Galaxies - Basic Catalogs and Data Collections

From my (biased?) observer's point of view, data on galaxies is where we must start in understanding them. With electronic collections, getting the numbers is easier than ever before. However, it still helps to know where the numbers come from, and what the initial selection of various catalogs was. In particular, just how a catalog sample was compiled may have a great influence on what one can learn from it. Surface-brightness and flux selection criteria are especially important in this regard. Furthermore, there is a big difference between an object list and a catalog including uniform physical data for each entry.

To catalog something, we must see that (1) an object is there and (2) it is not something else. For galaxies, we must detect them at some wavelength and distinguish them from stars, planetary nebulae, galactic cirrus, and navigation satellites. Most surveys to this point have been done in the optical, until recently from examination of photographic plates. This implies certain detection limits: (1) Anything less than about 1 arcsecond in size looks like a star on normal ground-based images, and can be distinguished only by nonstellar colors or spectrum (QSOs, compact galaxies; this worked fine for the SDSS, for example). HST imagery shows that there are lots of faint galaxies at high redshift which could masquerade as faint stars from the ground (in practice this enters in significant numbers only for magnitudes fainter than V=20, although such rarities as "peas" may be somewhat brighter). (2) Any object with surface brightness too low (much less than about 1% of the night-sky brightness for routine kinds of data) would only be found when near enough for individual stars to be seen (this is how the Sculptor dwarf galaxy was discovered). Surface brightness (received flux per unit solid angle) is often measured in the definitely non-SI units of magnitude per square arcsecond for optical and near-IR passbands; one also encounters MJy/steradian, S(10) (the light of a V=10 star per square degree), and very occasionally the SI unit of a nit (candela per square meter). Major components in the optical are airglow (much in isolated atomic and molecular emission lines) and scattered starlight from Solar System dust (zodiaal light) and interstellar dust. From a dark site, the dust/airglow contributions are about equal in the V band. In the near-IR, airglow molecular emission (particularly OH bands, the "OH forest") become extremely bright; gains of many thousands in sensitivity are possible by going to space. Details on typical sky brightnesses may be found in Leinert et al. 1998 (A&ASuppl 127, 1), especially their Table 4. Representative dark-sky values in magnitudes per square arcsecond are 22.5-23.1 in the B band and 21.5-22.0 in V. About a factor of 2 variation occurs with the solar cycle, with airglow faintest at sunspot minimum. Arp took advantage of this for many of his peculiar-galaxy photographs.

These limits translate to biases on the kinds of galaxies found by a particular technique; see the discussion in Mihalas and Binney (p. 371) on the detection problem and The Light of the Night Sky by Roach and Gordon (Reidel, 1973) for discussion of sources of confusing light (airglow, scattered sunlight and starlight, and unresolved galaxy light). Disney (1976, Nature 263, 573) described a visibility function for galaxies incorporating these quantities. Arp noted this problem in the Atlas of Peculiar Galaxies, and the issue is discussed at some length by Mihalas and Binney (their fig. 5-47, p. 372). The plot below was made from the 1036 galaxies in the RC2 with known integrated B magnitude, redshift, and characteristic diameter D25, using the same reference lines for surface-brightness limits as were adopted by Mihalas and Binney.

The upper dashed line indicates where confusion with foreground Galactic stars sets in (depending on the image quality; HST compact galaxies occur with effective diameter of order 2 kpc and MB up to -21 or so). The lower limit is roughly where most of a galaxy falls below the surface-brightness threshold set by the night-sky brightness (usually a few percent of the sky intensity). Objects have now been found outside these limits. Compact objects cannot escape detection in complete spectroscopic studies of faint samples, while low-surface-brightness (LSB) objects require very deep imaging studies. Numerous LSB dwarfs are known, and the census is not regarded as complete even within the Local Group. Photographic amplification techniques have led to the discovery of LSB giant galaxies (Bothun et al. 1987 AJ 94,23; Impey and Bothun 1989 ApJ 341, 89; Bothun et al. 1990 ApJ 360, 427). These systems are very luminous but so large as to have escaped previous detection. It is quite fair to question how far our understanding of galaxy formation and evolution is influenced by selection effects in what we consider typical galaxy properties to be. Zwicky used his "morphological approach" to ask what kinds of galaxies might exist, on the premise that all physical objects not prohibited from existing must be out there, and positing not only dwarf and compact galaxies, but pygmy and gnome objects as well. Others have referred to this procedure as dividing parameter space into Zwicky boxes and ruling out the ones that violate physical law.

