208 research outputs found
Comparing Masses in Literature (CoMaLit)-I. Bias and scatter in weak lensing and X-ray mass estimates of clusters
The first building block to use galaxy clusters in astrophysics and cosmology
is the accurate determination of their mass. Two of the most well regarded
direct mass estimators are based on weak lensing (WL) determinations or X-ray
analyses assuming hydrostatic equilibrium (HE). By comparing these two mass
measurements in samples of rich clusters, we determined the intrinsic scatters,
15 per cent for WL masses and
25 per cent for HE masses. The certain assessment of
the bias is hampered by differences as large as 40 per cent in either WL
or HE mass estimates reported by different groups. If the intrinsic scatter in
the mass estimate is not considered, the slope of any scaling relation
`observable--mass' is biased towards shallower values, whereas the intrinsic
scatter of the scaling is over-estimated.Comment: 14 pages, 7 figures; v2: 16 pages, 8 figures, MNRAS in press; results
unchanged; extended presentation of the statistical method and of the
correlations; products from the CoMaLit series are hosted and updated at
http://pico.bo.astro.it/~sereno/CoMaLi
Note on a polytropic beta-model to fit the X-ray surface brightness of clusters of galaxies
In this note, I suggest that the beta-model used to fit the X-ray surface
brightness profiles of extended sources, like groups and clusters of galaxies,
has to be corrected when the counts are collected in a wide energy band
comparable to the mean temperature of the source and a significant gradient in
the gas temperature is observed. I present a revised version of the beta-model
for the X-ray brightness that applies to a intracluster gas with temperature
and density related by a polytropic equation and extends the standard version
that is strictly valid for an isothermal gas. Given a temperature gradient
observed through an energy window of 1--10 keV typical for the new generation
of X-ray observatories, the beta parameter can change systematically up to 20
per cent from the value obtained under isothermal assumption, i.e. by an amount
larger that any statistical uncertainty obtained from the present data. Within
the virial regions of typical clusters of galaxies, these systematic
corrections affect the total gravitating mass estimate by 5--10 per cent, the
gas mass by 10--30 per cent and the gas fraction value up to 50 per cent, when
compared to the measurements obtained under the isothermal assumption.Comment: 5 pages, references added, version printed on MNRAS. Also available
at http://www-xray.ast.cam.ac.uk/~settori/paper.htm
CoMaLit - II. The scaling relation between mass and Sunyaev-Zel'dovich signal for Planck selected galaxy clusters
We discuss the scaling relation between mass and integrated Compton parameter
of a sample of galaxy clusters from the all-sky {\it Planck} Sunyaev-Zel'dovich
catalogue. Masses were measured with either weak lensing, caustics techniques,
or assuming hydrostatic equilibrium. The retrieved - relation
does not strongly depend on the calibration sample. We found a slope of
1.4-1.9, in agreement with self-similar predictions, with an intrinsic scatter
of per cent. The absolute calibration of the relation can not be
ascertained due to systematic differences of 20-40 per cent in mass
estimates reported by distinct groups. Due to the scatter, the slope of the
conditional scaling relation, to be used in cosmological studies of number
counts, is shallower, 1.1-1.6. The regression methods employed account
for intrinsic scatter in the mass measurements too. We found that Planck mass
estimates suffer from a mass dependent bias.Comment: 14 pages, 7 figures; v2: 17 pages, 11 figures; MNRAS in press,
results unchanged; extended discussion of the Planck calibration sample;
added discussion of conditional vs symmetric scaling relations and of mixture
of Gaussian functions as distribution of the independent variable; products
from the CoMaLit series at http://pico.bo.astro.it/~sereno/CoMaLi
On the Discrepancy between Theoretical and X-Ray Concentration-Mass Relations for Galaxy Clusters
[Abridged] In the past 15 years, the concentration-mass relation has been
investigated diffusely in theoretical studies. On the other hand, only recently
has this relation been derived from X-ray observations. When that happened, the
results caused a certain level of concern: the X-ray normalizations and slopes
were found significantly dissimilar from those predicted by theory.
