1,001 research outputs found
A numerical fit of analytical to simulated density profiles in dark matter haloes
Analytical and geometrical properties of generalized power-law (GPL) density
profiles are investigated in detail. In particular, a one-to-one correspondence
is found between mathematical parameters and geometrical parameters. Then GPL
density profiles are compared with simulated dark haloes (SDH) density
profiles, and nonlinear least-absolute values and least-squares fits involving
the above mentioned five parameters (RFSM5 method) are prescribed. More
specifically, the sum of absolute values or squares of absolute logarithmic
residuals is evaluated on a large number of points making a 5-dimension
hypergrid, through a few iterations. The size is progressively reduced around a
fiducial minimum, and superpositions on nodes of earlier hypergrids are
avoided. An application is made to a sample of 17 SDHs on the scale of cluster
of galaxies, within a flat CDM cosmological model (Rasia et al. 2004).
In dealing with the mean SDH density profile, a virial radius, averaged over
the whole sample, is assigned, which allows the calculation of the remaining
parameters. Using a RFSM5 method provides a better fit with respect to other
methods. No evident correlation is found between SDH dynamical state (relaxed
or merging) and asymptotic inner slope of the logarithmic density profile or
(for SDH comparable virial masses) scaled radius. Mean values and standard
deviations of some parameters are calculated, and a comparison with previous
results is made with regard to the scaled radius. A certain degree of
degeneracy is found in fitting GPL to SDH density profiles. If it is intrinsic
to the RFSM5 method or it could be reduced by the next generation of
high-resolution simulations, still remains an open question.Comment: 44 pages, 6 figures, updated version with recent results from
high-resolution simulations (Diemand et al. 2004; Reed et al. 2005) included
in the discussion; accepted for publication on SAJ (Serbian Astronomical
Journal
Halo Mass Profiles and Low Surface Brightness Galaxies Rotation Curves
A recent study has claimed that the rotation curve shapes and mass densities
of Low Surface Brightness (LSB) galaxies are largely consistent with
CDM predictions, in contrast to a large body of observational work. I
demonstrate that the method used to derive this conclusion is incapable of
distinguishing the characteristic steep CDM mass-density distribution from the
core-dominated mass-density distributions found observationally: even
core-dominated pseudo-isothermal haloes would be inferred to be consistent with
CDM. This method can therefore make no definitive statements on the
(dis)agreement between the data and CDM simulations. After introducing an
additional criterion that does take the slope of the mass-distribution into
account I find that only about a quarter of the LSB galaxies investigated are
possibly consistent with CDM. However, for most of these the fit parameters are
so weakly constrained that this is not a strong conclusion. Only 3 out of 52
galaxies have tightly constrained solutions consistent with CDM. Two
of these galaxies are likely dominated by stars, leaving only one possible dark
matter dominated, CDM-consistent candidate, forming a mere 2 per cent of the
total sample. These conclusions are based on comparison of data and simulations
at identical radii and fits to the entire rotation curves. LSB galaxies that
are consistent with CDM simulations, if they exist, seem to be rare indeed.Comment: Accepted for publication in Astrophysical Journa
A model for the postcollapse equilibrium of cosmological structure: truncated isothermal spheres from top-hat density perturbations
The postcollapse structure of objects which form by gravitational
condensation out of the expanding cosmological background universe is a key
element in the theory of galaxy formation. Towards this end, we have
reconsidered the outcome of the nonlinear growth of a uniform, spherical
density perturbation in an unperturbed background universe - the cosmological
``top-hat'' problem. We adopt the usual assumption that the collapse to
infinite density at a finite time predicted by the top-hat solution is
interrupted by a rapid virialization caused by the growth of small-scale
inhomogeneities in the initial perturbation. We replace the standard
description of the postcollapse object as a uniform sphere in virial
equilibrium by a more self-consistent one as a truncated, nonsingular,
isothermal sphere in virial and hydrostatic equilibrium, including for the
first time a proper treatment of the finite-pressure boundary condition on the
sphere. The results differ significantly from both the uniform sphere and the
singular isothermal sphere approximations for the postcollapse objects. These
results will have a significant effect on a wide range of applications of the
Press-Schechter and other semi-analytical models to cosmology. The truncated
isothermal sphere solution presented here predicts the virial temperature and
integrated mass distribution of the X-ray clusters formed in the CDM model as
found by detailed, 3D, numerical gas and N-body dynamical simulations
remarkably well. This solution allows us to derive analytically the
numerically-calibrated mass-temperature and radius-temperature scaling laws for
X-ray clusters which were derived empirically by Evrard, Metzler and Navarro
from simulation results for the CDM model. (Shortened)Comment: 29 pages, 7 ps figures, MNRAS-style, LaTeX. Accepted for publication
in MNRAS. Minor revisions only (including additional panel in Fig.3 and
additional comparison with X-ray cluster simulations
Reconstructing the Initial Density Field of the Local Universe: Method and Test with Mock Catalogs
Our research objective in this paper is to reconstruct an initial linear
density field, which follows the multivariate Gaussian distribution with
variances given by the linear power spectrum of the current CDM model and
evolves through gravitational instability to the present-day density field in
the local Universe. For this purpose, we develop a Hamiltonian Markov Chain
Monte Carlo method to obtain the linear density field from a posterior
probability function that consists of two components: a prior of a Gaussian
density field with a given linear spectrum, and a likelihood term that is given
by the current density field. The present-day density field can be
reconstructed from galaxy groups using the method developed in Wang et al.