The giant LSB galaxies are well represented by Malin 1, an object hiding behind the Virgo Cluster. These images are from a single B-band image provided by Greg Bothun, first with a typical intensity mapping and then with a high-contrast stretch about the sky level. The faint galaxy disk almost fills the frame, but at more usual thresholds it appears to be a much smaller and fainter system.

At the other end of the scale, QSOs furnish a test of our ability to find unresolved extragalactic objects (without necessarily suggesting that they are the only kinds of compact extragalactic objects). The first discoveries were as identifications of radio sources (3C 48, 3C 273), with Zwicky compact galaxies not far behind. The most productive optical search techniques for these are multicolor surveys (looking in some n-dimensional color-index space for things that fall off the sequence defined by galactic stars) or slitless spectroscopy, in which emission lines or peculiar continuum shapes may be identified wholesale. X-ray source identifications may be the most fruitful of all, especially since large-area deep surveys are in hand. (For examples of multiwavelength ID, see the extensive compendium of quasar candidates put together by a New Zealand amateur!)

Space observations (or those with adaptive optics) help most for angularly small objects (HST resolves numerous compact galaxies that look stellar from the ground). The background at V is not much darker from Earth orbit than from a dark ground site, since half the sky light there comes from sunlight or other starlight scattered by zodiacal or interstellar grains. The situation improves dramatically either into the UV or the far red.

Searches for some subclasses of galaxies can be more efficient, approaching the flux-limited ideal. This includes, for example, infrared- or radio-bright galaxies. In these instances, confusion with galactic objects (i.e. stars) is minimized, so that high spatial resolution is not required to recognize galaxies and they may be identified with rather coarse beams, provided the galaxies have high enough integrated fluxes at the relevant frequency. Large beam sizes reduce the risk of losing objects due to limited surface-brightness sensitivity.

A basic question arises at this point - just what is a galaxy? For theorists of galaxy formation, it is a collapsing high-density region of gas. For observers, we might require that the gas at some time formed some stars - but then what about the few intergalactic H I clouds? When do they cease being protogalaxies and become gas-rich dwarfs? Does a lump of material torn out of a spiral during a tidal encounter deserve the name? Is it then newly formed? Kinematics suggest that a galaxy is collected in a dark-matter potential well, while even the biggest star clusters are not - that is, globular clusters have no dark-matter problem, while essentially all galaxies do (even if they're no brighter than an individual globular cluster).

Thought experiment: In our night sky (with skyglow and looking no fainter than, say, 19th visual magnitude) stars are much more prominent than galaxies. Why is this, and what would we have to change to reverse the situation?

Optical surveys that are complete within any sort of useful limits were long based on Schmidt sky-survey plates - from the Palomar, ESO, or UK Schmidts. Such surveys include the UGC, MCG, and ESO/Uppsala catalogs (see reference list) - but most of the 107 or so galaxies on these plates are too faint to be of more than statistical interest. These surveys are based on visual inspection, so that optical catalogs now lag behind radio and IR surveys. There has long been a crying need for a digital optical sky survey, filled to an important extent by the Sloan Digital Sky Survey (SDSS). Optical surveys are sensitive to the intermediate-temperature stellar component exemplified by older main-sequence stars (like the Sun, which is not exactly a coincidence) and K-type red giants. The old populations, and dusty ones, are well sampled by near-IR surveys, of which 2MASS has now surveyed the whole sky with modest surface-brightness sensitivity. The optical and near-IR bands are special for galaxies in that all other bands are dominated by the effects of recent star formation or nuclear activity, so much of galaxy history is written at these intermediate waveengths.

I describe first the specific catalogs containing most well-studied galaxies and active nuclei, then point to useful (mostly electronic, by now) compendia that every astronomer working on galaxies should have in their bookmark list.

