We conclude this review by again referring to
Figure 1. These results on
the space density of galaxies as a function of central surface brightness
require a basic adjustment in the way we think about galaxies. Much of
the current thought is implicitly one dimensional, with one parameter
(like luminosity or morphology) dominating the way problems are approached.
This is no longer sufficient. Surface brightness selection effects have
been severe. Our results now indicate that up to 50% of the general
galaxy population is in the form of galaxies with central surface
brightness below 22.0 mag arcsec-2. Moreover, the space
density remains flat out to the limits of the data, and the space
density of the lowest surface brightness disks (µ
0 
25.0 mag
arcsec-2, the limits of current data) is vastly higher than
would have been anticipated based on Freeman's Law.
LSB galaxies offer a new window onto galaxy evolution. Because of this, the quest to find LSB galaxies continues. Over the next few years we hope to extend our sensitivity by using wide field CCD surveys of the sky at dark sites. These and new surveys by other groups should extend the current data by two magnitudes down to µ 0 = 27.0 mag arcsec-2. The major goals of these new surveys are to determine if the space density of galaxies as a function of µ 0 continues to remain flat over a factor of 100 in µ 0, and to detect what we so far have failed to detect in large numbers - the red LSB population that must result from the faded remnants of galaxies that no longer can form stars. In this regard, the Sloan Digital Sky Survey will be extremely helpful if its automatic image recognition system can reliably detect LSB galaxies. At levels well below the sky brightness, however, LSB galaxies are often defined by disconnected regions of pixel intensity. While the human eye is rather good at finding these, its not clear if that particular algorithm can be reproduced in a machine.
Despite the power of CCD surveys and automated detection algorithms, we are inevitably brought back to the starting point of this review. Messier cataloged galaxies long before their extragalactic nature was understood, and it is ironic that his catalog was a reject list of stationary objects for comet-hunters to avoid. We now know a lot more about about galaxies, but the selection biases that have operated for 200 years have not been fully overcome. It would be unwise to presume that we have yet revealed the true population of galaxies.
We leave the reader with a small list of things that we hoped they have learned from this review article:
(1) LSB galaxies exist. Lurking beneath the brightness of the night sky are real galaxies with evolutionary histories substantially different from the processes that produced the Hubble Sequence of spirals. In particular, LSBs evolve at a significantly slower rate and may well experience star formation outside of the molecular cloud environment.
(2) Surface brightness selection effects have been severe. A proper accounting of them has increased the local number density of galaxies and steepened the faint end slope of the galaxy luminosity function. Despite this progress, these selection effects still exist and thus we do not yet have a representative, volume limited sample of nearby galaxies.
(3) LSB galaxies span the entire galactic mass range. They are not
exclusively low mass galaxies but include the most massive disk
galaxies discovered to date (e.g., Malin 1). LSB disks likely are the
manifestation of 1-2
isolated peaks in the initial density
fluctuation spectrum. These lower density peaks have longer collapse
times and trace the mass distribution in a relatively unbiased way.
(4) LSB galaxies are embedded in dark matter halos which are of lower density and more extended than HSB galaxy halos. In this sense, disk galaxy surface mass density and subsequent evolution may be predetermined by the form of the dark matter halo. Surface mass density appears to be the single biggest driver of disk galaxy evolution.
ACKNOWLEDGMENTS
A number of people have helped to support this project over the last decade. We gratefully acknowledge Mike Disney for making us think, David Malin for his wizardry and patience with us, Jim Schombert for being there, Steve Strom for pointing the way, Jay Gallagher for telling one of us (G.D.B.) to work on something ``hard'', Mark Cornell for assistance with the Texas observations, and Allan Sandage for originally showcasing the smudge galaxies. We gratefully acknowledge support from the NSF under grant AST-9005115 and AST-9003158 without which, this project could never have been sustained.