144 research outputs found
The Cosmic Web from Perturbation Theory
Context: Analyzing the large-scale structure (LSS) with galaxy surveys
demands accurate structure formation models. Such models should ideally be fast
and have a clear theoretical framework to rapidly scan a variety of
cosmological parameter spaces without requiring large training data sets. Aims:
This study aims to extend Lagrangian perturbation theory (LPT), including
viscosity and vorticity, to reproduce the cosmic evolution from dark matter
N-body calculations at the field level. Methods: We extend LPT to an Eulerian
framework, dubbed eALPT. An ultraviolet regularisation through the spherical
collapse model provided by Augmented LPT, turns out to be crucial at low
redshifts. This enables modelling the stress tensor, with this introducing
vorticity. The model has two free parameters apart from the choice of
cosmology, redshift snapshots, cosmic volume, and the number of
particles-cells. Results: We find that the cross-correlation of the dark matter
distribution as compared to N-body solvers increases at Mpc
and from 55\% with the Zel'dovich approximation (70\% with
ALPT), to 95\% with three timesteps eALPT, and power spectra within
percentage accuracy up to Mpc.Comment: 6 pages, 3 figure
All-sky angular power spectra from cleaned WISESuperCOSMOS galaxy number counts
Aiming to extract cosmological information from linear scales of the
WISESuperCOSMOS photometric redshift catalog, we perform a
characterization of the systematic effects associated with stellar content,
evidencing the presence of contamination and obscuration. We create an
integrated model for these effects (which together we call `usurper
contamination'), devise a method to remove both of them simultaneously and show
its functionality by applying it to a set of mock catalogs. When administered
to WISESuperCOSMOS data, our method shows to improve the measurements
of angular power spectra on scales and the extraction of
cosmological parameters therefrom, even though a significant excess of power
remains at these scales. When ignoring scales , we still find strong
indications of systematics, albeit these can be localized in the southern
equatorial hemisphere. An independent analysis of the northern hemisphere at
agrees with a CDM model with parameters from the Planck
satellite and gives and
at 95% confidence limit when combined with priors
on , and .Comment: 33 pages, 20 figures, 5 tables. Accepted for publication in JCA
Ellipsoidal configurations in the de Sitter spacetime
The cosmological constant modifies certain properties of large
astrophysical rotating configurations with ellipsoidal geometries, provided the
objects are not too compact. Assuming an equilibrium configuration and so using
the tensor virial equation with we explore several equilibrium
properties of homogeneous rotating ellipsoids. One shows that the bifurcation
point, which in the oblate case distinguishes the Maclaurin ellipsoid from the
Jacobi ellipsoid, is sensitive to the cosmological constant. Adding to that,
the cosmological constant allows triaxial configurations of equilibrium
rotating the minor axis as solutions of the virial equations. The significance
of the result lies in the fact that minor axis rotation is indeed found in
nature. Being impossible for the oblate case, it is permissible for prolate
geometries, with zero and positive. For the triaxial case, however,
an equilibrium solution is found only for non-zero positive . Finally,
we solve the tensor virial equation for the angular velocity and display
special effects of the cosmological constant there.Comment: 15 pages, 11 figures, published in Class. Quant. Grav. References
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Equilibrium of large astrophysical structures in the Newton-Hooke spacetime
Using the scalar and tensor virial equations, the Lane-Emden equation
expressing the hydrostatic equilibrium and small oscillations around the
equilibrium, we show how the cosmological constant affects various
astrophysical quantities important for large matter conglomeration in the
universe. Among others we examine the effect of on the polytropic
equation of state for spherically symmetric objects and find non-negligible
results in certain realistic cases. We calculate the angular velocity for
non-spherical oblate configurations which demonstrates a clear effect of
on high eccentricity objects. We show that for oblate as well as
prolate ellipsoids the cosmological constant influences the critical mass and
the temperature of the astrophysical object. These and other results show that
the effect of is large for flat astrophysical bodies.Comment: References adde
Minimum mass-radius ratio for charged gravitational objects
We rigorously prove that for compact charged general relativistic objects
there is a lower bound for the mass-radius ratio. This result follows from the
same Buchdahl type inequality for charged objects, which has been extensively
used for the proof of the existence of an upper bound for the mass-radius
ratio. The effect of the vacuum energy (a cosmological constant) on the minimum
mass is also taken into account. Several bounds on the total charge, mass and
the vacuum energy for compact charged objects are obtained from the study of
the Ricci scalar invariants. The total energy (including the gravitational one)
and the stability of the objects with minimum mass-radius ratio is also
considered, leading to a representation of the mass and radius of the charged
objects with minimum mass-radius ratio in terms of the charge and vacuum energy
only.Comment: 19 pages, accepted by GRG, references corrected and adde
Impact of baryons on the cluster mass function and cosmological parameter determination
Recent results by the Planck collaboration have shown that cosmological
parameters derived from the cosmic microwave background anisotropies and
