (cf. Burstein 1982). more extensive analyses based on extended Η ι rotation curves of good quality combined with detailed surface Β. High-Resolution Studies and Narrow-Band Imaging photometry are necessary to elucidate the nature of dark The importance of high resolution is readily recogniz- halos. Studies on dark matter in dwarf galaxies are re- able if we compare, for example, two images of NGC 4156 viewed by Kormendy (1987a). taken under different seeing conditions (Nieto and Ti- Another approach to probe the gravitational potential ennot 1984). A large compilation of images and brightness far into halos is to measure the rotation of polar rings profiles of QSO nuclei and host galaxies by Hutchings et observed in some SO galaxies. Whitmore, McElroy, and al. (1984) may be noted as a typical example that took full Schweizer (1987) measured the rotation velocities of polar advantage of high resolution. rings and stellar components in the disks of three SO One tantalizing question that has a direct relevance to galaxies and found that the shape of equipotential surfaces high resolution has been whether supermassive objects is close to spherical. This approach appears to be promis- such as black holes are present in the center of galaxies. ing since dozens of polar-ring galaxies are known to exist Schweizer (1979, 1981) showed that measured properties (Schweizer, Whitmore, and Rubin 1983). of cores of elliptical galaxies are seriously degraded by seeing and other scattering processes. Recently, how- IV. Future Prospects ever, Lauer (1985&) and Kormendy (1985a, fc) success- A. Infrared Surface Photometry fully resolved cores of ellipticals and bulges of disk galax- Surface photometry of galaxies in near-infrared wave- ies. Use of CCDs at focal planes with sufficient scales lengths (1-3 μιη) has several advantages over that in under excellent seeing conditions was crucial to the suc- optical (0.4-0.8 μιη) wavelengths. First, effects of inter- cess of these studies. Kormendy (1985a) found that two

ellipticals, M 87 and NGC 3379, which are suspected of manifolds of disk galaxies are found to have two dominant having black holes (e.g.. Young et al. 1978; Dressier 1980; dimensions (Bujarrabal, Guibert, and Balkowski 1981; Nieto and Vidal 1984¾ ; Bendinelli, Parmeggiani, and Whitmore 1984), while those of ellipticals have one domi- Zavatti 1984), did not show any evidence of their presence nant and a secondary dimension (Efstathiou and Fall in the core profiles. Recently, Kormendy (1987¾) has 1984). Watanabe, Kodaira, and Okamura (1985). per- found evidence for black holes in M 32 and M 31 based on formed the analysis on a sample consisting of galaxies of all high-resolution kinematical observations (see also Tonry morphological types using surface photometric parame- 1987). Problems concerning high-resolution imaging ters in a single color band only. They found the same from the ground were reviewed by Woolf (1982). results as in previous studies and suggested that the The Hubble Space Telescope wide-field/planetary cam- structure of galaxies can be properly described by surface era will soon produce a flood of images with resolutions at photometric parameters alone or, in other words, there least one order of magnitude better than those attainable are tight correlations between surface photometric from the ground. This means that the frontier of space will parameters and kinematical parameters. With this in be pushed more than 10 times farther away. Accordingly, mind, Kodaira et al. (1983) presented the diameter versus applications of even conventional methods of surface pho- surface-brightness diagram (DSBD) as a proposed quanti- tometry to these images would shed new light on the tative classification system. structure and evolution of distant galaxies. Further, de- A recent comprehensive study for ellipticals by Djor- tailed studies of dense cores of nearby galaxies based on govski and Davis (1987) shows that ellipticals form a two- hyperresolution images, together with kinematical data of dimensional family and that in terms of observational good resolution (e.g., Kormendy 1987&), would yield parameters they are most properly described by central new results which have strong influences on theories of velocity dispersion and mean surface brightness (see also galaxy formation and evolution. Burstein et al. 1987). The thickness of the fundamental Narrow-band imaging by CCDs or even by photo- plane is found to be entirely due to measurement errors. graphic plates at the fast (prime) focus of future large This suggests a strong regularity in the process of forma- telescopes will become very important. Imaging Fabry- tion of ellipticals. Perot spectrometers such as TAURUS (Atherton et al. All the above studies are limited to normal, i.e., nonac- 1982) will be useful in some cases. Such observations will tive and nondwarf, galaxies and some are subject to un- produce important data on detailed velocity fields (e.g., known selection effects. An analysis of a large complete Teuben et al. 1986) and star forming/nuclear activities sample including both giant and dwarf galaxies of all (e.g., Courtès et al. 1987) in nearby galaxies, and metallic- morphological types is one of the very important targets of ity gradients in early-type galaxies (Thomsen and Baum future studies of galaxies. Such a study would enable us to 1987), etc. discriminate the regular trend of galaxy formation from noise and to assess the effect of environments and large- C. Quantitative Classification: Minimal Manifold scale structure of the universe on individual galaxy struc- of Galaxies ture. The study of galaxies is still in its infancy (Whitmore 1984). We have not found a diagram, or a set of diagrams, A remarkable number of colleagues responded to my by which structure, formation, and evolution of galaxies request for reprints and preprints. Many of them are can be interpreted on a physical basis similar to the H-R members of the Working Group on Galaxy Photometry diagram for stars. This is because we do not know what and Spectrophotometry in IAU Commission 28. They are the most fundamental properties are and whether what too numerous to name here but I am very grateful to all of we measure is "noise" or simply reflections of the more them. Some of them also kindly gave permission to use fundamental properties, in spite of recent rather ample figures in their papers. I especially thank M. Capaccioli, outflow of observational data. The problem is then to C. Carignan, G. de Vaucouleurs, J. S. Gallagher, S. M. determine how many, and which, physical quantities are Kent, K. Kodaira, J.-L. Nieto, F. Schweizer, Β. Takase, necessary and sufficient to describe a family of galaxies and M. Watanabe for reading the early version of the (Djorgovski and Davis 1987). It is stated as the problem of manuscript and giving many informative comments and the minimal manifold of galaxies (Djorgovski 1987). Iden- suggestions for revision. I would also like to express my tification of such fundamental quantities and examination sincere thanks to S. Ichikawa, M. lye, M. Hamabe, N. of their interrelationships will eventually elucidate the Takato, and T. Aoki for useful discussion on CCD data nature of galaxies. Development of an objective quantita- reduction; K. Tarusawa, S. Ichikawa, and N. Takato for tive classification system is the first step toward the dis- help in preparing data for Figures 4 and 5; and K. Ko- tant goal. daira, J. E. Hesser, J. S. Gallagher, K. Wakamatsu, and Since the pioneering study by Brosche (1973), a num- staff members of the Kiso Observatory for encourage- ber of studies have been directed to this problem. The ment. M. Othman kindly edited the manuscript at the

