487 research outputs found
The bias field of dark matter haloes
This paper presents a stochastic approach to the clustering evolution of dark
matter haloes in the Universe. Haloes, identified by a Press-Schechter-type
algorithm in Lagrangian space, are described in terms of `counting fields',
acting as non-linear operators on the underlying Gaussian density fluctuations.
By ensemble averaging these counting fields, the standard Press-Schechter mass
function as well as analytic expressions for the halo correlation function and
corresponding bias factors of linear theory are obtained, thereby extending the
recent results by Mo and White. The non-linear evolution of our halo population
is then followed by solving the continuity equation, under the sole hypothesis
that haloes move by the action of gravity. This leads to an exact and general
formula for the bias field of dark matter haloes, defined as the local ratio
between their number density contrast and the mass density fluctuation. Besides
being a function of position and `observation' redshift, this random field
depends upon the mass and formation epoch of the objects and is both non-linear
and non-local. The latter features are expected to leave a detectable imprint
on the spatial clustering of galaxies, as described, for instance, by
statistics like bispectrum and skewness. Our algorithm may have several
interesting applications, among which the possibility of generating mock halo
catalogues from low-resolution N-body simulations.Comment: 23 pages, LaTeX (included psfig.tex), 4 figures. Few comments and
references have been added, and minor typos and errors corrected. This
version matches the refereed one, in press in MNRA
Physical constraints on the halo mass function
We analyse the effect of two relevant physical constraints on the mass
multiplicity function of dark matter halos in a Press--Schechter type
algorithm. Considering the random--walk of linear Gaussian density fluctuations
as a function of the smoothing scale, we simultaneously i) account for mass
semi--positivity and ii) avoid the cloud--in--cloud problem. It is shown that
the former constraint implies a severe cutoff of low--mass objects, balanced by
an increase on larger mass scales. The analysis is performed both for
scale--free power--spectra and for the standard cold dark matter model. Our
approach shows that the well--known ``infrared" divergence of the standard
Press--Schechter mass function is caused by unphysical, negative mass events
which inevitably occur in a Gaussian distribution of density fluctuations.Comment: Revised version (accepted for publication in MNRAS) including a new
comparison with numerical results, a new appendix and new references.
uuencoded gzip'ed tar archive containing many LaTex files (the main file is
mass.tex). 16 pages with 6 figures (all included
The bispectrum of redshifted 21-cm fluctuations from the dark ages
Brightness-temperature fluctuations in the redshifted 21-cm background from
the cosmic dark ages are generated by irregularities in the gas-density
distribution and can then be used to determine the statistical properties of
density fluctuations in the early Universe. We first derive the most general
expansion of brightness-temperature fluctuations up to second order in terms of
all the possible sources of spatial fluctuations. We then focus on the
three-point statistics and compute the angular bispectrum of
brightness-temperature fluctuations generated prior to the epoch of hydrogen
reionization. For simplicity, we neglect redshift-space distortions. We find
that low-frequency radio experiments with arcmin angular resolution can easily
detect non-Gaussianity produced by non-linear gravity with high signal-to-noise
ratio. The bispectrum thus provides a unique test of the gravitational
instability scenario for structure formation, and can be used to measure the
cosmological parameters. Detecting the signature of primordial non-Gaussianity
produced during or right after an inflationary period is more challenging but
still possible. An ideal experiment limited by cosmic variance only and with an
angular resolution of a few arcsec has the potential to detect primordial
non-Gaussianity with a non-linearity parameter of f_NL ~ 1. Additional sources
of error as weak lensing and an imperfect foreground subtraction could severely
hamper the detection of primordial non-Gaussianity which will benefit from the
use of optimal estimators combined with tomographic techniques.Comment: 15 pages, 4 figures, revised version accepted for publication in ApJ
(contains an improved discussion of gas temperature fluctuations
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.
On the spatial distribution of dark matter halos
We study the spatial distribution of dark matter halos in the Universe in
terms of their number density contrast, related to the underlying dark matter
fluctuation via a non-local and non-linear bias random field. The description
of the matter dynamics is simplified by adopting the `truncated' Zel'dovich
approximation to obtain both analytical results and simulated maps. The halo
number density field in our maps and its probability distribution reproduce
with excellent accuracy those of halos in a high-resolution N-body simulation
with the same initial conditions. Our non-linear and non-local bias
prescription matches the N-body halo distribution better than any Eulerian
linear and local bias.Comment: 4 pages, LaTeX (uses emulateapj; included psfig.tex), 3 figures, 1
table. Shortened version, matching the size requirements of ApJ Letters.
