434 research outputs found
ROSAT PSPC observations of the outer regions of the Perseus cluster of galaxies
We present an analysis of four off-axis ROSAT PSPC observations of the
Perseus cluster of galaxies (Abell~426). We detect the surface brightness
profile to a radius of 80 arcmin ( Mpc) from the X-ray
peak. The profile is measured in various sectors and in three different energy
bands. Firstly, a colour analysis highlights a slight variation of over
the region, and cool components in the core and in the eastern sector. We apply
the -model to the profiles from different sectors and present a solution
to the, so-called, -problem. The residuals from an azimuthally-averaged
profile highlight extended emission both in the East and in the West, with
estimated luminosities of about 8 and 1 , respectively.
We fit several models to the surface brightness profile, including the one
obtained from the Navarro, Frenk and White (1995) potential. We obtain the best
fit with the gas distribution described by a power law in the inner, cooling
region and a -model for the extended emission. Through the best-fit
results and the constraints from the deprojection of the surface brightness
profiles, we define the radius where the overdensity inside the cluster is 200
times the critical value, , at Mpc. Within Mpc (), the total mass in the Perseus cluster is
and its gas fraction is about 30 per cent.Comment: 21 pages, 23 figures; accepted for publication in MNRAS; also
available at http://www-xray.ast.cam.ac.uk/~settori/paper.htm
Properties of gas clumps and gas clumping factor in the intra cluster medium
The spatial distribution of gas matter inside galaxy clusters is not
completely smooth, but may host gas clumps associated with substructures. These
overdense gas substructures are generally a source of unresolved bias of X-ray
observations towards high density gas, but their bright luminosity peaks may be
resolved sources within the ICM, that deep X-ray exposures may be (already)
capable to detect. In this paper we aim at investigating both features, using a
set of high-resolution cosmological simulations with ENZO. First, we monitor
how the bias by unresolved gas clumping may yield incorrect estimates of global
cluster parameters and affects the measurements of baryon fractions by X-ray
observations. We find that based on X-ray observations of narrow radial strips,
it is difficult to recover the real baryon fraction to better than 10 - 20
percent uncertainty. Second, we investigated the possibility of observing
bright X-ray clumps in the nearby Universe (z<=0.3). We produced simple mock
X-ray observations for several instruments (XMM, Suzaku and ROSAT) and
extracted the statistics of potentially detectable bright clumps. Some of the
brightest clumps predicted by simulations may already have been already
detected in X- ray images with a large field of view. However, their small
projected size makes it difficult to prove their existence based on X-ray
morphology only. Preheating, AGN feedback and cosmic rays are found to have
little impact on the statistical properties of gas clumps.Comment: 17 pages, 11 figures. MNRAS accepte
Pointing to the minimum scatter: the generalized scaling relations for galaxy clusters
We introduce a generalized scaling law, M_tot = 10^K A^a B^b, to look for the
minimum scatter in reconstructing the total mass of hydrodynamically simulated
X-ray galaxy clusters, given gas mass M_gas, luminosity L and temperature T. We
find a locus in the plane of the logarithmic slopes and of the scaling
relations where the scatter in mass is minimized. This locus corresponds to b_M
= -3/2 a_M +3/2 and b_L = -2 a_L +3/2 for A=M_gas and L, respectively, and B=T.
Along these axes, all the known scaling relations can be identified (at
different levels of scatter), plus a new one defined as M_tot ~ (LT)^(1/2).
Simple formula to evaluate the expected evolution with redshift in the
self-similar scenario are provided. In this scenario, no evolution of the
scaling relations is predicted for the cases (b_M=0, a_M=1) and (b_L=7/2,
a_L=-1), respectively. Once the single quantities are normalized to the average
values of the sample under considerations, the normalizations K corresponding
to the region with minimum scatter are very close to zero. The combination of
these relations allows to reduce the number of free parameters of the fitting
function that relates X-ray observables to the total mass and includes the
self-similar redshift evolution.Comment: 6 pages, 3 figures. MNRAS in pres
Large-scale inhomogeneities of the intracluster medium: improving mass estimates using the observed azimuthal scatter
Using a set of hydrodynamical simulations of 62 galaxy clusters and groups we
study the ICM of inhomogeneities, focusing on the ones on the large scale that,
unlike clumps, are the most difficult to identify. To this purpose we introduce
the concept of residual clumpiness, C_R, that quantifies the large-scale
inhomogeneity of the ICM. After showing that this quantity can be robustly
defined for relaxed systems, we characterize how it varies with radius, mass
and dynamical state of the halo. Most importantly, we observe that it
introduces an overestimate in the determination of the density profile from the
X-ray emission, which translates into a systematic overestimate of 6 (12)% in
the measurement of M_gas at R_200 for our relaxed (perturbed) cluster sample.
