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Theoretical models of the halo occupation distribution : separating central and satellite galaxies.

Zheng, Z. and Berlind, A.A. and Weinberg, D.H. and Benson, A.J. and Baugh, C.M. and Cole, S and Davé, R. and Frenk, C.S. and Katz, N. and Lacey, C.G. (2005) 'Theoretical models of the halo occupation distribution : separating central and satellite galaxies.', Astrophysical journal., 633 (2). pp. 791-809.


The halo occupation distribution (HOD) describes the relation between galaxies and dark matter at the level of individual dark matter halos. The properties of galaxies residing at the centers of halos differ from those of satellite galaxies because of differences in their formation histories. Using a smoothed particle hydrodynamics (SPH) simulation and a semianalytic (SA) galaxy formation model, we examine the separate contributions of central and satellite galaxies to the HOD, more specifically to the probability P(N|M) that a halo of virial mass M contains N galaxies of a particular class. In agreement with earlier results for dark matter subhalos, we find that the mean occupation function langNrangM for galaxies above a baryonic mass threshold can be approximated by a step function for central galaxies plus a power law for satellites and that the distribution of satellite numbers is close to Poisson at fixed halo mass. Since the number of central galaxies is always zero or one, the width of P(N|M) is narrower than a Poisson distribution at low N and approaches Poisson at high N. For galaxy samples defined by different baryonic mass thresholds, there is a nearly linear relation between the minimum halo mass Mmin required to host a central galaxy and the mass M1 at which an average halo hosts one satellite, with M1 ≈ 14Mmin (SPH) or M1 ≈ 18Mmin (SA). The stellar population age of central galaxies correlates with halo mass, and this correlation explains much of the age dependence of the galaxy HOD. The mean occupation number of young galaxies exhibits a local minimum at M ~ 10Mmin where halos are too massive to host a young central galaxy but not massive enough to host satellites. Using the SA model, we show that the conditional galaxy mass function at fixed halo mass cannot be described by a Schechter function because central galaxies produce a "bump" at high masses. We suggest parameterizations for the HOD and the conditional luminosity function that can be used to model observed galaxy clustering. Many of our predictions are in good agreement with recent results inferred from clustering in the Sloan Digital Sky Survey.

Item Type:Article
Keywords:Cosmology: theory, Galaxies: formation, Galaxies: halos, Large-scale structure of universe.
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Publisher statement:© 2005. The American Astronomical Society. All rights reserved.
Date accepted:No date available
Date deposited:11 February 2015
Date of first online publication:November 2005
Date first made open access:No date available

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