Astronomers typically use galaxies as markers of space in many of the standard cosmological tests. Galaxies are easy to detect. Notably, the upcoming Sloan Digital Sky survey will produce digital information for about 100 million of them. All optical surveys will undercount galaxies that are diffuse or small or unevolved (in the sense of having converted a small fraction of the gas mass into stars). While cold gas can be detected effectively by the 21 cm line of neutral hydrogen, the limited bandwidth and sensitivity of ``blind'' radio surveys mean that they probe relatively small volumes (Schneider, Spitzak, & Rosenberg 1998). Figure 2 shows that galaxies from the APM survey (Impey et al. 1996) have a strong trend of increasing gas richness with lower surface brightness (McGaugh & de Blok 1997). In this diagram, there is strong surface brightness selection - galaxies fainter than 10% of the sky level are under-represented and galaxies fainter than 1% of the sky level do not make it into most optical catalogs. The high surface brightness galaxies have have HI column densities of a few times 1021 atoms cm-2 and the low surface brightness galaxies have HI columns ten times lower. At any particular surface brightness, radio surveys only detect the most gas-rich galaxies. Below a few times 1019 atoms cm-2, disk star formation is inhibited (Kennicutt 1989), and we know very little about the universe at these low column densities.
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| Figure 2. The gas mass fraction of LSB galaxies from the APM survey increases with decreasing surface brightness. There are selection effects against finding objects that are diffuse in either gas or stars. |