Publications of the Astronomical Society of the Pacific Vol. 109 1997

Publications of the Astronomical Society of the Pacific Vol. 109 1997 July No. 737 Publications of the Astronomical Society of the Pacific 109: 745-758, 1997 July Invited Review Paper Low-Surface-Brightness Galaxies: Hidden Galaxies Revealed Greg Bothun Department of Physics, University of Oregon, Eugene, Oregon 97403 Electronic mail: [email protected] Chris Impey Steward Observatory, University of Arizona, Tucson, Arizona 85721 Electronic mail: [email protected] Stacy McGaugh Department of Terrestrial Magnetism, Carnegie Institution of Washington, DC 20005 Electronic mail: [email protected] Received 1997 February 12; accepted 1997 April 29 ABSTRACT. In 20 years, low-surface-brightness (LSB) galaxies have evolved from being an idiosyncratic notion to being one of the major baryonic repositories in the Universe. The story of their discovery and the characterization of their properties is told here. Their recovery from the noise of the night-sky background is a strong testament to the severity of surface-brightness selection effects. LSB galaxies have a number of remarkable properties which distinguish them from the more familiar Hubble sequence of spirals. The two most important are (1) they evolve at a significantly slower rate and may well experience star formation outside of the molecular-cloud environment, (2) they are embedded in dark-matter halos which are of lower density and more extended than the halos around high-surface-brightness (HSB) disk galaxies. Compared to HSB disks, LSB disks are strongly dark-matter dominated at all radii and show a systematic increase in M/L with decreasing central surface brightness. In addition, the recognition that large numbers of LSB galaxies actually exist has changed the form of the galaxy luminosity function and has clearly increased the space density of galaxies at ζ = 0. Recent CCD surveys have uncovered a population of red LSB disks that may be related to the excess of faint blue galaxies detected at moderate redshifts. LSB galaxies offer us a new window into galaxy evolution and formation which is every bit as important as those processes which have produced easy-to-detect galaxies. Indeed, the apparent youth of some LSB galaxies suggest that galaxy formation is a greatly extended process. While the discovery of LSB galaxies has led to new insights, it remains unwise to presume that we now have a representative sample which encompasses all galaxy types and forms. 1. INTRODUCTION ' 'Are there diffuse nebulae that cannot be cataloged because Through the noisy haze of sky photons, astronomers they remain masked by the night sky?" For the case of since the time of Messier have detected and cataloged the galaxy detection, this question is quite relevant in the con- positions and shapes of diffuse, resolved objects known as text of the Cosmological Principle, a corollary of which nebulae. The cataloger well knows the limits on sensitivity asserts that all observers in the universe should construct posed by the observing environment, yet these limits are similar catalogs of galaxies. If this were not the case, then rarely quantified and passed on to the next generation of different observers might have biased views and informa- astronomers. Indeed, if Messier were alive in today's light- tion about (1) the nature of the general galaxy population in polluted world, his catalog would certainly be much sparser the Universe, (2) the three-dimensional distribution of gal- since he could only catalog the nebulae he is able to see. axies, and (3) the amount of baryonic matter that is con- Given this basic constraint, the natural question to ask is tained in galactic potentials. On the largest scales, we ex- 745 © 1997. Astronomical Society of the Pacific © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 746 BOTHUN ET AL. (e.g.. Fish 1964). Like any law, it was apparently made to be broken. This review, 20 years after Disney's original analysis, shows that his basic argument has been vindicated. Selection effects have been severe and as a result no representative sample of nearby galaxies has yet been com- piled, cataloged, and investigated. □ Phülipps et al. (1987) Δ Davies (1990) fluí selected The most dramatic confirmation that these selection ef- O Davies (1990) diameter " ■ Schombert et al. (1Θ92) fects are real and significant has been provided by Mc- ▲ Sprayberry (1994) Gaugh et al. (1995) and is reproduced here in Fig. 1. Ten • de Jong (1996) years of hunting for galaxies of low surface brightness 25 24 23 22 (LSB) has revealed a surprising result which subverts the -2 μ0 {Β mag arcsec ) conventional wisdom as embodied by Freeman's Law. Fig- Fig. 1—The space density of galaxies as a function of central surface ure 1 shows that up to 50% of the general population of brightness. LSB objects appear to the left in this diagram. Raw