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System SURFACE PHOTOMETRY OF GALAXIES 541 initial stage of preparation. Figures 4 and 5 were pro- Burstein, D., Rubin, V. C., Thonnard, N., and Ford, K. W., Jr. 1982, duced at the Kiso Observatory using the facilities and the ApJ., 253, 70. Buta, R. 1986, Ap. J. Suppl., 61, 631. software library SPIRAL for astronomical image process- 1987a, Ap. J. Suppl., 64, 1. ing (Ichikawa et al. 19S7b). This work is supported by 1987b, Ap. J. Suppl, 64, 383. grant-in-aid No. 59065002 and, in part, No. 60420001 and Caldwell, N. 1983, A./., 88, 804. No. 60460007, from the Ministry of Education, Science, 1987, A J., 94, 1116. Caldwell, N., and Bothun, G. D. 1987, AJ., 94, 1126. and Culture. Cannon, R. D. 1979, in Photometry, Kinematics, and Dynamics of Galaxies, ed. D. S. Evans (Austin: University of Texas Press), p. 27. REFERENCES Capaccioli, M. 1984, in Data Analysis in Astronomy, ed. V. di Gesù et Aaronson, M., Huchra, J., and Mould, J. 1979, Ap.J., 229, 1. al. (New York: Plenum Press), p. 363. Aaronson, M., Mould, J., and Huchra, J. 1980, Ap. J., 237, 655. 1985, in New Aspects of Galaxy Photometry, ed. J.-L. Nieto, Abies, H. D. 1971, Pub. U.S. Naval Obs., Vol 20, Part IV. Lecture Notes in Physics, Vol. 232 (Berlin: Springer), p. 53. Adamson, A. J., Adams, D. J., and Warwick, R. S. 1987, M.N.R.A.S., 1987, in 1AU Symposium 127, Structure and Dynamics of 224, 367. Elliptical Galaxies, ed. T. de Zeeuw (Dordrecht: Reidel), p. 47. Allen, C. W. 1973, Astrophysical Quantities (3d ed.; London: Athlone Capaccioli, M., and de Vaucouleurs, G. 1983, Ap. J. Suppl., 52, 465 Press), p. 134. (CdV). Arp, H., and Lorre, J. 1976, Ap. J., 210, 58. Capaccioli, M., Held, Ε. V., and Nieto, J.-L. 1987, A J., 94, 1519. Atherton, P. D., Taylor, K., Pike, C. D., Harmer, F. W., Parker, Capaccioli, M., Held, E.V., and Rampazzo, R. 1984, Asir. Αρ., 135, 89. N. M., and Hook, R. N. 1982, M.N.R.A.S., 201, 661. Carignan, C. 1985, Ap.J. Suppl, 58, 107. Bahcall, J. N., and Casertano, S. 1985, Ap. J. (Letters), 293, L7. Carignan, C., and Freeman, K. C. 1985, Ap.J., 294, 494. Baluteau, J.-P., and D'Odorico, S., eds. 1986, The Optimization of the Carlberg, R. G. 1984α, Αρ. /., 286, 403. Use of CCD Detectors in Astronomy (Garching: European Southern 1984b, Αρ. J., 286, 416. Observatory). Carter, D. 1979, M.N.R.A.S., 186, 897. Barbon, R., Benacchio, L., and Capaccioli, M. 1976, Mem. Soc. Astr. 1987, Ap.J., 312, 514. Italy, 47, 263. Carter, D., and Dixon, K. L. 1978, A./., 83, 574. Barbon, R., Benacchio, L., Capaccioli, M., and Rampazzo, R. 1984, Carter, D., Jorden, P. R., Thorne, D. J., Wall, J. V., and Straede, J. C. Asir. Αρ., 137, 166. 1983, M.N.R.A.S., 205, 377. Barbon, R., Capaccioli, M., and Rampazzo, R. 1982, Asir. Αρ., 115, Cohen, J. G. 1986, A.J.,92, 1039. 388. Corwin, H. G., Jr., de Vaucouleurs, Α., and de Vaucouleurs, G. 1985, Baum, W. Α., Thomsen, Β., and Kreidl, T. J. 1981, Froc. SPIE, 290, 24. The University of Texas Monographs in Astronomy, No. 4 (Austin: Baum, W. Α., Thomsen, B., and Morgan, B. L. 1986, Ap.J., 301, 83. Department of Astronomy, University of Texas). Baumgart, C. W., and Peterson, C. J. 1986, Pub. A.S.P., 98, 56. Courtès, G., Petit, H., Sivan, J.-P., Dodonov, S., and Petit, M. 1987, Bendinelli, O., Parmeggiani, G., and Zavatti, F. 1984, Asir. Αρ., 140, Asir. Αρ., 174, 28. 174. Dainty, J. C., and Shaw, R. 1974, Image Science (London: Academic Benedict, G. F. 1980, A.J., 85, 513. Press). 1982, AJ., 87, 76. Daly, P. N., Phillips, S., and Disney, M. J. 1987, Asir. Αρ., 68, 33. Binggeli, B., Sandage, Α., and Tammann, G. Α. 1985, A.J., 90, 1681. Davies, R. L. 1987, in IAU Symposium 127, Structure and Dynamics of Binggeli, Β., Sandage, Α., andTarenghi, M. 1984, A.J., 89, 64. Elliptical Galaxies, ed. T. de Zeeuw (Dordrecht: Reidel), p. 63. Binney, J. 1982, Ann. Rev. Astr. Αρ., 20, 399. Davies, R. L., and Birkinshaw, M. 1986, Ap.J. (Letters), 303, L45. Blackman, C. P. 1979, in Photometry, Kinematics, and Dynamics of Davis, L. E., Cawson, M., Davies, R. L., and Illingworth, G. 1985, Galaxies, ed. D. S. Evans (Austin: University of Texas Press), p. 135. Α./., 90, 169. 1983, M.N.R.A.S., 202, 379. Davoust, Ε., and Pence, W. D. 1982, Asir. Αρ. Suppl, 49, 631. Boroson, T. A. 1981, Ap.J. Suppl., 46, 177. Dawe, J. Α., Coyte, E., and Metcalfe, N. 1984, in Astronomical Photog- Boroson, T. Α., and Thompson, I. B. 1987, A.J., 93, 33. raphy 1984, ed. M. E. Sim and K. Ishida (Edinburgh: Royal Obser- Boroson, T. Α., Strom, Κ. M., and Strom, S. Ε. 1983α, Αρ. J., 274, 39. vatory), p. 59. Boroson, Τ. Α., Thompson, L B., and Shectman, S. A. 1983^, A.J., 88, de Vaucouleurs, G. 1948, Ann. d'Ap., 11, 247. 