154 research outputs found
Joint deprojection of Sunyaev-Zeldovich and X-ray images of galaxy clusters
We present two non-parametric deprojection methods aimed at recovering the
three-dimensional density and temperature profiles of galaxy clusters from
spatially resolved thermal Sunyaev-Zeldovich (tSZ) and X-ray surface brightness
maps, thus avoiding the use of X-ray spectroscopic data. In both methods,
clusters are assumed to be spherically symmetric and modeled with an onion-skin
structure. The first method follows a direct geometrical approach. The second
method is based on the maximization of a single joint (tSZ and X-ray)
likelihood function, which allows one to fit simultaneously the two signals by
following a Monte Carlo Markov Chain approach. These techniques are tested
against a set of cosmological simulations of clusters, with and without
instrumental noise. We project each cluster along the three orthogonal
directions defined by the principal axes of the momentum of inertia tensor.
This enables us to check any bias in the deprojection associated to the cluster
elongation along the line of sight. After averaging over all the three
projection directions, we find an overall good reconstruction, with a small
(<~10 per cent) overestimate of the gas density profile. This turns into a
comparable overestimate of the gas mass within the virial radius, which we
ascribe to the presence of residual gas clumping. Apart from this small bias
the reconstruction has an intrinsic scatter of about 5 per cent, which is
dominated by gas clumpiness. Cluster elongation along the line of sight biases
the deprojected temperature profile upwards at r<~0.2r_vir and downwards at
larger radii. A comparable bias is also found in the deprojected temperature
profile. Overall, this turns into a systematic underestimate of the gas mass,
up to 10 percent. (Abridged)Comment: 17 pages, 15 figures, accepted by MNRA
Cluster Morphologies and Model-independent Y_(SZ) Estimates from Bolocam Sunyaev-Zel'dovich Images
We present initial results from our ongoing program to image the Sunyaev-Zel'dovich (SZ) effect in galaxy clusters at 143 GHz using Bolocam; five clusters and one blank field are described in this manuscript. The images have a resolution of 58 arcsec and a radius of ≃ 6-7 arcmin, which is approximately r_(500)-2r_(500) for these clusters. We effectively high-pass filter our data in order to subtract noise sourced by atmospheric fluctuations, but we are able to obtain unbiased images of the clusters by deconvolving the effects of this filter. The beam-smoothed rms is ≃ 10 μK_(CMB) in these images; with this sensitivity, we are able to detect the SZ signal to beyond r_(500) in binned radial profiles. We have fit our images to beta and Nagai models, fixing spherical symmetry or allowing for ellipticity in the plane of the sky, and we find that the best-fit parameter values are in general consistent with those obtained from other X-ray and SZ data. Our data show no clear preference for the Nagai model or the beta model due to the limited spatial dynamic range of our images. However, our data show a definitive preference for elliptical models over spherical models, quantified by an F ratio of ≃ 20 for the two models. The weighted mean ellipticity of the five clusters is ϵ = 0.27 ± 0.03, consistent with results from X-ray data. Additionally, we obtain model-independent estimates of Y_(500), the integrated SZ y-parameter over the cluster face to a radius of r_(500), with systematics-dominated uncertainties of ≃ 10%. Our Y_(500) values, which are free from the biases associated with model-derived Y_(500) values, scale with cluster mass in a way that is consistent with both self-similar predictions and expectations of a ≃ 10% intrinsic scatter
Studying the properties of galaxy cluster morphology estimators
X-ray observations of galaxy clusters reveal a large range of morphologies
with various degrees of disturbance, showing that the assumptions of
hydrostatic equilibrium and spherical shape which are used to determine the
cluster mass from X-ray data are not always satisfied. It is therefore
important for the understanding of cluster properties as well as for
cosmological applications to detect and quantify substructure in X-ray images
of galaxy clusters. Two promising methods to do so are power ratios and center
shifts. Since these estimators can be heavily affected by Poisson noise and
X-ray background, we performed an extensive analysis of their statistical
properties using a large sample of simulated X-ray observations of clusters
from hydrodynamical simulations. We quantify the measurement bias and error in
detail and give ranges where morphological analysis is feasible. A new,
computationally fast method to correct for the Poisson bias and the X-ray
background contribution in power ratio and center shift measurements is
presented and tested for typical XMM-Newton observational data sets. We studied
the morphology of 121 simulated cluster images and establish structure
boundaries to divide samples into relaxed, mildly disturbed and disturbed
clusters. In addition, we present a new morphology estimator - the peak of the
0.3-1 r500 P3/P0 profile to better identify merging clusters. The analysis
methods were applied to a sample of 80 galaxy clusters observed with
XMM-Newton. We give structure parameters (P3/P0 in r500, w and P3/P0_max) for
all 80 observed clusters. Using our definition of the P3/P0 (w) substructure
boundary, we find 41% (47%) of our observed clusters to be disturbed.Comment: Replaced to match version published in A&A, Eq. 1 correcte