Basic Object Catalogs


  • NGC = New General Catalog. Includes star clusters, galaxies, gaseous nebulae, and hallucinations, mostly from the visual surveys by the Herschels. Produced by J. Dreyer in the 1880s. The modern version is the Revised New General Catalog of Nonstellar Astronomical Objects, Sulentic and Tifft, Univ. Arizona 1973. The entries are ordered by right ascension at epoch 1855, which makes things somewhat confusing in current coordinates (especially when there are large declination jumps between objects). ( Electronic version at the CDS)

  • IC = Index Catalog, supplement to the above, with another 7000+ entries. The NGC 2000 compilation includes both NGC and IC lists.

  • 3C = Third Cambridge Catalog of radio sources selected at 178 MHz. Optical identifications of extragalactic sources are given by Spinrad, Djorgovski, and Aguilar 1985 PASP 97, 932. ( CDS electronic version)

    Bright Galaxies:

  • Reference Catalog of Bright Galaxies, G. and A. de Vaucouleurs, U. Texas, Austin 1964. Includes literature citations and descriptions of individual structures. Although superseded by the RC2 and RC3, online data are still available.
  • Second Reference Catalog of Bright Galaxies (RC2), de Vaucouleurs, de Vaucouleurs, and Corwin, U. Texas, Austin 1976. Much updated version, with redshifts, magnitudes, and classifications. Long the basic reference. CDS digital version
  • Third Reference Catalog of Bright Galaxies (RC3), de Vaucouleurs et al., Springer 1991. Beyond belief, best approached on disk. Three volumes on paper. Comparing the three RC versions gives some sense of how much data collection has accelerated in the last three decades. CDS files of the revised edition. The mix of different kinds of designations, plus the disorder among NGC and IC systems produced by precession since their original epochs, makes finding a given object by name interesting. On Unix systems, grep is your friend. And most of the time, NED is an even better friend.
  • Revised Shapley-Ames Catalog of Bright Galaxies (RSA), A. Sandage and G. Tammann, Carnegie Inst. of Washington 1981. Complete data for 1300 galaxies brighter than B=13.2, with a selection of photographs illustrating Sandage's version of the Hubble system. CDS files
  • Uppsala General Catalog of Galaxies (UGC), P. Nilsson, 1973, Uppsala Obs. Publ. 12942 galaxies larger than 1 arcminute in diameter from a full examination of the Palomar Sky Survey. Covers the sky north of -2.5°. Digital version
  • ESO/Uppsala Survey of the ESO(B) Atlas, Lauberts 1982 , ESO Garching. Galaxies, nebulae, and star clusters south of -17°, from examination of the ESO Quick Blue Survey. Classifications compatible with the UGC. Lauberts and Valentijn (ESO, 1989) did densitometry of the plates for these galaxies, producing an extensive photometric catalog including shape and orientation parameters. Digital version
  • Morphological Catalog of Galaxies (Morfologicheskii Katalog Galaktik, lacking a good Cyrillic font), Vorontsov-Velyaminov and coworkers, 4 vol., Shternberg State Astronomical Institute, Moscow. Objects north of -33° from PSS. Uses VV's own system of classification, sometimes wonderfully descriptive but not easy to mesh with standard Hubble types. Strictly keyed to PSS fields - no running numbers, so a typical name is MCG (leading minus signs are significant; for example, MCG +5-03-013 and MCG -5-03-013 are both Seyfert galaxies, a wonderful chance for total confusion). The VV designations have never been very popular with Western astronomers, in part because the morphological designations could hardly have been designed to be less friendly for ASCII coding and searching, and because the lack of a single running designation similarly made them unnecessarily difficult for early digital work. A fifth volume of the MCG covers southern galaxies to declination -58° from the PSS red-light extension, changing the magnitude scale. The online version does not include the original set of nontypographical symbols used for some of the classifications, and splits Volume 5 as separate files.
  • Catalog of Galaxies and Clusters of Galaxies, Zwicky and coworkers. Yet another examination of the PSS, with emphasis on groups and clusters. These are the blue books, in six volumes. The CDS version includes data on individual galaxies.
  • Catalog of Selected Compact Galaxies and of Post-Eruptive Galaxies, Zwicky 1971, Speich, Zürich. The red book; a real zoo of normal galaxies, interacting systems, Seyferts, compact dwarfs,... The introduction is amusing, and might be considered libellous today. Watch especially for "scatter-brained", "appalling naivety of the theoreticians", "high pope of American Astronomy", and "autistic interpretations". This is finally available in electronic form.

