Interpreting the Clustering of Distant Red Galaxies Page: 2 of 10
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FIG. 1. Panel (a): Halo occupation functions produced by the abundance matching method described in the text. Gray symbols represent all halos
(parent+sub) while white symbols represent only the subhalos. The two simulations have two different cosmologies, so they have been shifted to a common mass
scale to demonstrate self-similarity. The solid and dotted curves represent the HOD used in all analytic calculations henceforth. Panel (b): Comparison between
the clustering in the L120 box and the analytic model. The HOD for the analytic model is obtained from fitting the simulation results (e.g., panel a, but shifted to
the proper mass scale). The solid line is the full correlation function, and the dotted line the one-halo term.2. METHODS
2.1. Definition of the Term "Halo"
Because we will be dealing with halos that exist in various
environments, it is important to have a clear definition of a
halo and discuss what it implies. We assume that all galaxies
live at the center of a virialized clump of dark matter. That
dark matter clump may be isolated or it may exist within the
virial radius of a larger structure. Therefore we will use the
term halo to refer to an object that is distinct; i.e., it does not
exist within the virial radius of another object. These objects
typically have a mean overdensity of ~200 times that of the
background universe. We refer to objects inside the virial ra-
dius of halos as subhalos. We use the term galactic halos to
refer to all halos, both halos and subhalos, that contain galax-
ies at their center.
2.2. Halo Occupation from Simulations
Although collisionless N-body simulations do not include
any baryon physics, one can associate the likely sites of
galaxy formation with the dark matter halos and subhalos
within a simulation. Several recent studies have demonstrated
the robustness of this assumption by comparing the cluster-
ing of galaxies to that of a sample of galactic halos with the
same space density; i.e., galaxies brighter than a given lumi-
nosity threshold compared with galactic halos more massive
than a threshold that yields the same abundance. Conroy et al.
(2006) found that the predicted galactic halo clustering was
consistent with galaxy two-point clustering measurements
from z = 0 to z = 5 (see also Kravtsov et al. 2004; Wang et al.
2006). Marin et al. (2008) extended this to measurements of
the galaxy three-point correlation function as well.
For the purpose of this paper, we use high-resolution cos-
mological N-body simulations to guide our choice of the halo
occupation of all galaxies (DRGs and non-DRGs). The HOD
for all galaxies is then fixed by making use of the luminosity
function of all galaxies (see below), and we focus our effortIABLE 1
LIST OF SIMULATIONS
Lbox (h-1 Mpc) (Q.,c-8,ns) mp [h-1 Mo] zout
120 (0.3,0.9,1.0) 1.07 x 109 2.0
160 (0.24,0.75,0.95) 2.54 x 108 2.5
1000 (0.27,0.8,0.95) 6.98 x 1010 2.5
NOTE. Each simulation will be referred to in the text by its box
size. All simulations were performed with the ART code of Kravtsov et al.
(1997). The L120 and L1000 simulations have been described in
Tinker et al. (2008a).
on constraining the DRG HOD from the clustering data.
Just knowing where the galaxies are, however, doesn't
identify which ones are red. Before creating a model for
the halo occupation of DRGs, we first use the subhalo
abundance matching technique (SHAM) to set the occu-
pation of all galaxies down to the completeness limit of
Q08. The space density of DRGs in the Q08 sample is
nDRG = 6.5 x 10-4 (h-1 Mpc)-3 down to their completeness
limit of K < 21. Using the z ~ 2.3 luminosity function
of Marchesini et al. (2007), the space density of all galaxies
is 1.5 x 10-3 (h-1 Mpc)-3 at the same magnitude threshold,
MR = -22.3, yielding a DRG fraction of 44%.1 Whenever re-
ferring to the sample of all galaxies, we mean all galaxies
(DRGs and non-DRGs) down to the completeness limit of the
Q08 sample.
Figure la shows the halo occupation functions, (N)M, of
1 As Q08 point out, the space density of DRGs from Marchesini et al.
(2007) is slightly lower than that of the larger Q08 sample. Thus, whenever
using the Marchesini et al. (2007) luminosity functions and data, a correction
factor of 6.5/5 ~ 1.3 is applied. This increases the published number density
of galaxies brighter than MR = -22.3 in Marchesini et al. (2007) from 1.2 x
10-3 to 1.5 x 10-3 (h-1 Mpc)-3.I I I
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Tinker, Jeremy L.; Wechsler, Risa H. & Zheng, Zheng. Interpreting the Clustering of Distant Red Galaxies, article, August 3, 2009; United States. (https://digital.library.unt.edu/ark:/67531/metadc930489/m1/2/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.