At the same time, the increase of C_R with radius introduces also a ~2%
systematic underestimate in the measurement of the hydrostatic-equilibrium mass
(M_he), which adds to the previous one generating a systematic ~8.5%
overestimate in f_gas in our relaxed sample. Since the residual clumpiness of
the ICM is not directly observable, we study its correlation with the azimuthal
scatter in the X-ray surface brightness of the halo and in the y-parameter
profiles. We find that their correlation is highly significant (r_S = 0.6-0.7),
allowing to define the azimuthal scatter measured in the X-ray surface
brightness profile and in the y-parameter as robust proxies of C_R. After
providing a function that connects the two quantities, we obtain that
correcting the observed gas density profiles using the azimuthal scatter
eliminates the bias in the measurement of M_gas for relaxed objects, which
becomes (0+/-2)% up to 2R_200, and reduces it by a factor of 3 for perturbed
ones. This method allows also to eliminate the systematics on the measurements
of M_he and f_gas, although a significant halo to halo scatter remains.
(abridged)Comment: 18 pages, 17 figures, 3 tables. Submitted to MNRAS, revised after
referee's comment
Mass profiles and concentration-dark matter relation in X-ray luminous galaxy clusters
(Abriged) Assuming that the hydrostatic equilibrium holds between the
intracluster medium and the gravitational potential, we constrain the NFW
profiles in a sample of 44 X-ray luminous galaxy clusters observed with
XMM-Newton in the redshift range 0.1-0.3. We evaluate several systematic
uncertainties that affect our reconstruction of the X-ray masses. We measure
the concentration c200, the dark mass M200 and the gas mass fraction within
R500 in all the objects of our sample, providing the largest dataset of mass
parameters for galaxy clusters in this redshift range. We confirm that a tight
correlation between c200 and M200 is present and in good agreement with the
predictions from numerical simulations and previous observations. When we
consider a subsample of relaxed clusters that host a Low-Entropy-Core (LEC), we
measure a flatter c-M relation with a total scatter that is lower by 40 per
cent. From the distribution of the estimates of c200 and M200, with associated
statistical (15-25%) and systematic (5-15%) errors, we use the predicted values
from semi-analytic prescriptions calibrated through N-body numerical runs and
measure sigma_8*Omega_m^(0.60+-0.03)= 0.45+-0.01 (at 2 sigma level, statistical
only) for the subsample of the clusters where the mass reconstruction has been
obtained more robustly, and sigma_8*Omega_m^(0.56+-0.04) = 0.39+-0.02 for the
subsample of the 11 more relaxed LEC objects. With the further constraint from
the fgas distribution in our sample, we break the degeneracy in the
sigma_8-Omega_m plane and obtain the best-fit values sigma_8~1.0+-0.2
(0.75+-0.18 when the subsample of the more relaxed objects is considered) and
Omega_m = 0.26+-0.01.Comment: 21 pages. A&A in press. Minor revisions to match accepted version.
Corrected 2nd and 3rd column in Table 3, and equation (A.4
Helium abundance (and ) in X-COP galaxy clusters
We present the constraints on the helium abundance in 12 X-ray luminous
galaxy clusters that have been mapped in their X-ray and Sunyaev-Zeldovich (SZ)
signals out to for the XMM-Newton Cluster Outskirts Project (X-COP).
The unprecedented precision available for the estimate of allows us to
investigate how much the reconstructed X-ray and SZ signals are consistent with
the expected ratio between helium and proton densities of 0.08-0.1. We find
that a around 70 km/s/Mpc is preferred from our measurements, with lower
values of as requested from the Planck collaboration (67 km/s/Mpc)
requiring a 34% higher value of . On the other hand, higher values of ,
as obtained by measurements in the local universe, impose , from the
primordial nucleosynthesis calculations and current solar abundances, reduced
by 37--44\%.Comment: 7 pages. To appear as proceedings article for the XMM-Newton Workshop
"Astrophysics of Hot Plasma in Extended X-ray Sources" held at European Space
Astronomy Centre, Madrid, Spain, on 12-14 June 201
Gas clumping in galaxy clusters
The reconstruction of galaxy cluster's gas density profiles is usually
performed by assuming spherical symmetry and averaging the observed X-ray
emission in circular annuli. In the case of a very inhomogeneous and asymmetric
gas distribution, this method has been shown to return biased results in
numerical simulations because of the dependence of the X-ray emissivity.