    Pictures of galaxies:
  • Hubble Atlas of Galaxies, Sandage 1961, Carnegie Inst. of Washington. Basic reference for the Hubble classification scheme.
  • Atlas of Peculiar Galaxies, Arp 1966, Caltech, Pasadena (also in reduced size in ApJSuppl 14,1). Picked for bizarre appearance - interacting systems, plumes, dust lanes, twisted arms. Very deep Palomar photographs. Scanned images of high quality are available from NED. The photographic plates for the Atlas and the care of their preparation for publication represented a high point in the photography of galaxies.
  • Atlas and Catalog of Interacting Galaxies (Atlas i katalog vzaimodeistvuyuschikh galaktik), Vorontsov-Velyaminov, Shternberg Institute (1959, part 1) and A& A Suppl 28,1 (1977, part 2). 355 and 500 systems, respectively, identified by sequential VV numbers. More extensive than the Arp atlas, illustrated from the PSS, includes many probably non-interacting systems such as clumpy irregular galaxies. Very idiosyncratic arrangement and interpretation. Once again, NED level 5 provides a scanned version
  • A Catalog of Selected Southern Peculiar Galaxies and Associations, Arp and Madore, Cambridge 1987 (2 vol.). From a complete search of the SERC(J) plates, with accurate positions. Vol. 2 contains nice photos from the Schmidt plates of a large, representative sample of these. This one also exists in digital and scanned form at NED.
  • Atlas of Galaxies Useful for Measuring the Cosmological Distance Scale, Sandage and Bedke, NASA SP-496 (1988). The book big enough to be your coffee table. Mostly designed to show partial resolution into stars for HST planning, but has large-scale images of lots of nearby galaxies and allows an excellent impression of Sandage's classification scheme.
  • High-quality photographs of many Virgo galaxies appeared in Sandage and Bedke 1985 AJ 90,1992; 90,2001; and 90,2006. Dwarf members were illustrated in Sandage and Binggeli 1984 AJ 89, 919, and spiral luminosity classes were covered by Sandage, Binggeli, and Tammann 1985 AJ 90, 395.
  • Atlas de Galaxias Australes, Sersic 1967, Obs. Astronomico, Cordoba, Argentina. Before the southern Schmidt surveys, these were the only usable photographs of many southern galaxies.
  • Galaxies, Ferris 1982, Sierra Club Books. Gorgeous photos, enough to make people understand why galaxies are so fascinating.
  • Carnegie Atlas of Galaxies, A. Sandage and J. Bedke (1994, Carnegie Inst. of Washington). Two immense volumes, virtually all galaxies in the Revised Shapley-Ames catalog from large-scale photographs taken at Palomar, Mt. Wilson, or Las Campanas. The biggest printed collection (1168 galaxies) available to refine your classification talents.
  • de Vaucouleurs Atlas of Galaxies, collected by Buta, Corwin, and Odewahn, Cambridge 2007.

    See the bright-galaxy compendia, plus these specific collections:

  • Huchra, Davis, Latham and Tonry 1983, ApJSuppl 52, 89. First part of the CfA survey, 2401 Zwicky and UGC galaxies brighter than MZw=14.5.
  • Gisler and Friel, Index of Galaxy Spectra, 1979 Pachart, Tucson. 2004 galaxies, surveys previously published data.
  • Palumbo, Tanzella-Nitti, and Vettolani, Catalog of Radial Velocities of Galaxies, 1983, Gordon and Breach. 8250 galaxies from the literature.

    Many sources from the NSSDC (and NED, if you can tell which consensus value is listed) are useful for current redshift-survey results, which have passed the 3-million-galaxy mark, rising so fast that electronic retrieval is the only way to keep up. The Sloan Digital Sky Survey (SDSS) alone yielded 106 galaxy redshifts in the northern galactic cap in its first incarnation, and the AAT 2dF survey delivered more than 220,000 in two southern fields.