cluster number counts are in tension, with the latter preferring lower values
of the matter density parameter, , and power spectrum
amplitude, . Motivated by this, we investigate the extent to which
the tension may be ameliorated once the effect of baryonic depletion on the
cluster mass function is taken into account. We use the large-volume Millennium
Gas simulations in our study, including one where the gas is pre-heated at high
redshift and one where the gas is heated by stars and active galactic nuclei
(in the latter, the self-gravity of the baryons and radiative cooling are
omitted). In both cases, the cluster baryon fractions are in reasonably good
agreement with the data at low redshift, showing significant depletion of
baryons with respect to the cosmic mean. As a result, it is found that the
cluster abundance in these simulations is around 15 per cent lower than the
commonly-adopted fit to dark matter simulations by Tinker et al (2008) for the
mass range . Ignoring this effect
produces a significant artificial shift in cosmological parameters which can be
expressed as at
(the median redshift of the cluster sample) for the
feedback model. While this shift is not sufficient to fully explain the
discrepancy, it is clear that such an effect cannot be
ignored in future precision measurements of cosmological parameters with
clusters. Finally, we outline a simple, model-independent procedure that
attempts to correct for the effect of baryonic depletion and show that it works
if the baryon-dark matter back-reaction is negligible.Comment: 10 pages, 5 figures, Accepted by MNRA
DESI Mock Challenge: Halo and galaxy catalogs with the bias assignment method
We present a novel approach to the construction of mock galaxy catalogues for
large-scale structure analysis based on the distribution of dark matter halos
obtained with effective bias models at the field level. We aim to produce mock
galaxy catalogues capable of generating accurate covariance matrices for a
number of cosmological probes that are expected to be measured in current and
forthcoming galaxy redshift surveys (e.g. two- and three-point statistics). We
use the bias assignment method (BAM) to model the statistics of halo
distribution through a learning algorithm using a few detailed -body
simulations, and approximated gravity solvers based on Lagrangian perturbation
theory. Using specific models of halo occupation distributions, we generate
galaxy mocks with the expected number density and central-satellite fraction of
emission-line galaxies, which are a key target of the DESI experiment. BAM
generates mock catalogues with per cent accuracy in a number of summary
statistics, such as the abundance, the two- and three-point statistics of halo
distributions, both in real and redshift space. In particular, the mock galaxy
catalogues display accuracy in the multipoles of the power
spectrum up to scales of . We show that covariance
matrices of two- and three-point statistics obtained with BAM display a similar
structure to the reference simulation. BAM offers an efficient way to produce
mock halo catalogues with accurate two- and three-point statistics, and is able
to generate a variety of multi-tracer catalogues with precise covariance
matrices of several cosmological probes. We discuss future developments of the
algorithm towards mock production in DESI and other galaxy-redshift surveys.
(Abridged)Comment: Accepted for publication at A&
Characterising superclusters with the galaxy cluster distribution
G. Chon, et al., “Characterising superclusters with the galaxy cluster distribution”, Astronomy & Astrophysics, Vol. 567, August 2014. This version of record is available at: https://www.aanda.org/articles/aa/abs/2014/07/aa24047-14/aa24047-14.html Reproduced with Permission from Astronomy and Astrophysics, © ESO 2014.Superclusters are the largest observed matter density structures in the Universe. Recently, we presented the first supercluster catalogue constructed with a well-defined selection function based on the X-ray flux-limited cluster survey, REFLEX II. To construct the sample we proposed a concept to find large objects with a minimum overdensity such that it can be expected that most of their mass will collapse in the future. The main goal is to provide support for our concept here by using simulation that we can, on the basis of our observational sample of X-ray clusters, construct a supercluster sample defined by a certain minimum overdensity. On this sample we also test how superclusters trace the underlying dark matter distribution. Our results confirm that an overdensity in the number of clusters is tightly correlated with an overdensity of the dark matter distribution. This enables us to define superclusters within which most of the mass will collapse in the future. We also obtain first-order mass estimates of superclusters on the basis of the properties of the member clusters. We also show that in this context the ratio of the cluster number density and dark matter mass density is consistent with the theoretically expected cluster bias. Our previous work provided evidence that superclusters are a special environment in which the density structures of the dark matter grow differently from those in the field, as characterised by the X-ray luminosity function. Here we confirm for the first time that this originates from a top-heavy mass function at high statistical significance that is provided by a Kolmogorov-Smirnov test. We also find in close agreement with observations that the superclusters only occupy a small volume of a few per cent, but contain more than half of the clusters in the present-day Universe.Peer reviewe
Comparing approximate methods for mock catalogues and covariance matrices - I. Correlation function
Computational astrophysic
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