Mergers and bias in a cold dark matter cosmology

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Abstract

A dissipative N-body code is used to follow the evolution of a cold dark matter spectrum in an {OMEGA} = 1, h = 0.5 background using cubes 40, 80, and 200 Mpc on a side. Half the 524,288 particles are treated as a gas that can cool and turn into stars. This is accomplished by using a simple gasdynamical scheme that serves to break the clustering hierarchy by reducing the merger cross section of the galaxies. The clustering bias is found to be relatively small at the time identified as the current epoch; the extreme range of values is from 1.0 to 1.9, dependent on galaxy luminosity, with a preferred measured bias value of 1.5. The clustering of the galaxies is only weakly dependent on minor variations in the gas physics and resolution effects. On scales much longer than the correlation length the galaxies have streaming velocities essentially equal to those in the dark matter, in agreement with linear theory. The pairwise relative velocities of galaxies are typically a factor of 2 smaller than the dark particle velocities, so that virial estimates of the total mass density are {OMEGA} ~ 0.2. The number density of galaxies as a function of redshift is well fitted by the statistics of peaks, using a sharp threshold having v_t_ = 1.44(1 + z)/δ_0_(M), where δ_0_(M) is the linear extrapolation of the initial rms density fluctuation on mass scale M. The dark halo number density-redshift relation is fitted with the same v_t_ using the Press-Schechter model. The distinctions between the galaxies and the dark halos are largely a result of differing merger rates. The pairwise velocities of dark halos suggest that halos simply fall together and merge without orbiting, whereas galaxies survive 2-4 times longer statistically. Part of the reason for the low bias is that the stars form near a redshift of 5, whereas bright galaxies form in abundance in mergers much later, around a redshift of 1.5.

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