THE SPATIAL CORRELATION-PROPERTIES OF GALAXY HALOS IN A COLD DARK MATTER UNIVERSE

Using high-resolution N-body simulations, the two-point spatial correlation function, xi(r), of dark galaxy halos in a cold dark matter universe is determined as a function of halo mass and local environment. A linear bias model is a good description of the formation of halos with overdensity approximately 200, but not for halos with overdensities approximately 2000 and approximately 70. If we define sigma8 to be the mass fluctuation in a sphere of 8 h-1 Mpc, then in a universe in which 1/sigma8 = 2.5, high-mass halos are more biased compared to the mass than low-mass halos, while in a universe in which 1/sigma8 = 1.5 high- and low-mass halos are biased to the same degree. Assuming a constant mass to light ratio and that one luminous galaxy would form in each of our halos, this implies that the degree to which bright galaxies are more strongly clustered than faint galaxies is a function of sigma8. In order to determine sigma8 for the mass in the universe it is therefore essential to compare xi(r) for faint galaxies to xi(r) for bright galaxies. The function xi(r) for low-mass halos shows more evolution with redshift than xi(r) for high-mass halos, but less than linear theory predicts. Thus we expect xi(r) for faint galaxies to show more evolution with redshift than xi(r) for bright galaxies. Compared to a fiducial correlation function computed without regard to environmental effects, xi(r) for halos in high-density regions is flatter than the fiducial correlation function, xi(r) for halos in average-density regions is similar to the fiducial correlation function, and xi(r) for halos in low-density regions has a lower amplitude and is steeper than the fiducial correlation function. The fiducial correlation function of halos of overdensity approximately 2000 is completely dominated by halos in high-density regions and for halos of overdensities approximately 200 and approximately 70 the fiducial correlation function is dominated by halos in high-density regions, but there is a contribution from halos in average-density regions. For both the mass and halos, the function g(r) = 1 + xi(r) is well-fit by two power laws with a characteristic scale, r(b), at which the function changes from one power law to the other. The function g(r) for the mass does not resemble g(r) for the halos, and both evolve differently. The difference in slope from one power law to the other for g(r) for the mass increases as the simulation evolves, while it decreases for the halos. Neither g(r) for the mass nor g(r) for the halos is a good fit to g(r) observed for galaxies. The indices of the two power laws observed for galaxies can be matched by g(r) for halos of overdensity approximately 200 at 1/sigma8 = 2.5, however r(b) for these halos is a factor of 2.5 smaller than r(b) observed for galaxies.