1707. 1958, Ap.J., 128, 465. Bosma, Α. 1981, A.J., 86, 1825. 1959a, Handbuch der Physik, 53, 311. Bosma, Α., and van der Kruit, P. C. 1979, Astr. Αρ., 79, 281. 1959b, Ap.J., 130, 728. Bothun, G. D., Mould, J. R., Caldwell, N., and MacGillivray, Η. T. 1968, Appl. Optics, 7, 1513. 1986, A./., 92, 1007. 1975, Ap.J. Suppl, 29, 193. Bothun, G. D., Mould, J. R., Wirth, Α., and Caldwell, N. 1985, AJ., 1977, Ap.J. Suppl, 33, 211. 90, 697. 1979, in Photometry, Kinematics, and Dynamics of Galaxies, Brosche, P. 1973, Asir. Αρ., 23, 259. ed. D. S. Evans (Austin: University ofTexas Press), p. 1 (dV79). Bujarrabal, V., Guibert, J., and Balkowski, C. 1981, Asir. Αρ., 104, 1. 1984, in IAU Colloquium 78, Astronomy with Schmidt-Type Burkhead, M. S. 1978, Ap.J. Suppl, 38, 147. Telescopes, ed. M. Capaccioli (Dordrecht: Reidel), p. 367. Burkhead, M. S., and Burgess, R. D. 1973, A.J., 78, 606. 1987, in IAU Symposium 127, Structure and Dynamics of Burkhead, M. S., and Kalinowski, J. K. 1974, A./., 79, 835. Elliptical Galaxies, ed. T. de Zeeuw (Dordrecht: Reidel), p. 3. Burstein, D. 1979, Ap. J. Suppl., 41, 435. de Vaucouleurs, G., and Abies, H. D. 1968, Ap.J., 151, 105. 1982, Ap./.,253, 539. de Vaucouleurs, G., and Capaccioli, M. 1979, Ap. /. Suppl, 40, 699. Burstein, D., and Rubin, V. C. 1985, Ap. J., 297, 423. de Vaucouleurs, G., and Freeman, K. C. 1972, Vistas Astr., 14, 163. Burstein, D., Davies, R. L., Dressler, Α., Faber, S. M., Stone, R. P. S., de Vaucouleurs, G., and Nieto, J.-L. 1978, Ap. /., 220, 449. Lynden-Bell, D., Terlevich, R. J., and Wegner, G. 1987, Ap. J. de Vaucouleurs, G., de Vaucouleurs, Α., and Corwin, H. G.; Jr. 1976, Suppl., 64, 601. Second Reference Catalogue ofRright Galaxies (Austin: University

of Texas Press). 94. Dixon, M. E., Ford, V. L., and Robertson, J. W. 1972, Ap. J., 174, 17. Hunter, D. Α., and Gallagher, J. S. III 1985, Αρ./. Suppl., 58, 533. Djorgovski, S. 1984, in Proc. Workshop on Improvements to Photome- Hutchings, J. B., Crampton, D., Campbell, B., Duncan, D., and try, NASA CP-2350, p. 152. Glendenning, B. 1984, Ap. /. Suppl., 55, 319. 1985, Ph.D. dissertation. University of California, Berkeley. Ichikawa, S. 1987, doctoral dissertation. University of Tokyo. 1987, in IAU Symposium 118, Instrumentation and Research Ichikawa, S., Okamura, S., Kaneko, N., Nishimura, M., and Toyama, Programmes for Small Telescopes, ed. J. B. Hearnshaw and P. L. Κ. 1987a, Pub. Asir. Soc. ]apan, 39, 411. Cottrell (Dordrecht: Reidel), p. 243. Ichikawa, S., Okamura, S., Watanabe, M., Hamabe, M., Aoki, T., and Djorgovski, S., and Davis, M. 1987, Ap. /., 313, 59. Kodaira, K. 1987¿?, Annals Tokyo Astr. Obs., 21, 285. Djorgovski, S., and Ebneter, K. 1987, in IAU Symposium 118, Instru- Ichikawa, S., Wakamatsu, K., and Okamura, S. 1986, Ap./. Suppl., 60, mentation and Research Programmes for Small Telescopes, ed. J. B. 475. Hearnshaw and P. L. Cottrell (Dordrecht: Reidel), p. 277. Jedrzejewski, R. I. 1987, M.N.R.A.S., 226, 747. Dressier, A. 1980, Ap.]. (Letters), 240, Lll. Jedrzejewski, R. L, Davies, R. L., and Illingworth, G. 1987, A./., 94, Dressier, A., and Sandage, A. 1983, Ap. J., 265, 664. 1508. Dressier, Α., Faber, S. M., Burstein, D., Davies, R. L., Lynden-Bell, Jensen, Ε. B., Talbot, R. J., Jr., and Dufour, R. J. 1981, Ap./., 243, 716. D., Terlevich, R. J., and Wegner, G. 1987, Ap. J. (Letters), 313, Johnson, H. L. 1962, in Astronomical Techniques, ed. W, A. Hiltner, L37. Stars and Stellar Systems, Vol. II (Chicago: University of Chicago Duval, M. F., and Monnet, G. 1985, Asir. Αρ. Suppl., 61, 141. Press), p. 157. Efstathiou, G., and Fall, S. M. 1984, M.N.R.A.S., 206, 453. Jones, W. B., Gallet, R. M., Obitts, D., and de Vaucouleurs, G. 1967, Elmegreen, B. G., and Elmegreen, D. M. 1985, Ap. J., 288, 438. Pub. Astr. Dept. Univ. Texas, Ser. 2, Vol. 1, No. 8. Elmegreen, D. M. 1981, Ap.]. Suppl., 47, 229. Kalnajs, A. 1983, in IAU Symposium 100, Internal Kinematics and Elmegreen, D. M., and Elmegreen, B. G. 1982, M.N.R.A.S., 201, Dynamics of Galaxies, ed. E. Athanassoula (Dordrecht: Reidel), 1021. p. 87. 