    Active Galactic Nuclei:
  • Handbook of Quasistellar and BL Lacertae Objects, E.R. Craine 1977, Pachart, Tucson. Finding charts, fluxes, positions, references; much less necessary now than it was before the Web.
  • A Revised and Updated Catalog of Quasi-Stellar Objects, Hewitt and Burbidge 1993, APJSuppl 87, 451. Basic data and citations for 7225 QSOs and 90 BL Lac objects.
  • Optical Catalog of Radio Galaxies, Burbidge and Crowne 1979, APJSuppl 40, 583. Similar data for radio galaxies.
  • Catalogue of Quasars and Active Nuclei, Veron-Cetty and Veron, 12th edition, Astron. Astrophys. (in press?), 2006. Also (and most usefully) available electronically . Basic positional, flux, and type information for 85221 QSOs, 21737 active nuclei, and 1122 BL Lac objects. Some of the low-level AGN classifications at least in the early editions were fantasy - if it was ever called so by anyone, it will appear here. Did you know that the Andromeda Galaxy is a Seyfert 2? QSOs are now being identified at a huge rate, especially from followup of the FIRST radio survey and ROSAT all-sky survey in soft X-rays, plus new optical multicolor work such as the Hamburg effort. Earlier catalogs do not appear to have been as complete as we thought.
  • Catalogue of Seyfert Galaxies and Related Objects, Kaneko 1986, Hokkaido Univ., Sapporo. 560 objects; similarly loose definition of "related".
  • A Catalogue of Seyfert Galaxies, Lipovetskii, Neizvestnii, and Neizvestnaya, Publ. Special Astrophys. Obs. (USSR) vol. 55, 1987. Includes 959 objects, with emission-line, IR, and radio fluxes, plus morphological data on host galaxies.
  • An Optical Catalogue of Extragalactic Emission-Line Objects Similar to Quasi-Stellar Objects, Hewitt and Burbidge ApJSuppl 75, 297 (1991). Contorted way of saying Seyfert galaxies. 935 entries.
  • Handbook of Radio Sources, part 1, Pacholczyk 1978, Pachart, Tucson. Selected extragalactic radio sources from 0-12 hours, with maps, fluxes, and references. This was pre-VLA; still waiting for part 2.
  • Palomar-Green Bright Quasar Survey: Schmidt and Green 1983, ApJ 269, 352 and Green, Schmidt, and Liebert 1986 ApJSuppl 61, 305. A complete search for UV-excess objects brighter than B=16 in the northern sky, with complete confirming spectroscopy. These objects have very extensive further work (UV, X-ray, radio, variability, line profiles) because it is the largest sample of bright AGN.

    Markarian UV-excess galaxies are important, since they contain a large fraction of Seyfert 1 objects and actively star-forming systems. There were 15 published lists from the original Byurakan objective-prism surveys, all collected by Markarian, Lipovetskii, Stepanian, Erastova, and Shapovalova, Publ. Special Astrophys. Obs. (USSR), vol. 62, 1989. I have a local copy of their data table taken from a diskette provided by Lipovetskii. Cross-correlation with additional catalogs (notably the IRAS survey) was done by Mazzarella and Balzano 1986, ApJSuppl 62, 751.

    Additional surveys for UV-excess and emission-line galaxies have been done by several groups:

  • Kazarian 1979-1982, in Astrofizika 15,5; 15, 193; 16,17 and 18, 512. High-surface-brightness galaxies based on catalog photometry and sizes.
  • Arakelian high-surface-brightness galaxies, Soobsch. Byurakan Obs. 47,3 (1975). High-surface-brightness galaxies based on catalog photometry and sizes, includes some prominent Seyferts.
  • Kiso UV-excess lists: Noguchi, Maehara, and Kondo, Ann. Tokyo Astrophys. Obs. ser II 18, 53; Kondo, Noguchi, and Maehara, idem. 20, 130; Takase, Noguchi, and Maehara 1982, idem. 19, 440.
  • Michigan-Tololo emission-line survey: ApJSuppl 34, 95 (1977); 35, 197(1977); 35, 203 (1977); 36, 587 (1978); 45, 113 (1981).
  • KPNO International Spectroscopic Survey (KISS): Salzer et al. 2000, AJ 120, 80 and later lists.
  • Madrid emission-line survey: Zamorano et al. 1994 ApJS 95, 387; 1996 ApJS 105, 343; Alonso et al. 1999 ApJS 122, 415

