Time evolution of galaxy formation and bias in cosmological simulations

The clustering of galaxies relative to the underlying mass distribution dec lines with cosmic time for three reasons. First, nonlinear peaks become les s rare events as the density field evolves. Second, the densest regions sto p forming new galaxies because their gas becomes too hot to cool and collap se. Third, after galaxies form, they are subject to the same gravitational forces as the dark matter, and thus they tend to trace the dark matter dist ribution more closely with time; in this sense, they are gravitationally "d ebiased." In order to illustrate these effects, we perform a large-scale hy drodynamic cosmological simulation of a A cold dark matter (Lambda CDM) mod el with Omega(0) = 0.37 and examine the statistics of delta*(r, z), the den sity held of recently formed galaxies at position r and redshift z. We find that the bias of recently formed galaxies b* equivalent to [delta*(2)](1/2 )/[delta(2)](1/2), where delta is the mass overdensity, evolves from b* equ ivalent to similar to 4.5 at z = 3 to b* similar to 1 at z = 0, on 8 h(-1) Mpc comoving scales. The correlation coefficient r* equivalent to [delta de lta*]/[delta(2)](1/2)[delta*(2)](1/2) evolves from r* similar to 0.9 at z = 3 to r* similar to 0.3 at z = 0. That is, as gas in the universe heats up and prevents star formation, the star-forming galaxies become poorer tracer s of the mass density field. We show that the linear continuity equation is a good approximation for describing the gravitational debiasing, even on n onlinear scales. The most interesting observational consequence of the simu lations is that the linear regression of the galaxy formation density held on the galaxy density held, b(*g)r(*g) = [delta*delta(g)]/[delta(g)(2)], ev olves from about 0.9 at z = 1 to 0.35 at z = 0. Measuring this evolution, w hich should be possible using the Sloan Digital Sky Survey, would place con straints on models for galaxy formation. In addition, we evaluate the effec ts of the evolution of galaxy formation on estimates of Omega from cluster mass-to-light ratios, finding that while Omega(z) increases with z, the est imate Omega(est)(z) actually decreases. This effect is due to the combinati on of galaxy bias and the relative fading of cluster galaxies with respect to held galaxies. Finally, these effects provide a possible explanation for the Butcher-Oemler effect, the excess of blue galaxies in clusters at reds hift z similar to 0.5.