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Through the noisy haze of sky photons, astronomers since the time of Messier have detected and cataloged the positions and shapes of diffuse, resolved objects known as nebulae. The cataloger well knows the limits on sensitivity posed by the observing environment, yet these limits are rarely quantified and passed on to the next generation of astronomers. Indeed, if Messier were alive in today's light polluted world, his catalog would certainly be much more sparse since he could only catalog the nebulae he could see. Given this basic constraint, the natural question to ask is ``Are there diffuse nebulae that cannot be cataloged because they remain masked by the night sky?'' For the case of galaxy detection, this question is quite relevant in the context of the Cosmological Principle, a corollary of which asserts that all observers in the universe should construct similar catalogs of galaxies. If this were not the case, then different observers might have biased views and information about (1) the nature of the general galaxy population in the Universe, (2) the three dimensional distribution of galaxies, and (3) the amount of baryonic matter that is contained in galactic potentials. On the largest scales, we expect the universe to exhibit a homogeneous appearance, but our only signposts for matter are the galaxies whose light we detect with optical telescopes against a noisy background of finite brightness. Given this condition one can easily conceive of observing environments that would make galaxy detection difficult.

For example, suppose that we lived on a planet that was located in the inner regions of an elliptical galaxy. The high stellar density would produce a night sky background that would be relatively bright and therefore not conducive to the discovery of galaxies. Similarly, if the Solar System in its journey around the galaxy were unlucky enough to be located near or in a Giant Molecular Cloud (GMC) at the same time that evolutionary processes produced telescopes on the Earth, then our observational horizon would be severely limited by the local dust associated with the GMC. As it is, we are fortunate enough to be located at a relatively dust free area ~ 2.5 scale lengths from the center of a spiral galaxy. At this distance, the local surface brightness of the projected galactic disk is ~ 24 mag arcsec-2 in the blue. Thus in the direction the galactic poles, the galactic stellar density along the line of site is relatively low which affords us a relatively dark window to peer out of in hopes of discovering diffuse objects. Does this relatively unobscured view guarantee that earth-bound extragalactic astronomers are able to detect a representative sample of galaxies?

The idea that the night sky emission places limits on the kinds of galaxies which can be detected was first commented on by Zwicky (1957). The first quantitative analysis of the potential magnitude of this selection effect was presented by Disney (1976). Disney's efforts were largely motivated by the discovery of Freeman (1970) that spiral galaxies seemed to exhibit a constant central surface brightness (µ0 in mag arcsec-2) in the blue. The formal value found by Freeman was µ0 = 21.65 ± 0.35 for a sample of a few dozen spirals. This constancy of µ0 became known as ``Freeman's Law'' which strictly applies to only disk galaxies although an analogous law for ellipticals also exists (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 compiled, cataloged and investigated.

The most dramatic confirmation that these selection effects are real and significant has been provided by McGaugh et al. (1995) and is reproduced here in Figure 1. Ten years of hunting for galaxies of low surface brightness (LSB) has revealed a surprising result which subverts the conventional wisdom as embodied by Freeman's Law. Figure 1 shows that up to 50% of the general population of galaxies resides in a continuous tail extending towards low µ0. Thus, the space density of LSBs is significant. This conclusion has also been reached by Dalcanton et al. (1997) from a study of 7 LSB galaxies detected in the Palomar 5-m transit scan data. With measured redshifts they assigned a tentative space density of 0.08+0.08-0.03 h3100 Mpc-3 for galaxies with µ0 fainter than 23.5 mab arcsec-2, where h100 = H0 / 100.

Figure 1
Figure 1. The space density of galaxies as a function of central surface brightness. LSB objects appear to the left in this diagram. Raw counts from the indicated surveys have been converted to space density through the use of volumetric corrections discussed here and in more detail in McGaugh et al. 1995. The solid line shows the surface brightness distribution which Freeman's Law suggests. The flat line fit to the data, from McGaugh (1996), has a space density which is 6 orders of magnitude higher than predicted from Freeman's Law.

As emphasized by McGaugh et al. (1995), the most physically reasonable approach in converting raw counts to space densities is to assume that scale length and absolute galaxy magnitude are uncorrelated. As shown explicitly below, this results in smaller volumes being accessible to surveys for LSB galaxies compared to surveys for ``normal'' or high surface brightness (HSB) galaxies. It is HSB galaxies that define the Hubble Sequence from which Freeman (1970) derived his sample. The space distribution of galaxies as a function of µ0 after the volume sampling correction has been applied produces the distribution shown in Figure 1. The dark point defined by the Schombert et al. (1992) survey has a space density which is 105 times higher than the extrapolation of Freeman's Law would predict. Factors of 105 are significant. The space density derived by Dalcanton et al. (1997) is even higher than this, perhaps suggesting that galaxies become smaller at lower surface brightness. The opposite trend is seen in other data (i.e., LSB galaxies tend if anything to be larger; de Jong 1996); this illustrates the enormous uncertainty that remains in our knowledge of the local galaxy population. Nevertheless, the implication of Figure 1 is clear - very diffuse galaxies exist and they exist in large numbers. Their properties are only now being elucidated.

This review deals primarily with the physical properties of these newly discovered galaxies and their connection to galaxy evolution. For distance dependent quantities, we assume H0 = 100 kms-1 Mpc-1, and scale by h100 = (H0 / 100). A companion review (Impey and Bothun 1997) more fully details the selection effects that have previously prevented the discovery of LSBs, and how their actual discovery impacts the proper determination of the galaxy luminosity function and its relation to QSO absorption lines as well as deep galaxy surveys that have revealed an apparent excess of intermediate luminosity galaxies at intermediate redshifts. The overall context of this review is the idea that LSB disk galaxies represent a parallel track of galaxy evolution that is largely decoupled from the processes that have determined the Hubble sequence. While this has significant implications for galaxy formation scenarios and galaxy evolution the existence of LSB disks themselves is not a surprise. The interesting issue is epoch at which they begin to appear and proliferate. Eventually, when astration is complete in disks, the universe will contain nothing but LSB objects.

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