1984, Ap./. Suppl, 54, 127. Kent, S. M. 1983, Ap. /., 266, 562. Faber, S. M., 1977, in Evolution of Galaxies and Stellar Populations, 1984, Ap./. Suppl, 56, 105. ed. B. M. Tinsley and R. B. Larson (New Haven: Yale University 1985, Ap./. Suppl, 59, 115. Observatory), p. 157. 1986, A./., 91, 1301. Faber, S. M., and Gallagher, J. S. 1979, Ann. Rev. Asir. Αρ., 17, 135. 1987a, A.]., 93, 816. Faber, S. M., and Lin, D. Ν. C. 1983, Αρ. ]. (Letters), 266, L17. 1987k, A./., 94, 306. Feitzinger, J. V., and Galinski, T. 1986, Asir. Αρ., 167, 215. Kennicutt, R. C., Jr., and Edgar, B. K. 1986, Ap. /., 300, 132. Feitzinger, J. V., Nicolov, Α., Schmidt-Kaler, T., and Tennigkeit, J. King, I. R. 1966, A./., 71, 64. 1983, Asir. Αρ., 126, 352. 1971, Pub. A.S.F., 83, 199. Fort, Β. P. 1985, New Aspects of Galaxy Photometry, ed. J.-L. Nieto, 1978, Ap.]., 222, I. Lecture Notes in Physics, Vol. 232 (Berlin: Springer), p. 3. Kodaira, K., Okamura, S., and Watanabe, M. 1983, Ap. /. (Letters), Fort, B. P., Prieur, J.-L., Carter, D., Meatheringham, S. J., and 274, L49. Vigroux, L. 1986, Ap./., 306, 110. Kodaira, K., Watanabe, M., and Okamura, S. 1986, Αρ. /. Suppl, 62, Freeman, K. C. 1970, Ap.]., 160, 811. 703. Furenlid, I. 1978, in Modern Techniques in Astronomical Photography, Kormendy, J. 1973, A./., 78, 255. ed. R. M. West and J. L. Heudier (Geneva: ESO), p. 153. 1977a, Ap./., 214, 359. Gallagher, J. S. Ill 1986, Pub. A.S.P., 98, 81. 1977b, Ap.]., 217, 406. Gallagher, J. S., and Hudson, H. S. 1976a, Ap. /., 209, 389. 1977c, Ap.]., 218, 333. 1976b, Pub. A.S.P., 88, 824. 1979, Ap.]., 227, 714. Gallagher, J. S., and Hunter, D. A. 1981, A./., 86, 1312. 1980a, in Proc. ESO Workshop on Two-Dimensional Photome- Gallagher, J. S., and Wirth, A. 1980, Ap. /., 241, 567. try, ed. P. Crane and K. Kjär (Garching: ESO), p. 69. Gallagher, J. S., Faber, S. M., and Burstein, D. 1980, Ap./., 235, 743. 1980b, Proc. ESO Workshop on Two-Dimensional Photome- Gehren, T., Fried, J., Wehinger, P. Α., and Wyckoff, S. 1984, Ap. /., try, ed. P. Crane and K. Kjär (Garching: ESO), p. 191 (JK80). 278, 11. 1982, Morphology and Dynamics of Galaxies, ed. L. Martinet Gisler, G. R. 1979, Ap. /., 228, 385. and M. Mayor (Sauverny: Geneva Observatory), p. 75 (JK82). Grosb0l, P. J. 1985, Asir. Αρ. Suppl., 60, 261. 1985a, Ap. /. (Letters), 292, L9. 1986, in Optimization of the Use of CCD Detectors in Astron- 1985b, Ap.]., 295, 73. omy, ed. J.-P. Baluteau and S. D'Odorico (Garching: ESO), p. 93. 1987a, in IAU Symposium 117, Dark Matter in the Universe, Hackwell, J. Α., and Schweizer, F. 1983, Αρ. /., 265, 643. ed. J. Kormendy and G. R. Knapp (Dordrecht: Reidel), p. 139. Hamabe, Μ., and Kormendy, J. 1987, in IAU Symposium 127, Struc- 1987¿?, in IAU Symposium 127, Structure and Dynamics of ture and Dynamics of Elliptical Galaxies, ed. T. de Zeeuw (Dor- Elliptical Galaxies, ed. T. de Zeeuw (Dordrecht: Reidel), p. 17. drecht: Reidel), p. 379. Kormendy, J., and Bahcall, J. N. 1974, A./., 79, 671. Hausman, M. Α., and Ostriker, J. P. 1978, Ap. /., 224, 320. Kormendy, J., and Illingworth, G. 1983, Αρ. /., 265, 632. Hegyi, D. J., and Gerber, G. L. 1977, Ap. /. (Letters), 218, L7. Lachièze-Rey, M., Vigroux, L., and Souviron, J. 1985, Asir. Αρ., 150, Hegyi, D. J., and Olive, K. A. 1986, Ap. /., 303, 56. 62. Hickson, P., Fahlman, G. G., Auman, J. R., Walker, G. A. H., Menon, Lallemand, A. 1962, in Astronomical Techniques, ed. W. A. Hiltner, T. K., and Ninkov, Z. 1982, Ap. /., 258, 53. Stars and Stellar Systems, Vol. 2 (Chicago: University of Chicago Hoag, A. A. 1978, in Modem Techniques in Astronomical Photography, Press), p. 126. ed. R. M. West and J. L. Heudier (Geneva: ESO), p. 121. Lasker, B. M. 1983, in Astronomical Microdensitometry, ed. D. A. Hodge, P. W. 1971, Ann. Rev. Asir. Αρ., 9, 35. Klinglesmith, NASA CP-2317, p. 109. Hoessel, J. G. 1980, Αρ. /., 241, 493. Latham, D. W. 1969, AAS Photo-Bull, 1, 3. Hoessel, J. G., and Schneider, D. P. 1985, A./., 90, 1648. 1974, in Methods of Experimental Physics, Vol. 12, Part A, ed. Hoessel, J. G., Borne, K. D., and Schneider, D. P. 1985, Ap, /., 293, N. Carleton (New York and London: Academic Press), p. 221.

