Scenarios of Biased Galaxy Formation

The naive assumption that the galaxies trace the underlying mass distribution makes life easy: it enables us to estimate the total mass density in the universe and to interpret the large-scale structure based on the observed distribution of galaxies. But when assuming that the mass lies where we see light, aren’t we playing the role of the drunk who is searching for his lost keys under the street light because it is easier to look there? I would argue that based on the observed correlation of the type of a galaxy and its environment [1], it would be astonishing if galaxy formation itself was not affected by environmental effects segregating the galaxies from the underlying mass. I will first summarize the motivation behind this suspicion and then, together with Martin Rees in the following talk, discuss bias mechanisms in the various cosmogonies, arguing that a segregation of one sort or another is a natural outcome of almost every cosmogony (although the bias is not always of the desired sort). I will conclude by trying to point at key observational tests.