Reconstructing mass profiles of simulated galaxy clusters by combining Sunyaev-Zeldovich and X-ray images
We present a method to recover mass profiles of galaxy clusters by combining
data on thermal Sunyaev-Zeldovich (tSZ) and X-ray imaging, thereby avoiding to
use any information on X-ray spectroscopy. This method, which represents a
development of the geometrical deprojection technique presented in Ameglio et
al. (2007), implements the solution of the hydrostatic equilibrium equation. In
order to quantify the efficiency of our mass reconstructions, we apply our
technique to a set of hydrodynamical simulations of galaxy clusters. We propose
two versions of our method of mass reconstruction. Method 1 is completely
model-independent, while Method 2 assumes instead the analytic mass profile
proposed by Navarro et al. (1997) (NFW). We find that the main source of bias
in recovering the mass profiles is due to deviations from hydrostatic
equilibrium, which cause an underestimate of the mass of about 10 per cent at
r_500 and up to 20 per cent at the virial radius. Method 1 provides a
reconstructed mass which is biased low by about 10 per cent, with a 20 per cent
scatter, with respect to the true mass profiles. Method 2 proves to be more
stable, reducing the scatter to 10 per cent, but with a larger bias of 20 per
cent, mainly induced by the deviations from equilibrium in the outskirts. To
better understand the results of Method 2, we check how well it allows to
recover the relation between mass and concentration parameter. When analyzing
the 3D mass profiles we find that including in the fit the inner 5 per cent of
the virial radius biases high the halo concentration. Also, at a fixed mass,
hotter clusters tend to have larger concentration. Our procedure recovers the
concentration parameter essentially unbiased but with a scatter of about 50 per
cent.Comment: 13 pages, 11 figures, submitted to MNRA
Angular diameter distance estimates from the Sunyaev-Zel'dovich effect in hydrodynamical cluster simulations
The angular-diameter distance D_A of a galaxy cluster can be measuread by
combining its X-ray emission with the cosmic microwave background fluctution
due to the Sunyaev-Zeldovich effect. The application of this distance indicator
usually assumes that the cluster is spherically symmetric, the gas is
distributed according to the isothermal beta-model, and the X-ray temperature
is an unbiased measure of the electron temperature. We test these assumptions
with galaxy clusters extracted from an extended set of cosmological
N-body/hydrodynamical simulations of a LCDM concordance cosmology, which
include the effect of radiative cooling, star formation and energy feedback
from supernovae. We find that, due to the steep temperature gradients which are
present in the central regions of simulated clusters, the assumption of
isothermal gas leads to a significant underestimate of D_A. This bias is
efficiently corrected by using the polytropic version of the beta-model to
account for the presence of temperature gradients. In this case, once irregular
clusters are removed, the correct value of D_A is recovered with a ~ 5 per cent
accuracy on average, with a ~ 20 per cent intrinsic scatter due to cluster
asphericity. This result is valid when using either the electron temperature or
a spectroscopic-like temperature. When using instead the emission-weighted
definition for the temperature of the simulated clusters, D_A is biased low by
\~ 20 per cent. We discuss the implications of our results for an accurate
determination of the Hubble constant H_0 and of the density parameter Omega_m.
We find that H_0 can be potentially recovered with exquisite precision, while
the resulting estimate of Omega_m, which is unbiased, has typical errors
Delta(Omega_m) ~ 0.05.Comment: 12 pages, 10 figures, accepted by MNRAS, replaced to match accepted
versio
Substructures in hydrodynamical cluster simulations
The abundance and structure of dark matter subhalos has been analyzed
extensively in recent studies of dark matter-only simulations, but
comparatively little is known about the impact of baryonic physics on halo
substructures. We here extend the SUBFIND algorithm for substructure
identification such that it can be reliably applied to dissipative
hydrodynamical simulations that include star formation. This allows, in
particular, the identification of galaxies as substructures in simulations of
clusters of galaxies, and a determination of their content of gravitationally
bound stars, dark matter, and hot and cold gas. Using a large set of
cosmological cluster simulations, we present a detailed analysis of halo
substructures in hydrodynamical simulations of galaxy clusters, focusing in
particular on the influence both of radiative and non-radiative gas physics,
and of non-standard physics such as thermal conduction and feedback by galactic
outflows. We also examine the impact of numerical nuisance parameters such as
artificial viscosity parameterizations. We find that diffuse hot gas is
efficiently stripped from subhalos when they enter the highly pressurized
cluster atmosphere. This has the effect of decreasing the subhalo mass function
relative to a corresponding dark matter-only simulation. These effects are
mitigated in radiative runs, where baryons condense in the central subhalo
regions and form compact stellar cores. However, in all cases, only a very
small fraction, of the order of one percent, of subhalos within the cluster
virial radii preserve a gravitationally bound hot gaseous atmosphere.
