35,289 research outputs found

### Stochastic Biasing and Galaxy-Mass Density Relation in the Weakly Non-linear Regime

It is believed that the biasing of the galaxies plays an important role for
understanding the large-scale structure of the universe. In general, the
biasing of galaxy formation could be stochastic. Furthermore, the future galaxy
survey might allow us to explore the time evolution of the galaxy distribution.
In this paper, the analytic study of the galaxy-mass density relation and its
time evolution is presented within the framework of the stochastic biasing. In
the weakly non-linear regime, we derive a general formula for the galaxy-mass
density relation as a conditional mean using the Edgeworth expansion. The
resulting expression contains the joint moments of the total mass and galaxy
distributions. Using the perturbation theory, we investigate the time evolution
of the joint moments and examine the influence of the initial stochasticity on
the galaxy-mass density relation. The analysis shows that the galaxy-mass
density relation could be well-approximated by the linear relation. Compared
with the skewness of the galaxy distribution, we find that the estimation of
the higher order moments using the conditional mean could be affected by the
stochasticity. Therefore, the galaxy-mass density relation as a conditional
mean should be used with a caution as a tool for estimating the skewness and
the kurtosis.Comment: 22 pages, 7 Encapusulated Postscript Figures, aastex, The title and
the structure of the paper has been changed, Results and conclusions
unchanged, Accepted for publication in Ap

### The time-evolution of bias

We study the evolution of the bias factor b and the mass-galaxy correlation
coefficient r in a simple analytic model for galaxy formation and the
gravitational growth of clustering. The model shows that b and r can be
strongly time-dependent, but tend to approach unity even if galaxy formation
never ends as the gravitational growth of clustering debiases the older
galaxies. The presence of random fluctuations in the sites of galaxy formation
relative to the mass distribution can cause large and rapidly falling bias
values at high redshift.Comment: 4 pages, with 2 figures included. Typos corrected to match published
ApJL version. Color figure and links at http://www.sns.ias.edu/~max/bias.html
or from [email protected]

### Structure formation from non-Gaussian initial conditions: multivariate biasing, statistics, and comparison with N-body simulations

We study structure formation in the presence of primordial non-Gaussianity of
the local type with parameters f_NL and g_NL. We show that the distribution of
dark-matter halos is naturally described by a multivariate bias scheme where
the halo overdensity depends not only on the underlying matter density
fluctuation delta, but also on the Gaussian part of the primordial
gravitational potential phi. This corresponds to a non-local bias scheme in
terms of delta only. We derive the coefficients of the bias expansion as a
function of the halo mass by applying the peak-background split to common
parametrizations for the halo mass function in the non-Gaussian scenario. We
then compute the halo power spectrum and halo-matter cross spectrum in the
framework of Eulerian perturbation theory up to third order. Comparing our
results against N-body simulations, we find that our model accurately describes
the numerical data for wavenumbers k < 0.1-0.3 h/Mpc depending on redshift and
halo mass. In our multivariate approach, perturbations in the halo counts trace
phi on large scales and this explains why the halo and matter power spectra
show different asymptotic trends for k -> 0. This strongly scale-dependent bias
originates from terms at leading order in our expansion. This is different from
what happens using the standard univariate local bias where the scale-dependent
terms come from badly behaved higher-order corrections. On the other hand, our
biasing scheme reduces to the usual local bias on smaller scales where |phi| is
typically much smaller than the density perturbations. We finally discuss the
halo bispectrum in the context of multivariate biasing and show that, due to
its strong scale and shape dependence, it is a powerful tool for the detection
of primordial non-Gaussianity from future galaxy surveys.Comment: 26 pages, 16 figures. Minor modifications, version accepted by Phys.
Rev.