(2009a). Using a realistic mock SDSS DR7, obtained by populating dark matter
haloes in the Millennium simulation with galaxies, we show that our method can
effectively and accurately recover both the amplitudes and phases of the
initial, linear density field. To examine the accuracy of our method, we use
-body simulations to evolve these reconstructed initial conditions to the
present day. The resimulated density field thus obtained accurately matches the
original density field of the Millennium simulation in the density range 0.3 <=
rho/rho_mean <= 20 without any significant bias. Especially, the Fourier phases
of the resimulated density fields are tightly correlated with those of the
original simulation down to a scale corresponding to a wavenumber of ~ 1 h/Mpc,
much smaller than the translinear scale, which corresponds to a wavenumber of ~
0.15 h\Mpc.Comment: 43 pages, 15 figures, accepted for publication in Ap
The Three-Dimensional Shapes of Galaxy Clusters
While clusters of galaxies are considered one of the most important
cosmological probes, the standard spherical modelling of the dark matter and
the intracluster medium is only a rough approximation. Indeed, it is well
established both theoretically and observationally that galaxy clusters are
much better approximated as triaxial objects. However, investigating the
asphericity of galaxy clusters is still in its infancy. We review here this
topic which is currently gathering a growing interest from the cluster
community. We begin by introducing the triaxial geometry. Then we discuss the
topic of deprojection and demonstrate the need for combining different probes
of the cluster's potential. We discuss the different works that have been
addressing these issues. We present a general parametric framework intended to
simultaneously fit complementary data sets (X-ray, Sunyaev Zel'dovich and
lensing data). We discuss in details the case of Abell 1689 to show how
different models/data sets lead to different haloe parameters. We present the
results obtained from fitting a 3D NFW model to X-ray, SZ, and lensing data for
4 strong lensing clusters. We argue that a triaxial model generally allows to
lower the inferred value of the concentration parameter compared to a spherical
analysis. This may alleviate tensions regarding, e.g. the over-concentration
problem. However, we stress that predictions from numerical simulations rely on
a spherical analysis of triaxial halos. Given that triaxial analysis will have
a growing importance in the observational side, we advocate the need for
simulations to be analysed in the very same way, allowing reliable and
meaningful comparisons. Besides, methods intended to derive the three
dimensional shape of galaxy clusters should be extensively tested on simulated
multi-wavelength observations.Comment: (Biased) Review paper. Comments welcome. Accepted for publication in
Space Science Reviews. This is a product of the work done by an international
team at the International Space Science Institute (ISSI) in Bern on
"Astrophysics and Cosmology with Galaxy Clusters: the X-ray and lensing view
Dark Matter In Disk Galaxies II: Density Profiles as Constraints on Feedback Scenarios
The disparity between the density profiles of galactic dark matter haloes
predicted by dark matter only cosmological simulations and those inferred from
rotation curve decomposition, the so-called cusp-core problem, suggests that
baryonic physics has an impact on dark matter density in the central regions of
galaxies. Feedback from black holes, supernovae and massive stars may each play
a role by removing matter from the centre of the galaxy on shorter timescales
than the dynamical time of the dark matter halo. Our goal in this paper is to
determine constraints on such feedback scenarios based on the observed
properties of a set of nearby galaxies.
Using a Markov Chain Monte Carlo (MCMC) analysis of galactic rotation curves,
via a method developed in a previous paper, we constrain density profiles and
an estimated minimum radius for baryon influence, , which we couple with a
feedback model to give an estimate of the fraction of matter within that radius
that must be expelled to produce the presently observed halo profile. We show
that in the case of the gas rich dwarf irregular galaxy DDO 154, an outflow
from a central source (e.g. a black hole or star forming region) could produce
sufficient feedback on the halo without removing the disk gas.
We examine the rotation curves of 8 galaxies taken from the THINGS data set
and determine constraints on the radial density profiles of their dark matter
haloes. For some of the galaxies, both cored haloes and cosmological cusps are excluded. These intermediate central slopes require
baryonic feedback to be finely tuned. We also find for galaxies which exhibit
extended cores in their haloes (e.g. NGC 925), the use of a split power-law
halo profile yields models without the unphysical, sharp features seen in
models based on the Einasto profile.Comment: 17 pages, 19 figures Submitted to MNRA
The Birmingham-CfA cluster scaling project - I: gas fraction and the M-T relation
We have assembled a large sample of virialized systems, comprising 66 galaxy
clusters, groups and elliptical galaxies with high quality X-ray data. To each
system we have fitted analytical profiles describing the gas density and
temperature variation with radius, corrected for the effects of central gas
cooling. We present an analysis of the scaling properties of these systems and
focus in this paper on the gas distribution and M-T relation. In addition to
clusters and groups, our sample includes two early-type galaxies, carefully
selected to avoid contamination from group or cluster X-ray emission. We
compare the properties of these objects with those of more massive systems and
find evidence for a systematic difference between galaxy-sized haloes and
groups of a similar temperature. We derive a mean logarithmic slope of the M-T
relation within R_200 of 1.84+/-0.06, although there is some evidence of a
gradual steepening in the M-T relation, with decreasing mass. We recover a
similar slope using two additional methods of calculating the mean temperature.
