326 research outputs found
Are Halo and Galaxy Formation Histories Correlated?
The properties of dark matter halos, including mass growth, correlate with
larger scale environment at fixed mass, an effect known as assembly bias.
However, whether this environmental dependence manifests itself in galaxy
properties remains unclear. We apply a group-finding algorithm to DR7 of the
SDSS to estimate the halo mass of each galaxy and to decompose galaxies into
those that exist at the centers of distinct halos and those that orbit as
satellites within larger halos. Using the 4000-A break as a measure of star
formation history, we examine the correlation between the quenched fraction of
galaxies, f_q, and large-scale environment, rho. At all galaxy magnitudes,
there is a positive, monotonic relationship between f_q and rho. We use the
group catalog to decompose this correlation into the contribution from central
and satellite galaxies as a function of halo mass. Because satellites are more
likely to be quenched than central galaxies, the observed f_q-rho correlation
is primarily due to variations of the halo mass function with environment,
which causes a larger fraction of satellite galaxies at high rho. For low-mass
central galaxies (Mgal <~ 10^10.0 Msol/h^2), there is no correlation between
f_q and rho. These results are inconsistent with the strong assembly bias of
dark matter halos seen in this mass regime if recent galaxy growth at all
correlates with recent halo growth, as we demonstrate through a high resolution
N-body simulation. We also find that the mean stellar age of quenched central
galaxies is independent of rho at fixed Mgal, while the formation times of low
mass halos vary significantly. We conclude that the processes that halt the
star formation of low mass central galaxies are not correlated to the formation
histories of their host halos, and old galaxies do not reside preferentially in
old halos. (Abridged)Comment: 21 pages, submitted to MNRA
On the Halo Occupation of Dark Baryons
We introduce a new technique that adopts the halo occupation framework for
understanding the origin of QSO absorption-line systems. Our initial study
focuses specifically on MgII absorbers. We construct a model of the gaseous
content in which the absorption equivalent width W_r is determined by the the
amount of cold gas, in the form of discrete clouds, along a sightline through a
halo. The two quantities that we specify per halo in the model are (1) the mean
absorption strength per unit surface mass density A_W(M), and (2) the mean
covering factor kappa_g(M) of the gaseous clouds. These parameters determine
the conditional probability distribution of W_r as a function of halo mass,
P(W_r|M). Two empirical measurements are applied to constrain the model: (i)
the absorber frequency distribution function and (ii) the W_r-dependent
clustering amplitude. We find that the data demand a rapid transition in the
gas content of halos at ~10^11.5 Msol/h, below which halos contain
predominantly cold gas and beyond which gas becomes predominantly hot. In order
to reproduce the observed overall strong clustering of the absorbers and the
anti-correlation between W_r and halo mass M, roughly 5% of gas in halos up to
10^14 Msol/h is required to be cold. The gas covering factor is near unity over
a wide range of halo mass, supporting that Mg II systems probe an unbiased
sample of typical galaxies. We discuss the implications of our study in the
contexts of mass assembly of distant galaxies and the origin of QSO absorption
line systems.Comment: 15 emulateapj pages, 7 figures, replaced with revised version
incorporating referee's comment
Cosmological Constraints from Galaxy Clustering and the Mass-to-Number Ratio of Galaxy Clusters: Marginalizing over the Physics of Galaxy Formation
Many approaches to obtaining cosmological constraints rely on the connection
between galaxies and dark matter. However, the distribution of galaxies is
dependent on their formation and evolution as well as the cosmological model,
and galaxy formation is still not a well-constrained process. Thus, methods
that probe cosmology using galaxies as a tracer for dark matter must be able to
accurately estimate the cosmological parameters without knowing the details of
galaxy formation a priori. We apply this reasoning to the method of obtaining
and from galaxy clustering combined with the
mass-to-number ratio of galaxy clusters. To test the sensitivity of this method
to variations due to galaxy formation, we consider several different models
applied to the same cosmological dark matter simulation. The cosmological
parameters are then estimated using the observables in each model,
marginalizing over the parameters of the Halo Occupation Distribution (HOD). We
find that for models where the galaxies can be well represented by a
parameterized HOD, this method can successfully extract the desired
cosmological parameters for a wide range of galaxy formation prescriptions.Comment: 10 pages, 7 figures, Submitted to Ap
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