We propose a method to recover the true density profiles in the presence of
inhomogeneities, based on the derivation of the azimuthal median of the surface
brightness in concentric annuli. We demonstrate the performance of this method
with numerical simulations, and apply it to a sample of 31 galaxy clusters in
the redshift range 0.04-0.2 observed with ROSAT/PSPC. The clumping factors
recovered by comparing the mean and the median are mild and show a slight trend
of increasing bias with radius. For , we measure a clumping factor
, which indicates that the thermodynamic properties and
hydrostatic masses measured in this radial range are only mildly affected by
this effect. Comparing our results with three sets of hydrodynamical numerical
simulations, we found that non-radiative simulations significantly overestimate
the level of inhomogeneities in the ICM, while the runs including cooling, star
formation, and AGN feedback reproduce the observed trends closely. Our results
indicate that most of the accretion of X-ray emitting gas is taking place in
the diffuse, large-scale accretion patterns rather than in compact structures.Comment: 12 pages, 11 figures, accepted for publication in MNRAS.
Largely-improved version compared to v1, method and comparison with
simulations update
On the occurrence of Radio Halos in galaxy clusters - Insight from a mass-selected sample
Giant radio halos (RH) are diffuse Mpc-scale synchrotron sources detected in
a fraction of massive and merging galaxy clusters. An unbiased study of the
statistical properties of RHs is crucial to constrain their origin and
evolution. We aim at investigating the occurrence of RHs and its dependence on
the cluster mass in a SZ-selected sample of galaxy clusters, which is as close
as possible to be a mass-selected sample. Moreover, we analyse the connection
between RHs and merging clusters. We select from the Planck SZ catalogue
(Planck Collaboration XXIX 2014) clusters with
at z=0.08-0.33 and we search for the presence of RHs using the NVSS for z<0.2
and the GMRT RH survey (GRHS, Venturi et al. 2007, 2008) and its extension
(EGRHS, Kale et al. 2013, 2015) for 0.2<z<0.33. We use archival Chandra X-ray
data to derive information on the clusters dynamical status. We confirm that RH
clusters are merging systems while the majority of clusters without RH are
relaxed, thus supporting the idea that mergers play a fundamental role in the
generation of RHs. We find evidence for an increase of the fraction of clusters
with RHs with the cluster mass and this is in line with expectations derived on
the basis of the turbulence re-acceleration scenario. Finally, we discuss the
effect of the incompleteness of our sample on this result.Comment: 11 pages, 7 figures, accepted for publication in A&
On the connection between turbulent motions and particle acceleration in galaxy clusters
Giant radio halos are Mpc-scale diffuse radio sources associated with the
central regions of galaxy clusters. The most promising scenario to explain the
origin of these sources is that of turbulent re-acceleration, in which MeV
electrons injected throughout the formation history of galaxy clusters are
accelerated to higher energies by turbulent motions mostly induced by cluster
mergers. In this Letter, we use the amplitude of density fluctuations in the
intracluster medium as a proxy for the turbulent velocity and apply this
technique to a sample of 51 clusters with available radio data. Our results
indicate a segregation in the turbulent velocity of radio halo and radio quiet
clusters, with the turbulent velocity of the former being on average higher by
about a factor of two. The velocity dispersion recovered with this technique
correlates with the measured radio power through the relation , which implies that the radio power is
nearly proportional to the turbulent energy rate. Our results provide an
observational confirmation of a key prediction of the turbulent re-acceleration
model and possibly shed light on the origin of radio halos.Comment: Submitted to ApJ Letter
The gas distribution in the outer regions of galaxy clusters
We present the analysis of a local (z = 0.04 - 0.2) sample of 31 galaxy
clusters with the aim of measuring the density of the X-ray emitting gas in
cluster outskirts. We compare our results with numerical simulations to set
constraints on the azimuthal symmetry and gas clumping in the outer regions of
galaxy clusters. We exploit the large field-of-view and low instrumental
background of ROSAT/PSPC to trace the density of the intracluster gas out to
the virial radius. We perform a stacking of the density profiles to detect a
signal beyond r200 and measure the typical density and scatter in cluster
outskirts. We also compute the azimuthal scatter of the profiles with respect
to the mean value to look for deviations from spherical symmetry. Finally, we
compare our average density and scatter profiles with the results of numerical
simulations. As opposed to some recent Suzaku results, and confirming previous
evidence from ROSAT and Chandra, we observe a steepening of the density
profiles beyond \sim r500. Comparing our density profiles with simulations, we
find that non-radiative runs predict too steep density profiles, whereas runs
including additional physics and/or treating gas clumping are in better
agreement with the observed gas distribution. We report for the first time the
high-confidence detection of a systematic difference between cool-core and
non-cool core clusters beyond \sim 0.3r200, which we explain by a different
distribution of the gas in the two classes. Beyond \sim r500, galaxy clusters
deviate significantly from spherical symmetry, with only little differences
between relaxed and disturbed systems. We find good agreement between the
observed and predicted scatter profiles, but only when the 1% densest clumps
are filtered out in the simulations. [Abridged]Comment: The data for the average profiles and individual clusters can be
downloaded at:
http://www.isdc.unige.ch/~deckert/newsite/The_Planck_ROSAT_project.htm
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