For refcode 2003ApJ...593....1B: Please click here for ADS abstract
NED Abstract
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2003ApJ...593....1B
The Halo Occupation Distribution and the Physics of Galaxy Formation
Andreas A. Berlind, David H. Weinberg, Andrew J. Benson, Carlton M. Baugh,
Shaun Cole, Romeel Dave, Carlos S. Frenk, Adrian Jenkins, Neal Katz, and
Cedric G. Lacey
Abstract. The halo occupation distribution (HOD) describes the bias between
galaxies and dark matter by specifying (1) the probability P(NM) that a
halo of virial mass M contains N galaxies of a particular class and (2) the
relative spatial and velocity distributions of galaxies and dark matter
within halos. We calculate and compare the HODs predicted by a smoothed
particle hydrodynamics (SPH) simulation of a {LAMBDA}CDM cosmological model
(cold dark matter with a cosmological constant) and by a semianalytic
galaxy formation model applied to the same cosmology. Although the two
methods predict different galaxy mass functions, their HOD predictions for
samples of the same space density agree remarkably well. In a sample
defined by a baryonic mass threshold, the mean occupation function <N>_M_
exhibits a sharp cutoff at low halo masses, a slowly rising plateau in
which <N> climbs from 1 to 2 over nearly a decade in halo mass, and a more
steeply rising highoccupancy regime at high halo mass. In the
lowoccupancy regime, the factorial moments <N(N1)> and <N(N1)(N2)> are
well below the values of <N>^2^ and <N>^3^ expected for Poisson statistics,
with important consequences for the smallscale behavior of the two and
threepoint correlation functions. The HOD depends strongly on galaxy age,
with highmass halos populated mainly by old galaxies and lowmass halos by
young galaxies. The distribution of galaxies within SPH halos supports the
assumptions usually made in semianalytic calculations: the most massive
galaxy lies close to the halo center and moves near the halo's mean
velocity, while the remaining, satellite galaxies have the same radial
profile and velocity dispersion as the dark matter. The mean occupation at
fixed halo mass in the SPH simulation is independent of the halo's larger
scale environment, supporting both the merger tree approach of the
semianalytic method and the claim that the HOD provides a complete
statistical characterization of galaxy bias. We discuss the connections
between the predicted HODs and the galaxy formation physics incorporated in
the SPH and semianalytic approaches. These predictions offer useful
guidance to theoretical models of galaxy clustering, and they will be
tested empirically by ongoing analyses of galaxy redshift surveys. By
applying the HODs to a largevolume Nbody simulation, we show that both
methods predict slight departures from a powerlaw galaxy correlation
function, similar to features detected in recent observational analyses.
Keywords: Cosmology: Theory, Galaxies: Formation, Galaxies: Halos,
Cosmology: LargeScale Structure of Universe
Please click here for ADS abstract