Repeating the analysis with the assumption of isothermality, we find the slope
changes only slightly, to 1.89+/-0.04, but the normalization is increased by
30%. Correspondingly, the mean gas fraction within R_200 changes from
(0.13+/-0.01)h70^-1.5 to (0.11+/-0.01)h70^-1.5, for the isothermal case, with
the smaller fractional change reflecting different behaviour between hot and
cool systems. There is a strong correlation between the gas fraction within
0.3*R_200 and temperature. This reflects the strong (5.8 sigma) trend between
the gas density slope parameter, beta, and temperature, which has been found in
previous work. (abridged)Comment: 27 pages, accepted for publication in MNRAS; uses longtable.sty &
lscape.st
Cosmology in 2D: the concentration-mass relation for galaxy clusters
The aim of this work is to perform a systematic study of the measures of the
mass and concentration estimated by fitting the convergence profile of a large
sample of mock galaxy cluster size lenses, created with the publicly available
code MOKA. We found that the main contribution to the bias in mass and in
concentration is due to the halo triaxiality and second to the presence of
substructures within the host halo virial radius. We show that knowing the
cluster elongation along the line of sight helps in correcting the mass bias,
but still keeps a small negative bias for the concentration. If these mass and
concentration biases will characterize the galaxy cluster sample of a wide
field survey it will be difficult to well recover within one sigma the
cosmological parameters that mainly influence the c - M relation, using as
reference a 3D c - M relation measured in cosmological N-body simulation. In
this work we propose how to correct the c - M relation for projection effects
and for adiabatic contraction and suggest to use these as reference for real
observed data. Correcting mass and concentration estimates, as we propose,
gives a measurement of the cosmological parameter within 1 - {\sigma}
confidence contours.Comment: 18 pages, 14 figures - replaced to match the accepted version for
publication by MNRA
The strongest gravitational lenses: I. The statistical impact of cluster mergers
For more than a decade now, it has been controversial whether or not the high
rate of giant gravitational arcs and the largest observed Einstein radii are
consistent with the standard cosmological model. Recent studies indicate that
mergers provide an efficient mechanism to substantially increase the
strong-lensing efficiency of individual clusters. Based on purely semi-analytic
methods, we investigated the statistical impact of cluster mergers on the
distribution of the largest Einstein radii and the optical depth for giant
gravitational arcs of selected cluster samples. Analysing representative
all-sky realizations of clusters at redshifts z < 1 and assuming a constant
source redshift of z_s = 2.0, we find that mergers increase the number of
Einstein radii above 10 arcsec (20 arcsec) by ~ 35 % (~ 55 %). Exploiting the
tight correlation between Einstein radii and lensing cross sections, we infer
that the optical depth for giant gravitational arcs with a length-to-width
ratio > 7.5 of those clusters with Einstein radii above 10 arcsec (20 arcsec)
increases by ~ 45 % (85 %). Our findings suggest that cluster mergers
significantly influence in particular the statistical lensing properties of the
strongest gravitational lenses. We conclude that semi-analytic studies must
inevitably take these events into account before questioning the standard
cosmological model on the basis of the largest observed Einstein radii and the
statistics of giant gravitational arcs.Comment: 23 pages, 18 figures; accepted for publication in Astronomy and
Astrophysics; v2: minor corrections (added clarifying comments; added Fig.
19) to match the accepted versio
Dark matter haloes: an additional criterion for the choice of fitting density profiles
Simulated dark matter haloes are fitted by self-similar, universal density
profiles, where the scaled parameters depend only on a scaled (truncation)
radius which, in turn, is supposed to be independent on the mass and the
formation redshift. A criterion for the choice of the best fitting density
profile is proposed, with regard to a set of high-resolution simulations, where
some averaging procedure on scaled density profiles has been performed, in
connection with a number of fitting density profiles. An application is made to
a pair of sets each made of a dozen of high-resolution simulations, which are
available in literature, in connection with two currently used fitting density
profiles, where the dependence of the scaled radius on the mass and the
formation redshift, may be neglected to a first extent. Some features of the
early evolution of dark matter haloes represented by fitting density profiles,
are discussed in the limit of the spherical top-hat model.Comment: 62 pages, 9 figures, accepted for publication on SAJ (Serbian
Astronomical Journal), paragraph and reference added for section
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