1978, in Modern Techniques in Astronomical Photography, ed. Pedersen, H. 1986, in Optimization of the Use of CCD Detectors in R. M. West and J. L. Heudier (Geneva: ESO), p. 141. Astronomy, ed. J.-P. Baluteau and S. D'Odorico (Garching: ESO), Lauberts, A. 1982, The ESO/Uppsala Survey of the ESO(B) Atlas p. 111. (Garching: ESO). Pence, W. D., and Davoust, E. 1985, Asir. Αρ. Suppl., 60, 517. Lauer, T. R. 1985α, Αρ. J. Suppl, 57, 473. Pence, W. D., and de Vaucouleurs, G. 1985, Ap. /., 298, 560. 1985fr, Αρ. J., 292, 104. Picat, J.-P. 1985, New Aspects of Galaxy Photometry, ed. J.-L. Nieto, 1985c, M.N.R.A.S., 216, 429. Lecture Notes in Physics, Vol. 232, (Springer: Berlin), p. 21. Lawrance, J. L. 1973, in Astronomical Observations with Television- Price, J. S. 1985, Ap./., 297, 652. Type Sensors, ed. J. W. Glaspey and G. Α. H. Walker (Canada: Price, J. S., and Grasdalen, G. L. 1983, Ap. /., 275, 559. Institute of Astronomy and Space Science, UBC), p. 5. Prieto, M., Battaner, Ε., Sanchez, G., and Beckman, J. 1985, Asir. Αρ., Leach, R. 1981, Ap. J., 248, 485. 146, 297. Lehmann, H., and Häupl, W. 1986, AAS Photo-Bull, 43, 5. Reaves, G. 1983, Αρ. /. Suppl., 53, 375. Lilly, S. J., andPrestage, R. M. 1987, M.N.R.A.S., 225, 531. Reynolds, R. H. 1913, M.N.R.A.S., 74, 132. Lilly, S. J., McLean, I. S., and Longair, M. S. 1984, M.N.R.A.S., 209, Romanishin, W., Strom, Κ. M., and Strom, S. E. 1983, Ap. J. Suppl., 401. 53, 105. Longo, G., and de Vaucouleurs, A. 1983, The University of Texas Rubin, V. G., Burstein, D., Ford, W. K., Jr., andThonnard, N. 1985, Monographs in Astronomy, No. 3 (Austin: Department of Astron- Ap. /., 289, 81. omy, University of Texas). Rubin, V. G., Ford, W. K., Jr., and Thonnard, N. 1980, Ap. /., 238, Lugger, P. M. 1984, Ap. /., 286, 106. 471. MacGillivray, H. T., and Stobie, R. S. 1985, Vistas Astr., 27, 433. 1982, Ap./., 261, 439. Mackay, G. D. 1986, Ann. Rev. Astr. Αρ., 24, 255. Sadler, Ε. M., and Gerhard, Ο. Ε. 1985a, M.N.R.A.S., 214, 177. Malin, D. F. 1978, Nature, 276, 591. 1985fr, New Aspects of Galaxy Photometry, ed. J.-L. Nieto, 1981,/. Photographic Sei., 29, 199. Lecture Notes in Physics, Vol. 232, (Springer: Berlin), P. 269. 1982,/. Photographic Sei., 30, 87. Saito, M. 1979, Pub. Astr. Soc. Japan, 31, 181. Malin, D. F., and Garter, D. 1983, Αρ. /., 274, 534. Sandage, Α., and Binggeli, B. 1984, A./., 89, 919. Malumuth, Ε. Μ., and Kirshner, R. P. 1985, Ap. /., 291, 8. Sandage, Α., and Tammann, G. Α. 1981, A Revised Shapley-Ames Malumuth, Ε. M., and Richstone, D. O. 1984, Ap. /., 276, 413. Catalog of Bright Galaxies (Washington: Garnegie Institution of Martinez Roger, G., Phillips, J. P., and Sanchez Magro, G. 1986, Asir. Washington). Αρ., 161,237. Sandage, Α., and Visvanathan, N. 1978, Ap./., 223, 707. McMullan, D. 1980, in Proc. ESO Workshop on Two-Dimensional Sandage, Α., Binggeli, B., and Tammann, G. Α. 1985, Α./., 90, 1759. Photometry, ed. P. Grane and Κ. Kjär (Garching: ESO), p. 69. Sandage, Α. R. 1976, Α./., 81, 954. Meiseis, Α., and Ostriker, J. P. 1984, Α./., 89, 1451. Schild, R., Tresch-Fienberg, R., and Huchra, J. 1985, A./., 90, 441. Meurs, Ε. J. Α. 1986, in Optimization of the Use of CCD Detectors in Schneider, D. P., Gunn, J. E., and Hoessel, J. G. 1983, Ap. /., 268, Astronomy, ed. J.