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524-544, May 1988 SURFACE PHOTOMETRY of GALAXIES Publications of the Astronomical Society of the Pacific 100: 524-544, May 1988 SURFACE PHOTOMETRY OF GALAXIES* 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- 524 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System SURFACE PHOTOMETRY OF GALAXIES 525 planetary dust. 3. Unresolved faint stars and galaxies. These three components have roughly equal contribu- tions 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 back- ground 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.
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