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M. Blanton (Princeton University)
The Jeans criterion for the gravitational collapse of gas implies that the efficiency of galaxy formation depends on local temperature as well as local mass density. If this is the case, we show that the relationship between mass density and galaxy formation is a strong function of time and scale. The relationship depends on time because as the universe evolves, the densest regions become too hot to form galaxies. The scale-dependence arises because gas temperature is correlated with the large-scale density field. That is, on large scales, dense regions are hotter and the efficiency of galaxy formation is depressed; thus, the bias is reduced on large scales, even if it remains strong on small scales. We present hydrodynamic simulations illustrating these results. The segregation of spirals and ellipticals at z=0 is a natural consequence of this model; it makes the further prediction that this segregation should occur based on the local temperature rather than the local density. In addition, it explains the Butcher-Oemler effect (that is, there should be more young galaxies in clusters at high redshift, when temperatures there were cooler). In general, one should find that young galaxies and old galaxies are more correlated in the past than they are today. These predictions will be testable by examining future redshift surveys such as the Sloan Digital Sky Survey.
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