397 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
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
Gamma-Ray Bursts and Dark Energy - Dark Matter interaction
In this work Gamma Ray Burst (GRB) data is used to place constraints on a
putative coupling between dark energy and dark matter. Type Ia supernovae (SNe
Ia) constraints from the Sloan Digital Sky Survey II (SDSS-II) first-year
results, the cosmic microwave background radiation (CMBR) shift parameter from
WMAP seven year results and the baryon acoustic oscillation (BAO) peak from the
Sloan Digital Sky Survey (SDSS) are also discussed. The prospects for the field
are assessed, as more GRB events become available.Comment: 7 pages, 3 figures. Version to match the one published in Mon. Not.
R. Ast. So
Two Ways of Biasing Galaxy Formation
We calculate the galaxy bispectrum in both real and redshift space adopting
the most common prescriptions for local Eulerian biasing and Lagrangian
evolving-bias model. We show that the two biasing schemes make measurably
different predictions for these clustering statistics. The Eulerian
prescription implies that the galaxy distribution depends only on the
present-day local mass distribution, while its Lagrangian counterpart relates
the current galaxy distribution to the mass distribution at an earlier epoch
when galaxies first formed. Detailed measurement of the galaxy bispectrum (of
its reduced amplitude) can help establish whether galaxy positions are
determined by the current mass distribution or an earlier mass distribution.Comment: 6 pages, added one figure and revised; MNRAS Pink Pages (in press),
Latex file uses mn.st
On the Association of Gamma-ray Bursts with Massive Stars: Implications for Number Counts and Lensing Statistics
Recent evidence appears to link gamma-ray bursts (GRBs) to star-forming
regions in galaxies at cosmological distances. If short-lived massive stars are
the progenitors of GRBs, the rate of events per unit cosmological volume should
be an unbiased tracer (i.e. unaffected by dust obscuration and surface
brightness limits) of the cosmic history of star formation. Here we use
realistic estimates for the evolution of the stellar birthrate in galaxies to
model the number counts, redshift distribution, and time-delay factors of GRBs.
We present luminosity function fits to the BATSE log N-log P relation for
different redshift distributions of the bursts. Our results imply about 1-2
GRBs every one million Type II supernovae, and a characteristic
`isotropic-equivalent' burst luminosity in the range 3-20 x 1e51 ergs/s (for
H_0=65 km/s/Mpc). We compute the rate of multiple imaging of background GRBs
due to foreground mass condensations in a Lambda-dominated cold dark matter
cosmology, assuming that dark halos approximate singular isothermal spheres on
galaxy scales and Navarro-Frenk-White profiles on group/cluster scales, and are
distributed in mass according to the Press-Schechter model. We show that the
expected sensivity increase of Swift relative to BATSE could result in a few
strongly lensed individual bursts detected down to a photon flux of 0.1
phot/s/cm^2 in a 3-year survey. Because of the partial sky coverage, however,
it is unlikely that the Swift satellite will observe recurrent events (lensed
pairs).Comment: emulateapj, 13 pages, 7 figures, revised version accepted for
publication in the Ap
The Evolution of the Number Density of Large Disk Galaxies in COSMOS
We study a sample of approximately 16,500 galaxies with I_(ACS,AB) †22.5 in the central 38% of the COSMOS field, which are extracted from a catalog constructed from the Cycle 12 ACS F814W COSMOS data set. Structural information on the galaxies is derived by fitting single SĂ©rsic models to their two-dimensional surface brightness distributions. In this paper we focus on the disk galaxy population (as classified by the Zurich Estimator of Structural Types), and investigate the evolution of the number density of disk galaxies larger than approximately 5 kpc between redshift z ~ 1 and the present epoch. Specifically, we use the measurements of the half-light radii derived from the SĂ©rsic fits to construct, as a function of redshift, the size function Ί(r_(1/2), z) of both the total disk galaxy population and of disk galaxies split in four bins of bulge-to-disk ratio. In each redshift bin, the size function specifies the number of galaxies per unit comoving volume and per unit half-light radius r_(1/2). Furthermore, we use a selected sample of roughly 1800 SDSS galaxies to calibrate our results with respect to the local universe. We find the following: (1) The number density of