-P. Baluteau and S. D'Odorico (Garching: ESO), 476. p. 105. Schoening, W. E. 1976, AAS Photo-Bull., 11, 8. Michard, R. 1984, Astr. Ap. (Letters), 140, L39. 1978, in Modern Techniques in Astronomical Photography, ed. 1985, Astr. Ap. Suppl., 59, 205. R. M. West and J. L. Heudier (Geneva: ESO), p. 63. Mihalas, D., and Binney, J. 1981, Galactic Astronomy (2d ed.; San Schombert, J. M. 1986, Ap. /. Suppl., 60, 603. Francisco: Freeman and Go.), p. 302. 1987, Ap./. Suppl, 64, 643. Miller, J. S., Robinson, L. B., Wampler, E. J. 1979, Proc. SP1E, 172, Schombert, J. M., and Bothun, G. D. 1987, A./., 92, 60. 62. Schweizer, F. 1976, Αρ./. Suppl., 31, 313. Miller, R. H. 1963, Ap./., 137, 1049. 1979, Αρ./., 233, 23. Miller, R. H., and Prendergast, Κ. H. 1962, Ap.]., 136, 713. 1981, Α./., 86, 662. 1968, Ap./., 153, 35. Schweizer, F., and Ford, W. Κ., Jr. 1985, New Aspects of Galaxy Miller, W. G. 1977, Observatory Photographic Manual (Pasadena.·. Hale Photometry, ed. J.-L. Nieto, Lecture Notes in Physics, Vol. 232 Observatory). (Springer: Berlin), p. 145. Mizuno, T., and Hamajima, K. 1987, Pub. Astr. Soc. Japan, 39, 221. Schweizer, F., Whitmore, B. G., and Rubin, V. G. 1983, Α./., 88, 909. Morbey, G., and Morris, S. 1983, Ap. /., 274, 502. Shostak, G. S., and van der Kruit, P. G. 1984, Asir. Αρ., 132, 20. Nieto, J.-L. 1982, Asir. Αρ. Suppl., 47, 535. Sim, Μ. Ε. 1978, in Modern Techniques in Astronomical Photography, 1983, Asir. Αρ. Suppl., 53, 383. ed. R. M. West and J. L. Heudier (Geneva: ESO), p. 63. 1986, in Data Analysis in Astronomy II, ed. V. di Gesu et al. Simien, F., and de Vaucouleurs, G. 1986, Αρ. /., 302, 564. (New York: Plenum Press), p. 45. Skrutskie, M. F., Shure, Μ. Α., andBeckwith, S. 1985, Αρ./., 299, 303. Nieto, J.-L., and Prugniel, P. 1987a, in/AC/ Symposium 127, Structure Smith, Α. G., and Hoag, A. A. 1979, Ann. Rev. Astr. Αρ., 17, 43. and Dynamics of Elliptical Galaxies, ed. T. de Zeeuw (Dordrecht: Smith, R. M., Efstathiou, G., Ellis, R. S., Frenk, G. S., and Valentijin, Reidel), p. 99. E. A. 1985, M.N.R.A.S., 216, 71P. 19876, Asir. Αρ., 186, 30. Sparks, W. B., Wall, J. V., Thorne, D. J., Jorden, P. R., van Breda, Nieto, J.-L., andTiennot, L. 1984, Asir. Αρ., 131, 291. I. G., Rudd, P. J., and Jorgensen, Η. Ε. 1985, M.N.fí.A.S., 217, 87. Nieto, J.-L., and Vidal, J.-L. 1984α, Asir. Αρ., 135, 190. Strittmatter, P. Α. 1973, in Astronomical Observations with Television- 1984fr, M.N.R.A.S., 209, 21Ρ. Type Sensors, ed. J. W. Glaspey and G. A. H. Walker (Ganada: Nilson, P. 1973, Uppsala General Catalogue of Galaxies, Acta Universi- Institute of Astronomy and Space Science, UBG), p. 1. tatis Upsaliensis, Ser. V:A, Vol. 1. Strom, Κ. M., and Strom, S. E. 1977, in Evolution of Galaxies and Oemler, A. 1976, Ap. /., 209, 693. Stellar Populations, ed. Β. M. Tinsley and R. B. Larson (New Okamura, S. 1977, Annals Tokyo Astr. Obs., 16, 111. Haven: Yale University Observatory), p. 43. 