21,620 research outputs found

### Intrinsic correlation of halo ellipticity and its implications for large-scale weak lensing surveys

We use a large set of state-of-the-art cosmological N-body simulations [512^3
particles] to study the intrinsic ellipticity correlation functions of halos.
With the simulations of different resolutions, we find that the ellipticity
correlations converge once the halos have more than 160 members. For halos with
fewer members, the correlations are underestimated, and the underestimation
amounts to a factor of 2 when the halos have only 20 particles. After
correcting for the resolution effects, we show that the ellipticity
correlations of halos in the bigger box (L=300 mpc) agree very well with those
obtained in the smaller box (L=100 mpc). Combining these results from the
different simulation boxes, we present accurate fitting formulae for the
ellipticity correlation function c_{11}(r) and for the projected correlation
functions Sigma_{11}(r_p) and Sigma_{22}(r_p) over three orders of magnitude in
halo mass. The latter two functions are useful for predicting the contribution
of the intrinsic correlations to deep lensing surveys. With reasonable
assumptions for the redshift distribution of galaxies and for the mass of
galaxies, we find that the intrinsic ellipticity correlation can contribute
significantly not only to shallow surveys but also to deep surveys. Our results
indicate that previous similar studies significantly underestimated this
contribution for their limited simulation resolutions.Comment: 5 pages with 3 figures; minor revisions, accepted for publication in
MNRAS (Letters

### The density profile of equilibrium and non-equilibrium dark matter halos

We study the diversity of the density profiles of dark matter halos based on
a large set of high-resolution cosmological simulations of 256^3 particles. The
cosmological models include four scale-free models and three representative
cold dark matter models. The simulations have good force resolution, and there
are about 400 massive halos with more than 10^4 particles within the virial
radius in each cosmological model. Our unbiased selection of all massive halos
enables to quantify how well the bulk of dark matter halos can be described by
the Navarro, Frenk & White (NFW) profile which was established for equilibrium
halos. We find that about seventy percent of the halos can be fitted by the NFW
profile with a fitting residual dvi_{max} less than 30% in Omega_0=1 universes.
This percentage is higher in lower density cosmological models. The rest of the
halos exhibits larger deviations from the NFW profile for more significant
internal substructures. There is a considerable amount of variation in the
density profile even for the halos which can be fitted by the NFW profile (i.e.
dvi_{max}<0.30). The distribution of the profile parameter, the concentration
$c$, can be well described by a lognormal function with the mean value \bar c
slightly smaller (15%) than the NFW result and the dispersion \sigma_c in \ln c
about 0.25. The more virialized halos with dvi_{max}<0.15 have the mean value
\bar c in good agreement with the NFW result and a slightly smaller dispersion
\sigma_c (about 0.2). Our results can alleviate some of the conflicts found
recently between the theoretical NFW profile and observational results.
Implications for theoretical and observational studies of galaxy formation are
discussed.Comment: The final version accepted for publication in ApJ; one figure and one
paragraph added to demonstrate that all the conclusions of the first version
are solid to the resoltuion effects; 19 pages with 6 figure

### Determine the galaxy bias factors on large scales using bispectrum method

We study whether the bias factors of galaxies can be unbiasedly recovered
from their power spectra and bispectra. We use a set of numerical N-body
simulations and construct large mock galaxy catalogs based upon the
semi-analytical model of Croton et al. (2006). We measure the reduced bispectra
for galaxies of different luminosity, and determine the linear and first
nonlinear bias factors from their bispectra. We find that on large scales down
to that of the wavenumber k=0.1h/Mpc, the bias factors b1 and b2 are nearly
constant, and b1 obtained with the bispectrum method agrees very well with the
expected value. The nonlinear bias factor b2 is negative, except for the most
luminous galaxies with M<-23 which have a positive b2. The behavior of b2 of
galaxies is consistent with the b2 mass dependence of their host halos. We show
that it is essential to have an accurate estimation of the dark matter
bispectrum in order to have an unbiased measurement of b1 and b2. We also test
the analytical approach of incorporating halo occupation distribution to model
the galaxy power spectrum and bispectrum. The halo model predictions do not fit
the simulation results well on the precision requirement of current
cosmological studies.Comment: 9 pages, 8 figures, accepted for publication in Ap

### Modelling galaxy stellar mass evolution from z~0.8 to today

We apply the empirical method built for z=0 in the previous work of Wang et
al. to a higher redshift, to link galaxy stellar mass directly with its hosting
dark matter halo mass at z~0.8. The relation of the galaxy stellar mass and the
host halo mass M_infall is constrained by fitting both the stellar mass
function and the correlation functions at different stellar mass intervals of
the VVDS observation, where M_infall is the mass of the hosting halo at the
time when the galaxy was last the central galaxy. We find that for low mass
haloes, their residing central galaxies are less massive at high redshift than
those at low redshift. For high mass haloes, central galaxies in these haloes
at high redshift are a bit more massive than the galaxies at low redshift.
Satellite galaxies are less massive at earlier times, for any given mass of
hosting haloes. Fitting both the SDSS and VVDS observations simultaneously, we
also propose a unified model of the M_stars-M_infall relation, which describes
the evolution of central galaxy mass as a function of time. The stellar mass of
a satellite galaxy is determined by the same M_stars-M_infall relation of
central galaxies at the time when the galaxy is accreted. With these models, we
study the amount of galaxy stellar mass increased from z~0.8 to the present day
through galaxy mergers and star formation. Low mass galaxies gain their stellar
masses from z~0.8 to z=0 mainly through star formation. For galaxies of higher
mass, the increase of stellar mass solely through mergers from z=0.8 can make
the massive galaxies a factor ~2 larger than observed at z=0. We can also
predict stellar mass functions of redshifts up to z~3, and the results are
consistent with the latest observations.Comment: 12 pages, 10 figures, accepted for publication in MNRA

### The Scaling of the Redshift Power Spectrum: Observations from the Las Campanas Redshift Survey

In a recent paper we have studied the redshift power spectrum $P^S(k,\mu)$ in
three CDM models with the help of high resolution simulations. Here we apply
the method to the largest available redshift survey, the Las Campanas Redshift
Survey (LCRS). The basic model is to express $P^S(k,\mu)$ as a product of three
factors P^S(k,\mu)=P^R(k)(1+\beta\mu^2)^2 D(k,\mu). Here $\mu$ is the cosine of
the angle between the wave vector and the line of sight. The damping function
$D$ for the range of scales accessible to an accurate analysis of the LCRS is
well approximated by the Lorentz factor D=[1+{1\over
2}(k\mu\sigma_{12})^2]^{-1}. We have investigated different values for $\beta$
($\beta=0.4$, 0.5, 0.6), and measured $P^R(k)$ and $\sigma_{12}(k)$ from
$P^S(k,\mu)$ for different values of $\mu$. The velocity dispersion
$\sigma_{12}(k)$ is nearly a constant from $k=0.5$ to 3 \mpci. The average
value for this range is 510\pm 70 \kms. The power spectrum $P^R(k)$ decreases
with $k$ approximately with $k^{-1.7}$ for $k$ between 0.1 and 4 \mpci. The
statistical significance of the results, and the error bars, are found with the
help of mock samples constructed from a large set of high resolution
simulations. A flat, low-density ($\Omega_0=0.2$) CDM model can give a good fit
to the data, if a scale-dependent special bias scheme is used which we have
called the cluster-under-weighted bias (Jing et al.).Comment: accepted for publication in MNRAS, 20 pages with 7 figure

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