disk galaxies with intermediate sizes (r_(1/2) ~ 5-7 kpc) remains nearly constant from z ~ 1 to today. Unless the growth and destruction of such systems exactly balanced in the last eight billion years, they must have neither grown nor been destroyed over this period. (2) The number density of the largest disks (r_(1/2) > 7 kpc) decreases by a factor of about 2 out to z ~ 1. (3) There is a constancyâor even slight increaseâin the number density of large bulgeless disks out to z ~ 1; the deficit of large disks at early epochs seems to arise from a smaller number of bulged disks. Our results indicate that the bulk of the large disk galaxy population has completed its growth by z ~ 1 and support the theory that secular evolution processes produceâor at least add stellar mass toâthe bulge components of disk galaxies
COSMOS morphological classification with ZEST (the Zurich Estimator of Structural Types) and the evolution since z=1 of the Luminosity Function of early-, disk-, and irregular galaxies
(ABRIDGED) Motivated by the desire to reliably and automatically classify
structure of thousands of COSMOS galaxies, we present ZEST, the Zurich
Estimator of Structural Types. To classify galaxy structure, ZEST uses: (i)
Five non-parametric diagnostics: asymmetry, concentration, Gini coefficient,
2nd-order moment of the brightest 20% of galaxy pixels, and ellipticity; and
(ii) The exponent n of single--Sersic fits to the 2D surface brightness
distributions. To fully exploit the wealth of information while reducing the
redundancy present in these diagnostics, ZEST performs a principal component
(PC) Analysis. We use a sample of ~56,000 I<24 COSMOS galaxies to show that the
first three PCs fully describe the key aspects of the galaxy structure, i.e.,
to calibrate a three-dimensional classification grid of axis PC_1, PC_2, and
PC_3. We demonstrate the robustness of the ZEST grid on the z=0 sample of Frei
et al. (1996). The ZEST classification breaks most of the degeneracy between
different galaxy populations that affects morphological classifications based
on only some of the diagnostics included in ZEST. As a first application, we
present the evolution since z~1 of the Luminosity Functions of COSMOS galaxies
of early, disk and irregular galaxies and, for disk galaxies, of different
bulge-to-disk ratios. Overall, we find that the LF up to a redshift z=1 is
consistent with a pure-luminosity evolution (of about 0.95 magnitudes at z
\~0.7). We highlight however two trends, that are in general agreement with a
down-sizing scenario for galaxy formation: (1.) A deficit of a factor of about
two at z~0.7 of MB>-20.5 structurally--classified early--type galaxies; and
(2.) An excess of a factor of about three, at a similar redshift, of irregular
galaxies.Comment: Accepted for publication in the ApJ COSMOS special issue. A version
with high resolution figures is available at
http://www.exp-astro.phys.ethz.ch/scarlata/papers/ApJS_ZEST.pd
Testing the gravitational lensing explanation for the MgII problem in GRBs
Sixty percent of gamma-ray bursts (GRBs) reveal strong MgII absorbing
systems, which is a factor of ~2 times the rate seen along lines-of-sight to
quasars. The discrepancy in the covering factor is most likely to be the result
of either quasars being obscured due to dust, or the consequence of many GRBs
being strongly gravitationally lensed. We analyze observations of GRBs that
show strong foreground MgII absorption. We Monte-Carlo the distances between
foreground galaxies in the HUDF and lines of sight distributed randomly within
a radius derived from the covering factor, and find that galaxies are located
systematically closer to the position of the observed GRBs than expected for
random lines of sight. This over-density at small impact parameters is
statistically more robust than the well known excess of MgII absorbers among
GRB afterglow spectra, and presents a new puzzle for MgII absorption studies.
The over-density cannot be explained by obscuration in the GRB sample, but is a
natural consequence of gravitational lensing. Upon examining the particular
configurations of galaxies near a sample of GRBs with strong MgII absorption,
we find several intriguing lensing candidates. Our results suggest that lensing
provides a viable contribution to the observed enhancement of strong MgII
absorption along lines of sight to GRBs, and we outline the future observations
required to test this hypothesis conclusively.Comment: v2 includes major updates in response to referee's comments. 15
pages, 10 figures, accepted for publication in Ap
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