counts galaxies resides in a continuous tail extending towards low from the indicated surveys have been converted to space density through the use of volumetric corrections discussed here and in more detail in μ-ο. Thus, the space density of LSBs is significant. This McGaugh et al. (1995). The solid line shows the surface-brightness distri- conclusion has also been reached by Dalcanton et al. (1997) bution which Freeman's Law suggests. The flat line fit to the data, from from a study of seven LSB galaxies detected in the Palomar McGaugh (1996), has a space density which is 6 orders of magnitude 5-m transit scan data. With measured redshifts they as- higher than predicted from Freeman's Law. signed a tentative space density of O.OSÍ^os^ioo -3 -2 Mpc for galaxies with /¿0 fainter than 23.5 mag arcsec , pect the Universe to exhibit a homogeneous appearance, but where h ^ = í/q/IOO. our only signposts for matter are the galaxies whose light As emphasized by McGaugh et al. (1995), the most we detect with optical telescopes against a noisy back- physically reasonable approach in converting raw counts to ground of finite brightness. Given this condition one can space densities is to assume that scale length and absolute easily conceive of observing environments that would make galaxy magnitude are uncorrelated. As shown explicitly be- galaxy detection difficult. low, this results in smaller volumes being accessible to For example, suppose we lived on a planet that was surveys for LSB galaxies compared to surveys for "nor- located in the inner regions of an elliptical galaxy. The high mal" or high-surface-brightness (HSB) galaxies. It is HSB stellar density would produce a night-sky background that galaxies that define the Hubble Sequence from which Free- would be relatively bright and therefore not conducive to man (1970) derived his sample. The space distribution of the discovery of galaxies. Similarly, if the Solar System in galaxies as a function of μ o after the volume sampling its journey around the galaxy were unlucky enough to be correction has been applied produces the distribution shown located near or in a Giant Molecular Cloud (GMC) at the in Fig. 1. The dark point defined by the Schombert et al. 5 same time that evolutionary processes produced telescopes (1992) survey has a space density which is 10 times higher on the Earth, then our observational horizon would be se- than the extrapolation of Freeman's Law would predict. 5 verely limited by the local dust associated with the GMC. Factors of 10 are significant. The space density derived by As it is, we are fortunate enough to be located at a rela- Dalcanton et al. (1997) is even higher than this, perhaps tively dust free area ~ 2.5 scale lengths from the center of suggesting that galaxies become smaller at lower surface a spiral galaxy. At this distance, the local surface brightness brightness. The opposite trend is seen in other data (i.e., of the projected galactic disk is ~ 24 mag arc sec"2 in the LSB galaxies tend if anything to be larger; de Jong 1996); blue. Thus in the direction of the galactic poles, the galactic this illustrates the enormous uncertainty that remains in our stellar density along the line of site is relatively low which knowledge of the local galaxy population. Nevertheless, the affords us a relatively dark window to peer out of in hopes implication of Fig. 1 is clear—very diffuse galaxies exist of discovering diffuse objects. Does this relatively unob- and they exist in large numbers. Their properties are only scured view guarantee that Earth-bound extragalactic as- now being elucidated. tronomers are able to detect a representative sample of This review deals primarily with the physical properties galaxies? of these newly discovered galaxies and their connection to The idea that the night-sky emission places limits on the galaxy evolution. For distance dependent quantities, we as- -1 -1 kinds of galaxies which can be detected was first com- sume Hq = 100 kms Mpc , and scale by /i100 mented on by Zwicky (1957). The first quantitative analysis = (7/0/100). A companion review (Impey and Bothun of the potential magnitude of this selection effect was pre- 1997) more fully details the selection effects that have pre- sented by Disney (1976). Disney's efforts were largely mo- viously prevented the discovery of LSBs, and how their tivated by the discovery of Freeman (1970) that spiral gal- actual discovery impacts the proper determination of the axies seemed to exhibit a constant central surface brightness galaxy luminosity function and its relation to QSO absorp- (/¿o in mag arcsec-2) in the blue. The formal value found tion fines as well as deep-galaxy surveys that have revealed by Freeman was /¿0 = 21.65±0.35 for a sample of a few an apparent excess of intermediate luminosity galaxies at dozen spirals.

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