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I present results of a large area CCD survey for ultra-low surface brightness galaxies (ULSBs) that reaches central surface brightnesses of 27 magnitudes per square arcsecond in V. I have analyzed 18 square degrees of transit scan data, and found 36 candidate ULSB's with central surface brightnesses fainter than 23 magnitudes per square arcsecond and with angular scale lengths larger than 5 arcseconds. I show that the data is consistent with these galaxies being local and uniformly distributed, and with there being either a constant number or luminosity density of galaxies in every surface brightness interval between 23 and 27 magnitudes per square arcsecond. The implied number densities are consistently over a hundred times larger than the number density in L>0.1L* galaxies, and the luminosity density is anywhere between 4% and 100% of the luminosity density in cataloged galaxies. The associated mass density is likely to be even larger, due to the likely decrease in star formation efficiency with decreasing surface density. Therefore, ULSBs are likely to be the most numerous type of galaxy, and possibly make a significant contribution to the mass of the universe. Studies of the angular correlation function of ULSBs show them to be dramatically less correlated than bright galaxies, and the cross-correlation between ULSBs and Zwicky galaxies suggests that the typical ULSBs in typical sample have scale lengths of 1 kpc. The population of ULSBs is in many ways consistent with the predicted properties of the low mass end of the galaxy mass distribution. The survey also detects many other compact, low-surface brightness fluctuations which are likely candidate high-redshift clusters.
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