195 research outputs found

### Large-Scale Mass Power Spectrum from Peculiar Velocities

This is a brief progress report on a long-term collaborative project to
measure the power spectrum (PS) of mass density fluctuations from the Mark III
and the SFI catalogs of peculiar velocities. The PS is estimated by applying
maximum likelihood analysis, using generalized CDM models with and without COBE
normalization. The application to both catalogs yields fairly similar results
for the PS. The robust result is a relatively high PS, with
P(k)\Omega^{1.2}=(4.5+/-2.0)X10^3 (Mpc/h)^3 at k=0.1 h/Mpc. An extrapolation to
smaller scales using the different CDM models gives
\sigma_8\Omega^{0.6}=0.85+/-0.2. The general constraint on the combination of
cosmological parameters is of the sort \Omega \h_{50}^{\mu}
n^{\nu}=0.75+/-0.25, where \mu=1.3 and \nu=3.7,2.0 for \Lambda CDM models with
and without tensor fluctuations respectively. For open CDM, without tensor
fluctuations, the powers are \mu=0.9 and \nu=1.4.Comment: 3 pages, 1 figure, uses mprocl.sty. To appear in Proceedings of the
Eighth Marcel Grossmann Meetin

### The Growth of Galaxy Stellar Mass Within Dark Matter Halos

We study the evolution of stellar mass in galaxies as a function of host halo
mass, using the "MPA" and "Durham" semi-analytic models, implemented on the
Millennium Run simulation. The results from both models are similar. We find
that about 45% of the stellar mass in central galaxies in present-day halos
less massive than ~10^{12} Msun/h is already in place at z~1. This fraction
increases to ~65% for more massive halos. The peak of star formation efficiency
shifts toward lower mass halos from z~1 to z~0. The stellar mass in low-mass
halos grows mostly by star formation since z~1, while in high-mass halos most
of the stellar mass is assembled by mergers. These trends are clear indications
of "halo downsizing". We compare our findings to the results of the
phenomenological method developed by Zheng, Coil & Zehavi (2007). The
theoretical predictions are in qualitative agreement with these results,
however there are large discrepancies. The most significant one concerns the
amount of stars already in place in the progenitor galaxies at z~1, which is
about a factor of two larger in both semi-analytic models. We also use the
semi-analytic catalogs to test different assumptions made in that work, and
illustrate the importance of smooth accretion of dark matter when estimating
the mergers contribution. We demonstrate that methods studying galaxy evolution
from the galaxy-halo connection are powerful in constraining theoretical models
and can guide future efforts of modeling galaxy evolution. Conversely,
semi-analytic models serve an important role in improving such methods.Comment: 13 pages, 8 figures, submitted to Ap

### Properties and Origin of Galaxy Velocity Bias in the Illustris Simulation

We use the hydrodynamical galaxy formation simulations from the Illustris
suite to study the origin and properties of galaxy velocity bias, i.e., the
difference between the velocity distributions of galaxies and dark matter
inside halos. We find that galaxy velocity bias is a decreasing function of the
ratio of galaxy stellar mass to host halo mass. In general, central galaxies
are not at rest with respect to dark matter halos or the core of halos, with a
velocity dispersion above 0.04 times that of the dark matter. The central
galaxy velocity bias is found to be mostly caused by the close interactions
between the central and satellite galaxies. For satellite galaxies, the
velocity bias is related to their dynamical and tidal evolution history after
being accreted onto the host halos. It depends on the time after the accretion
and their distances from the halo centers, with massive satellites generally
moving more slowly than the dark matter. The results are in broad agreements
with those inferred from modeling small-scale redshift-space galaxy clustering
data, and the study can help improve models of redshift-space galaxy
clustering.Comment: 15 pages, 11 figures. Accepted for publication in Ap

### On the Mass-to-Light Ratio of Large Scale Structure

We examine the dependence of the mass-to-light (M/L) ratio of large-scale
structure on cosmological parameters, in models that are constrained to match
observations of the projected galaxy correlation function w(rp). For a sequence
of cosmological models with a fixed P(k) shape and increasing normalization
\sig8, we find parameters of the galaxy halo occupation distribution (HOD) that
reproduce SDSS measurements of w(rp) as a function of luminosity. Using these
HOD models we calculate mean M/L ratios as a function of halo mass and populate
halos of N-body simulations to compute M/L in larger scale environments,
including cluster infall regions. For all cosmological models, the M/L ratio in
high mass halos or high density regions is approximately independent of halo
mass or smoothing scale. However, the "plateau" value of M/L depends on \sig8
as well as \Omega_m, and it represents the universal mass-to-light ratio
only for models in which the galaxy correlation function is approximately
unbiased, i.e., with \sig8 ~ \sig8_gal. Our results for cluster mass halos
follow the trend M/L = 577(\Omega_m/0.3)(\sig8/0.9)^{1.7} h Msun/Lsun. Combined
with Carlberg et al.'s (1996) mean M/L ratio of CNOC galaxy clusters, this
relation implies (\sig8/0.9)(\Omega_m/0.3)^{0.6} = 0.75 +/- 0.06. M/L ratios of
clusters from the SDSS and CAIRNS surveys yield similar results. This
constraint is inconsistent with parameter values \Omega_m ~ 0.3, \sig8 ~ 0.9
favored by recent joint analyses of CMB measurements and other large-scale
structure data. We discuss possible resolutions, none of which seems entirely
satisfactory. Appendices present an improved formula for halo bias factors and
an improved analytic technique for calculating the galaxy correlation function
from a given cosmological model and HOD. (Abridged)Comment: Accepted to ApJ (v 630, no 2). Replaced with accepted versio