Accepted for publicatio
Orbital Parameters of Merging Dark Matter Halos
In order to specify cosmologically motivated initial conditions for major
galaxy mergers (mass ratios 4:1) that are supposed to explain the
formation of elliptical galaxies we study the orbital parameters of major
mergers of cold dark matter halos using a high-resolution cosmological
simulation. Almost half of all encounters are nearly parabolic with
eccentricities and no correlations between the halo spin planes
or the orbital planes. The pericentric argument shows no correlation
with the other orbital parameters and is distributed randomly. In addition we
find that 50 % of typical pericenter distances are larger than half the halo's
virial radii which is much larger than typically assumed in numerical
simulations of galaxy mergers. In contrast to the usual assumption made in
semi-analytic models of galaxy formation the circularities of major mergers are
found to be not randomly distributed but to peak around a value of . Additionally all results are independent of the minimum
progenitor mass and major merger definitions (i.e. mass ratios 4:1; 3:1;
2:1).Comment: 11 pages, 20 figures, replaced by version accepted to A&A, figure 1
low re
A gravitational lensing explanation for the excess of strong Mg-II absorbers in GRB afterglow spectra
GRB afterglows offer a probe of the intergalactic medium out to high redshift
which complements observations along more abundant quasar lines-of-sight.
Although both quasars and GRB afterglows should provide a-priori random
sight-lines through the intervening IGM, it has been observed that strong Mg-II
absorbers are twice as likely to be found along sight-lines toward GRBs.
Several proposals to reconcile this discrepancy have been put forward, but none
has been found sufficient to explain the magnitude of the effect. In this paper
we estimate the effect of gravitational lensing by galaxies and their
surrounding mass distributions on the statistics of Mg-II absorption. We find
that the multi-band magnification bias could be very strong in the
spectroscopic GRB afterglow population and that gravitational lensing can
explain the discrepancy in density of absorbers, for plausibly steep luminosity
functions. The model makes the prediction that approximately 20%-60% of the
spectroscopic afterglow sample (i.e. ~ 5-15 of 26 sources) would have been
multiply imaged, and hence result in repeating bursts. We show that despite
this large lensing fraction it is likely that none would yet have been
identified by chance owing to the finite sky coverage of GRB searches. We
predict that continued optical monitoring of the bright GRB afterglow locations
in the months and years following the initial decay would lead to
identification of lensed GRB afterglows. A confirmation of the lensing
hypothesis would allow us to constrain the GRB luminosity function down to
otherwise inaccessibly faint levels, with potential consequences for GRB
models.Comment: 8 pages, 3 figures. Submitted to MNRAS
On the Clustering of Sub-millimeter Galaxies
We measure the angular two-point correlation function of sub-millimeter
galaxies (SMGs) from 1.1-millimeter imaging of the COSMOS field with the AzTEC
camera and ASTE 10-meter telescope. These data yields one of the largest
contiguous samples of SMGs to date, covering an area of 0.72 degrees^2 down to
a 1.26 mJy/beam (1-sigma) limit, including 189 (328) sources with S/N greater
than 3.5 (3). We can only set upper limits to the correlation length r_0,
modeling the correlation function as a power-law with pre-assigned slope.
Assuming existing redshift distributions, we derive 68.3% confidence level
upper limits of r_0 < 6-8 h^-1 Mpc at 3.7 mJy, and r_0 < 11-12 h^-1 Mpc at 4.2
mJy. Although consistent with most previous estimates, these upper limits imply
that the real r_0 is likely smaller. This casts doubts on the robustness of
claims that SMGs are characterized by significantly stronger spatial
clustering, (and thus larger mass), than differently selected galaxies at
high-redshift. Using Monte Carlo simulations we show that even strongly
clustered distributions of galaxies can appear unclustered when sampled with
limited sensitivity and coarse angular resolution common to current
sub-millimeter surveys. The simulations, however, also show that unclustered
distributions can appear strongly clustered under these circumstances. From the
simulations, we predict that at our survey depth, a mapped area of two
degrees^2 is needed to reconstruct the correlation function, assuming smaller
beam sizes of future surveys (e.g. the Large Millimeter Telescope's 6" beam
size). At present, robust measures of the clustering strength of bright SMGs
appear to be below the reach of most observations.Comment: 23 pages, 8 figures, accepted for publication in The Astrophysical
Journa
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