### What Can the Distribution of Intergalactic Metals Tell us About the History of Cosmological Enrichment?

I study the relationship between the spatial distribution of intergalactic
metals and the masses and ejection energies of the sources that produced them.
Over a wide range of models, metal enrichment is dominated by the smallest
efficient sources, as the enriched volume scales roughly as E^{3/5} ~ M^{3/5}
while the number density of sources goes as 1/M. In all cases, the earliest
sources have the biggest impact, because fixed comoving distances correspond to
smaller physical distances at higher redshifts. This means that most of the
enriched volume is found around rare peaks, and intergalactic metals are
naturally highly clustered. Furthermore, this clustering is so strong as to
lead to a large overlap between individual bubbles. Thus the typical radius of
enriched z ~ 3 regions should be interpreted as a constraint on groupings of
sources rather than the ejection radius of a typical source. Similarly, the
clustering of enriched regions should be taken as a measurement of source bias
rather than mass.Comment: 10 pages, 2 figures, ApJL in pres

### The Accretion and Cooling of Preheated Gas in Dark Matter Halos

(abridged) We use a one-dimensional hydrodynamical code to investigate the
effects of preheating on gas accretion and cooling in cold dark matter halos.
In the absence of radiative cooling, preheating reduces the amount of gas that
can be accreted into a halo, and the accreted gas fraction is determined by the
ratio of the initial specific entropy of the gas to the virial entropy of the
halo. In the presence of radiative cooling, preheating affects the gas fraction
that can cool in two different ways. For small halos with masses <10^12Msun,
preheating suppresses gas accretion, but most of the accreted gas can cool. For
more massive halos, preheating not only reduces the amount of accreted gas, but
also reduces the cooling efficiency. For both small and massive halos, gas
cooling is delayed by preheating and in an inside-out fashion if the halo gas
is assumed to be a single-phase medium. However, cooling can occur over a wider
range of redshifts and radii, if we assume a multi-phase medium. As examples,
two specific preheating cases are investigated. In the first case, the
preheating entropy is assumed to be proportional to the virial entropy of the
halo, as expected from AGN feedback. Such preheating effectively suppresses
radiative cooling in halos with M>10^13Msun. We suggest that this may be the
reason why the stellar mass function of galaxies breaks sharply at the massive
end. Such preheating also helps create the hot diffused halos within which the
"radio mode" feedback of AGNs can act effectively. In the second case, we
assume the intergalactic medium is warm. Here the total amount of gas that can
cool in a halo scales with halo mass as ~M^2, as would be required to match the
observed stellar- and HI-mass functions in the current CDM model at the small
mass end.Comment: 14 pages, 13 figures, submitted to MNRA

### Accurate determination of the Lagrangian bias for the dark matter halos

We use a new method, the cross power spectrum between the linear density
field and the halo number density field, to measure the Lagrangian bias for
dark matter halos. The method has several important advantages over the
conventional correlation function analysis. By applying this method to a set of
high-resolution simulations of 256^3 particles, we have accurately determined
the Lagrangian bias, over 4 magnitudes in halo mass, for four scale-free models
with the index n=-0.5, -1.0, -1.5 and -2.0 and three typical CDM models. Our
result for massive halos with $M \ge M_*$ ($M_*$ is a characteristic non-linear
mass) is in very good agreement with the analytical formula of Mo & White for
the Lagrangian bias, but the analytical formula significantly underestimates
the Lagrangian clustering for the less massive halos $M < M_*. Our simulation
result however can be satisfactorily described, with an accuracy better than
15%, by the fitting formula of Jing for Eulerian bias under the assumption that
the Lagrangian clustering and the Eulerian clustering are related with a linear
mapping. It implies that it is the failure of the Press-Schechter theories for
describing the formation of small halos that leads to the inaccuracy of the Mo
& White formula for the Eulerian bias. The non-linear mapping between the
Lagrangian clustering and the Eulerian clustering, which was speculated as
another possible cause for the inaccuracy of the Mo & White formula, must at
most have a second-order effect. Our result indicates that the halo formation
model adopted by the Press-Schechter theories must be improved.Comment: Minor changes; accepted for publication in ApJ (Letters) ; 11 pages
with 2 figures include

- â€¦