(abridged)Comment: improved manuscript, to appear in MNRA
Substructure of the galaxy clusters in the REXCESS sample: observed statistics and comparison to numerical simulations
We study the substructure statistics of a representative sample of galaxy
clusters by means of two currently popular substructure characterisation
methods, power ratios and centroid shifts. We use the 31 clusters from the
REXCESS sample, compiled from the southern ROSAT All-Sky cluster survey REFLEX
with a morphologically unbiased selection in X-ray luminosity and redshift, all
of which have been reobserved with XMM-Newton. We investigate the uncertainties
of the substructure parameters and examine the dependence of the results on
projection effects, finding that the uncertainties of the parameters can be
quite substantial. Thus while the quantification of the dynamical state of
individual clusters with these parameters should be treated with extreme
caution, these substructure measures provide powerful statistical tools to
characterise trends of properties in large cluster samples. The centre shift
parameter, w, is found to be more sensitive in general. For the REXCESS sample
neither the occurence of substructure nor the presence of cool cores depends on
cluster mass. There is a significant anti-correlation between the existence of
substantial substructure and cool cores. The simulated clusters show on average
larger substructure parameters than the observed clusters, a trend that is
traced to the fact that cool regions are more pronounced in the simulated
clusters, leading to stronger substructure measures in merging clusters and
clusters with offset cores. Moreover, the frequency of cool regions is higher
in the simulations than in the observations, implying that the description of
the physical processes shaping cluster formation in the simulations requires
further improvement.Comment: Mauscript submitted to Astronomy and Astrophysics, 20 figure
Substructure and Scatter in the Mass-Temperature Relations of Simulated Clusters
Galaxy clusters exhibit regular scaling relations among their bulk
properties. These relations establish vital links between halo mass and cluster
observables. Precision cosmology studies that depend on these links benefit
from a better understanding of scatter in the mass-observable scaling
relations. Here we study the role of merger processes in introducing scatter
into the - relation, using a sample of 121 galaxy clusters
simulated with radiative cooling and supernova feedback, along with three
statistics previously proposed to measure X-ray surface brightness
substructure. These are the centroid variation (), the axial ratio (),
and the power ratios ( and ). We find that in this set of
simulated clusters, each substructure measure is correlated with a cluster's
departures and from the mean - relation, both for emission-weighted temperatures and for
spectroscopic-like temperatures , in the sense that clusters with
more substructure tend to be cooler at a given halo mass. In all cases, a
three-parameter fit to the - relation that includes substructure
information has less scatter than a two-parameter fit to the basic - relation.Comment: Accepted by ApJ, 10 pages, 10 figure
Sunyaev-Zel'dovich-measured Pressure Profiles from the Bolocam X-Ray/SZ Galaxy Cluster Sample
We describe Sunyaev-Zel'dovich (SZ) effect measurements and analysis of the intracluster medium (ICM) pressure profiles of a set of 45 massive galaxy clusters imaged using Bolocam at the Caltech Submillimeter Observatory. We deproject the average pressure profile of our sample into 13 logarithmically spaced radial bins between 0.07R_(500) and 3.5R_(500), and we find that a generalized Navarro, Frenk, and White (gNFW) profile describes our data with sufficient goodness-of-fit and best-fit parameters (C_(500), α, β, γ, P_0 = 1.18, 0.86, 3.67, 0.67, 4.29). We use X-ray data to define cool-core and disturbed subsamples of clusters, and we constrain the average pressure profiles of each of these subsamples. We find that, given the precision of our data, the average pressure profiles of disturbed and cool-core clusters are consistent with one another at R≳ 0.15R_(500), with cool-core systems showing indications of higher pressure at R≾ 0.15R_(500). In addition, for the first time, we place simultaneous constraints on the mass scaling of cluster pressure profiles, their ensemble mean profile, and their radius-dependent intrinsic scatter between 0.1R_(500) and 2.0R_(500). The scatter among profiles is minimized at radii between ≃ 0.2R_(500) and ≃ 0.5R_(500), with a value of ≃ 20%. These results for the intrinsic scatter are largely consistent with previous analyses, most of which have relied heavily on X-ray derived pressures of clusters at significantly lower masses and redshifts compared to our sample. Therefore, our data provide further evidence that cluster pressure profiles are largely universal with scatter of ≃ 20%-40% about the universal profile over a wide range of masses and redshifts
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