    Radio and X-ray surveys: these are by now so extensive that they are most sensibly approached digitally, from (for example) the FIRST, NVSS, and HEASARC WWW sites. The near-IR 2MASS survey covered the entire sky at a level detecting tens of thousands of galaxies from 1.2-2.2 microns, though the surface-brightness level is not great for galaxy structures. Looking at specific wavelength ranges:

  • Radio continuum surveys from about 6-90 cm are in excellent shape, from the 3C and Parkes surveys plus a number of small fields surveyed very deeply. Two different surveys at 20 cm (the NVSS and FIRST projects) have been carried out with the VLA, covering most of the accessible sky to flux limits typically a few mJy for point sources. A radio galaxy such as M87 could be found anywhere within z=10 by current techniques. These wavelengths are domanated by nonthermal processes such as synchrotron radiation.
  • The far-infrared survey by the Infrared Astronomical Satellite (IRAS) produced measurements from 12-100 m for over 250,000 galaxies, seen via thermal dust emission. This is a treasure trove for seekers both of the systematic and the exotic. At longer wavelengths, the ESA Infrared Space Observatory (ISO) mission provided dramatically improved sensitivity and somewhat better spatial resolution for individual targets, in turn extended dramatically by the detectors on the Spitzer Space Telescope. The near-infrared bands are sensitive to the cooler giant populations, and at longer wavelengths we see mainly interstellar dust heated by the assorted stellar components; at large redshifts all these contributions naturally slide to longer wavelengths. The IRAS survey has been repeated with much better resolution and sensitivity by the Akari mission. The WISE survey recently filled the gap between the ground-based 2MASS and far-IR IRAS and Akari data.
  • Note that the millimeter and submillimeter observations sample the long-wavelength tail of the thermal dust emission normally seen in the far-infrared.
  • Radio line work is more difficult because of extra requirements of angular resolution and the additional dimension of frequency space - that is, such observations must generally be targeted at some known galaxy, from optical or IR surveys. Hundreds (CO) or thousands (H I) of galaxies have been observed well enough to measure a total line flux and velocity profile. A few large-solid-angle surveys in H I have been conducted, looking for optically obscured galaxies behind the Milky Way. The problem here is that existing receivers can take in only a fairly narrow redshift range in a single observation. Submillimeter receivers are now being deployed which can take in a wide redshift range at once using CO lines.
  • X-ray surveys took a huge leap with ROSAT. These energies sample AGN, hot thermalized gas, and the X-ray binary population tying to the recent star-forming history.
  • UV surveys remained primitive until the start of the GALEX mission. A few galaxies were been photographed from sounding rockets, the Large Magellanic Cloud was imaged from the lunar surface by Apollo 16, and a few dozen galaxies were observed by the Astro missions. O'Connell has summarized a number of advantages to UV surveys, largely accruing from the very low scattered-light background around 2500 Angstroms and the sensitivity of UV data to (unreddened) hot stellar populations. HST parallel observations are slowly providing deep, high-resolution survey data over small areas of the sky.

    Electronic archives and retrieval

    The biggest news in data collections throughout astronomy has been, of course, network resources. For galaxies, the NASA Extragalactic Database (NED) maintained at IPAC has ways of searching for basic data and literature references by position or catalog designation (and even for objects close to a known one). Somewhat different searches (for example, by availability of 2D or 3D kinematic information) are available using HyperCat at (a descendant of LEDA, the Lyon Extragalactic Database). To deal with whole catalogs at once, you can retrieve them from the CDS in Strasbourg. The HEASARC "browse" interface includes powerful catalog search-and-plot routines.

    The primary literature interface has become the NASA Astrophysics Data System (ADS) archive search tool. There are emerging and powerful cross-links that let you enter the data unierse via an object name, location, literature mention, and emerge in one of the other kinds of interface. Mastering these techniques has become a basic professional skill, as we move toward the era a global Virtual Observatory (VO).

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