1978, Pub. Astr. Soc. Japan, 30, 91. Strom, Κ. M., and Strom, S. Ε. 1978a, A./., 83, 73. Okamura, S., Kodaira, K., and Watanabe, M. 1984, Ap./., 280, 7. 1978c, A./., 13, 1293. Ostriker, J. P., and Hausman, M. A. 1977, Ap. J. (Letters), 217, L125. Strom, Κ. M., Strom, S. E., Jensen, Ε. B., Moller, J., and Thompson,

L. A. 1977, Ap./., 212, 335. van,der Kruit, P. C. 1987, Asfr. Αρ., 173, 59. Strom, Κ. M., Strom, S. E., Wells, D. C., and Romanishin, W. 1978, van der Kruit, P. C., and Searle, L. 1981, Asir. Αρ., 95, 116. Ap./., 220, 62. 1982, Asir. Αρ., 110, 79. Strom, S. E., and Strom, Κ. M. 1978^, A./., 83, 732. Vigroux, L., and Nieto, J.-L. 1985, New Aspects of Galaxy Photometry, Strom, S. E., Strom, Κ. M., Goad, J. W., Vrba, F. J., and Rice, W. ed. J.-L. Nieto, Lecture Notes in Physics, Vol. 232 (Berlin: Spring- 1976, Ap./., 204, 684. er), p. 67. Sulentic, J. W., and Lorre, J. J. 1983, Asir. Αρ., 120, 36. Wakamatsu, K., and Hamabe, M. 1984, Ap. /. Suppl., 56, 283. Takase, Β., Kodaira, Κ., and Okamura, S., eds. 1984, An Atlas of Walterbos, R. A. M., and Kennicutt, R. C., Jr. 1987, Asir. Αρ. Suppl., Selected Galaxies (Tokyo and Holland; University of Tokyo Press and 69, 311. VNU Science Press). Watanabe, M., 1983, Annals Tokyo Astr. Obs., 19, 121. Talbot, R. J., Jensen, Ε. B., and Dufour, R. J. 1979, Ap. /., 229, 91. Watanabe, M., Kodaira, K., and Okamura, S. 1982, Ap./. Suppl., 50,1. Teuben, P. J., Sanders, R. H., Atherton, P. D., and van Albada, G. D. 1985, Ap./., 292, 72. 1986, M.N.R.A.S., 221, 1. Wevers, B. M. H. R., van der Kruit, P. C., and Allen, R. J. 1986, Asir. Thomsen, B., and Baum, W. A. 1987, Ap. /., 315, 460. Αρ. Suppl., 66, 505. Thomsen, B., and Frandsen, S. 1983, A./., 88, 789. Whitmore, B. C. 1984, Ap. /., 278, 61. Thuan, T. X., and Kormendy, J. 1977, Pub. A.S.P., 89, 466. Whitmore, B. C., McElroy, D., and Schweizer, F. 1987, Αρ. /., 314, Tonry, J. L. 1984, Ap. /., 279, 13. 439. 1985, A./., 90, 2431. Wirth, Α. 1981, Α./., 86, 981. 1987, Ap./., 322, 632. Wirth, Α., and Gallagher, J. S. 1984, Αρ. /., 282, 85. Toomre, A. 1977, Ann. Rev. Asir. Αρ., 15, 437. Woolf, Ν. J. 1982, Ann. Rev. Astr. Αρ., 20, 367. Tsubaki, T., and Engvold, O. 1975, AAS Photo-Bull., 9, 17. Wright, J. P., and Mackay, C. D. 1981, Proc. SPIE, 290, 160. Tyson, J. Α., Seitzer, P., Weymann, R. J., and Foltz, C. 1986, A./., 91, Yoshii, Y., and Arimoto, N. 1987, Asir. Αρ., 188, 13. 1274. Young, P., Gunn, J. E., Kristian, J., Oke, J. B., and Westphal, J. A. van Albada, T. S., Bahcall, J. N., Begeman, K., and Sanscisi, R. 1985, 1980, Ap./., 241, 507. Ap./., 295, 305. Young, P. J., Westphal, J. Α., Kristian, J., Wilson, G. P., and Landauer, van den Bergh, S. 1976, Ap. /., 206, 883. F. P. 1978, Αρ./., 221, 721.