### The Linear Point: A cleaner cosmological standard ruler

We show how a characteristic length scale imprinted in the galaxy two-point
correlation function, dubbed the "linear point", can serve as a comoving
cosmological standard ruler. In contrast to the Baryon Acoustic Oscillation
peak location, this scale is constant in redshift and is unaffected by
non-linear effects to within $0.5$ percent precision. We measure the location
of the linear point in the galaxy correlation function of the LOWZ and CMASS
samples from the Twelfth Data Release (DR12) of the Baryon Oscillation
Spectroscopic Survey (BOSS) collaboration. We combine our linear-point
measurement with cosmic-microwave-background constraints from the Planck
satellite to estimate the isotropic-volume distance $D_{V}(z)$, without relying
on a model-template or reconstruction method. We find $D_V(0.32)=1264\pm 28$
Mpc and $D_V(0.57)=2056\pm 22$ Mpc respectively, consistent with the quoted
values from the BOSS collaboration. This remarkable result suggests that all
the distance information contained in the baryon acoustic oscillations can be
conveniently compressed into the single length associated with the linear
point.Comment: The optimal two-point correlation function bin-size is employed.
Results are updated and the distance constraints are improve

### The Conditional Colour-Magnitude Distribution: I. A Comprehensive Model of the Colour-Magnitude-Halo Mass Distribution of Present-Day Galaxies

We formulate a model of the conditional colour-magnitude distribution (CCMD)
to describe the distribution of galaxy luminosity and colour as a function of
halo mass. It consists of two populations of different colour distributions,
dubbed pseudo-blue and pseudo-red, respectively, with each further separated
into central and satellite galaxies. We define a global parameterization of
these four colour-magnitude distributions and their dependence on halo mass,
and we infer parameter values by simultaneously fitting the space densities and
auto-correlation functions of 79 galaxy samples from the Sloan Digital Sky
Survey defined by fine bins in the colour-magnitude diagram (CMD). The model
deprojects the overall galaxy CMD, revealing its tomograph along the halo mass
direction. The bimodality of the colour distribution is driven by central
galaxies at most luminosities, though at low luminosities it is driven by the
difference between blue centrals and red satellites. For central galaxies, the
two pseudo-colour components are distinct and orthogonal to each other in the
CCMD: at fixed halo mass, pseudo-blue galaxies have a narrow luminosity range
and broad colour range, while pseudo-red galaxies have a narrow colour range
and broad luminosity range. For pseudo-blue centrals, luminosity correlates
tightly with halo mass, while for pseudo-red galaxies colour correlates more
tightly (redder galaxies in more massive haloes). The satellite fraction is
higher for redder and for fainter galaxies, with colour a stronger indicator
than luminosity. We discuss the implications of the results and further
applications of the CCMD model.Comment: 32 pages, 26 figures, accepted for publication in MNRA

### A New Method to Correct for Fiber Collisions in Galaxy Two-Point Statistics

In fiber-fed galaxy redshift surveys, the finite size of the fiber plugs
prevents two fibers from being placed too close to one another, limiting the
ability of studying galaxy clustering on all scales. We present a new method
for correcting such fiber collision effects in galaxy clustering statistics
based on spectroscopic observations. Our method makes use of observations in
tile overlap regions to measure the contributions from the collided population,
and to therefore recover the full clustering statistics. The method is rooted
in solid theoretical ground and is tested extensively on mock galaxy catalogs.
We demonstrate that our method can well recover the projected and the full
three-dimensional redshift-space two-point correlation functions on scales both
below and above the fiber collision scale, superior to the commonly used
nearest neighbor and angular correction methods. We discuss potential
systematic effects in our method. The statistical correction accuracy of our
method is only limited by sample variance, which scales down with (the square
root of) the volume probed. For a sample similar to the final SDSS-III BOSS
galaxy sample, the statistical correction error is expected to be at the level
of 1% on scales 0.1--30Mpc/h for the two-point correlation functions. The
systematic error only occurs on small scales, caused by non-perfect correction
of collision multiplets, and its magnitude is expected to be smaller than 5%.
Our correction method, which can be generalized to other clustering statistics
as well, enables more accurate measurements of full three-dimensional galaxy
clustering on all scales with galaxy redshift surveys. (abridged)Comment: ApJ accepted. Matched to accepted version(improvements on
systematics

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