We analyzed 52 objects, simulated each time with different physical recipes
for the baryonic component, as well as 60 synthetic X-ray images, to determine
if these discrepancies are real or artificial. In particular, we investigate
how the simulated concentration-mass relation depends (1) on the radial range
used to derive the concentration, (2) on the presence of baryons in the
simulations, and on the prescription used to reproduce the gas. Finally, we
evaluate (3) how the results differ when adopting an X-ray approach for the
analysis and (4) how the selection functions based on X-ray luminosity can
impact the results. All effects studied go in the direction of alleviating the
discrepancy between observations and simulations, although with different
significance: while the fitting radial range and the baryonic component play
only a minor role, the X-ray approach and selection function have profound
repercussion on the resulting concentration-mass relation.Comment: 15 pages, 11 figures, 3 tables, ApJ in press. Significant extension
of the study of the selection-function influence and more attentive treatment
of errors (results unchanged
Self-similarity of temperature profiles in distant galaxy clusters: the quest for a Universal law
We present the XMM-Newton temperature profiles of 12 bright clusters of
galaxies at 0.4<z<0.9, with 5<kT<11 keV. The normalized temperature profiles
(normalized by the mean temperature T500) are found to be generally
self-similar. The sample was subdivided in 5 cool-core (CC) and 7 non cool-core
(NCC) clusters, by introducing a pseudo-entropy ratio
sigma=(T_IN/T_OUT)X(EM_IN/EM_OUT)^-1/3 and defining the objects with sigma<0.6
as CC clusters and those with sigma>=0.6 as NCC clusters. The profiles of CC
and NCC clusters differ mainly in the central regions, with the latters
exhibiting a marginally flatter central profile. A significant dependence of
the temperature profiles on the pseudo-entropy ratio sigma is detected by
fitting a function of both r and sigma, showing an indication that the outer
part of the profiles becomes steeper for higher values of sigma (i.e.
transitioning towards the NCC clusters). No significant evidence of redshift
evolution could be found within the redshift range sampled by our clusters
(0.4<z<0.9). A comparison of our high-z sample with intermediate clusters at
0.1<z<0.3, showed how both the CC and NCC clusters temperature profiles have
experienced some sort of evolution. This can be due by the fact that higher z
clusters are at less advanced stage of their formation and did not have enough
time to create a relaxed structure, characterized by a central temperature dip
in CC clusters and by flatter profiles in NCC clusters. This is the first time
that a systematic study of the temperature profiles of galaxy clusters at z>0.4
has been attempted, as we were able to define the closest possible relation to
a Universal law for the temperature profiles of galaxy clusters at 0.1<z<0.9,
showing a dependence on both the state of relaxation of the clusters and the
redshift.Comment: 14 pages, 8 figures, A&A in press, minor changes (language editing
Mass, shape and thermal properties of A1689 by a multi-wavelength X-ray, lensing and Sunyaev-Zel'dovich analysis
Knowledge of mass and concentration of galaxy clusters is crucial to
understand their formation and evolution. Unbiased estimates require the
understanding of the shape and orientation of the halo as well as its
equilibrium status. We propose a novel method to determine the intrinsic
properties of galaxy clusters from a multi-wavelength data set spanning from
X-ray spectroscopic and photometric data to gravitational lensing to the
Sunyaev-Zel'dovich effect (SZe). The method relies on two quite non informative
geometrical assumptions: the distributions of total matter or gas are
approximately ellipsoidal and co-aligned; they have different, constant axial
ratios but share the same degree of triaxiality. Weak and strong lensing probe
the features of the total mass distribution in the plane of the sky. X-ray data
measure size and orientation of the gas in the plane of the sky. Comparison
with the SZ amplitude fixes the elongation of the gas along the line of sight.
These constraints are deprojected thanks to Bayesian inference. The mass
distribution is described as a Navarro-Frenk-White halo with arbitrary
orientation, gas density and temperature are modelled with parametric profiles.
We applied the method to Abell 1689. Independently of the priors, the cluster
is massive, M_{200}=(1.3+-0.2)*10^{15}M_sun, and over-concentrated,
c_{200}=8+-1, but still consistent with theoretical predictions. The total
matter is triaxial (minor to major axis ratio ~0.5+-0.1 exploiting priors from
N-body simulations) with the major axis nearly orientated along the line of
sight. The gas is rounder (minor to major axis ratio ~0.6+-0.1) and deviates
from hydrostatic equilibrium. The contribution of non-thermal pressure is
~20-50 per cent in inner regions, <~ 300 kpc, and ~25+-5 per cent at ~1.5 Mpc.Comment: 14 pages; MNRAS, in pres
The mass-concentration relation in lensing clusters: the role of statistical biases and selection effects
The relation between mass and concentration of galaxy clusters traces their
formation and evolution. Massive lensing clusters were observed to be
over-concentrated and following a steep scaling in tension with predictions
from the concordance CDM paradigm. We critically revise the relation
in the CLASH, the SGAS, the LOCUSS, and the high-redshift samples of weak
lensing clusters. Measurements of mass and concentration are anti-correlated,
which can bias the observed relation towards steeper values. We corrected for
this bias and compared the measured relation to theoretical predictions
accounting for halo triaxiality, adiabatic contraction of the halo, presence of
a dominant BCG and, mostly, selection effects in the observed sample. The
normalisation, the slope and the scatter of the expected relation are strongly
sample-dependent. For the considered samples, the predicted slope is much
steeper than that of the underlying relation characterising dark-matter only
clusters. We found that the correction for statistical and selection biases in
observed relations mostly solve the tension with the CDM model.Comment: 13 pages, 3 figures; v2: 14 pages, minor changes, in press on MNRA
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