The Identification of Lyα Absorbers at Low Redshift

Plots of equivalent width (or neutral hydrogen column density) versus impact parameter (between the galaxy and quasar line of sight) have been used to support a variety of conclusions about the relationship between galaxies and Ly\alpha absorbers. I 'observe' my simulated galaxies again (as in Linder 1998 astro-ph/9810162) except using M_B<-18. It is shown that an anticorrelation between impact parameter and column density can occur largely as a result of selection effects. The nearest galaxy is likely to be observed at an impact parameter aro und a few hundred kpc whether it gives rise to absorption or not, unless a strong absorption line is seen. Thus while it is clear that at least some stronger (>10^{16} cm^{-2}) absorption lines arise in galaxies, making such plots does not allow for a meaningful test of the relationship between galaxies and weaker absorbers. While there is no reason to think that gas around galaxies should be cut off at any particular column density, other tests, such as those in which the absorption cross sections of galaxies are examined for a wide range of galaxy properties, will be needed to establish the relationship between absorbers and galaxies.