398 research outputs found
The cluster gas mass fraction as a cosmological probe: a revised study
(Abriged) We present the analysis of the baryonic content of 52 X-ray
luminous galaxy clusters observed with Chandra in the redshift range 0.3-1.273.
We use the deprojected X-ray surface brightness profiles and the measured
values of the gas temperature to recover the gas and total mass profiles. By
assuming that galaxy clusters are representative of the cosmic baryon budget,
the distribution of the cluster baryon fraction in the hottest (T> 4 keV)
systems as a function of redshift is used to constrain the cosmological
parameters. We discuss how our constraints are affected by several systematics,
namely the isothermality, the assumed baryon fraction in stars, the depletion
parameter and the sample selection. By using only the cluster baryon fraction
as a proxy for the cosmological parameters, we obtain that Omega is very well
constrained at the value of 0.35 with a relative statistical uncertainty of 11%
(1 sigma level; w=-1) and a further systematic error of about (-6,+7)%. On the
other hand, constraints on Lambda (without the prior of flat geometry) and w
(using the prior of flat geometry) are definitely weaker due to the presence of
larger statistical and systematic uncertainties (of the order of 40 per cent on
Lambda and larger than 50 per cent on w). If the WMAP 5-year best-fit results
are assumed to fix the cosmological parameters, we limit the contributions
expected from non-thermal pressure support and ICM clumpiness to be lower than
about 10 per cent, leaving also room to accommodate baryons not accounted for
either in the X-ray emitting plasma or in stars of the order of 18 per cent of
the total cluster baryon budget.Comment: A&A in press. Accepted on March 28, 2009. Revised to match version in
prin
On the evolution of cooling cores in X-ray galaxy clusters
(Abridged) To define a framework for the formation and evolution of the
cooling cores in X-ray galaxy clusters, we study how the physical properties
change as function of the cosmic time in the inner regions of a 4 keV and 8 keV
galaxy cluster under the action of radiative cooling and gravity only. The
cooling radius, R_cool, defined as the radius at which the cooling time equals
the Universe age at given redshift, evolves from ~0.01 R200 at z>2, where the
structures begin their evolution, to ~0.05 R200 at z=0. The values measured at
0.01 R200 show an increase of about 15-20 per cent per Gyr in the gas density
and surface brightness and a decrease with a mean rate of 10 per cent per Gyr
in the gas temperature. The emission-weighted temperature diminishes by about
25 per cent and the bolometric X-ray luminosity rises by a factor ~2 after 10
Gyrs when all the cluster emission is considered in the computation. On the
contrary, when the core region within 0.15 R500 is excluded, the gas
temperature value does not change and the X-ray luminosity varies by 10-20 per
cent only. The cooling time and gas entropy radial profiles are well
represented by power-law functions. The behaviour of the inner slopes of the
gas temperature and density profiles are the most sensitive and unambiguous
tracers of an evolving cooling core. Their values after 10 Gyrs of radiative
losses, T_gas ~ r^0.4 and n_gas ~ r^(-1.2) for the hot (cool) object, are
remarkably in agreement with the observational constraints available for nearby
X-ray luminous cooling core clusters. Because our simulations do not consider
any AGN heating, they imply that the feedback process does not greatly alter
the gas density and temperature profiles as generated by radiative cooling
alone.Comment: 8 pages. MNRAS in pres
Detecting shocked intergalactic gas with X-ray and radio observations
Detecting the thermal and non-thermal emission from the shocked cosmic gas
surrounding large-scale structures represents a challenge for observations, as
well as a unique window into the physics of the warm-hot intergalactic medium.
In this work, we present synthetic radio and X-ray surveys of large
cosmological simulations in order to assess the chances of jointly detecting
the cosmic web in both frequency ranges. We then propose best observing
strategies tailored for existing (LOFAR, MWA and XMM) or future instruments
(SKA-LOW and SKA-MID, ATHENA and eROSITA). We find that the most promising
targets are the extreme peripheries of galaxy clusters in an early merging
stage, where the merger causes the fast compression of warm-hot gas onto the
virial region. By taking advantage of a detection in the radio band, future
deep X-ray observations will probe this gas in emission, and help us to study
plasma conditions in the dynamic warm-hot intergalactic medium with
unprecedented detail.Comment: 22 pages, 25 Figures. A\&A accepted, in press. Moderate revision
compared to version 1, with a few new figure
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
A Chandra archival study of the temperature and metal abundance profiles in hot Galaxy Clusters at 0.1 < z < 0.3
We present the analysis of the temperature and metallicity profiles of 12
galaxy clusters in the redshift range 0.1--0.3 selected from the Chandra
archive with at least ~20,000 net ACIS counts and kT>6 keV. We divide the
sample between 7 Cooling-Core (CC) and 5 Non-Cooling-Core (NCC) clusters
according to their central cooling time. We find that single power-laws can
describe properly both the temperature and metallicity profiles at radii larger
than 0.1 r_180 in both CC and NCC systems, showing the NCC objects steeper
profiles outwards. A significant deviation is only present in the inner 0.1
r_180. We perform a comparison of our sample with the De Grandi & Molendi
BeppoSAX sample of local CC and NCC clusters, finding a complete agreement in
the CC cluster profile and a marginally higher value (at ~1sigma) in the inner
regions of the NCC clusters. The slope of the power-law describing kT(r) within
0.1 r_180 correlates strongly with the ratio between the cooling time and the
age of the Universe at the cluster redshift, being the slope >0 and
tau_c/tau_age<=0.6 in CC systems.Comment: 12 pages, 6 figures, Accepted for publication by the Astrophysical
Journa
Weighing simulated galaxy clusters using lensing and X-ray
We aim at investigating potential biases in lensing and X-ray methods to
measure the cluster mass profiles. We do so by performing realistic simulations
of lensing and X-ray observations that are subsequently analyzed using
observational techniques. The resulting mass estimates are compared among them
and with the input models. Three clusters obtained from state-of-the-art
hydrodynamical simulations, each of which has been projected along three
independent lines-of-sight, are used for this analysis. We find that strong
lensing models can be trusted over a limited region around the cluster core.
Extrapolating the strong lensing mass models to outside the Einstein ring can
lead to significant biases in the mass estimates, if the BCG is not modeled
properly for example. Weak lensing mass measurements can be largely affected by
substructures, depending on the method implemented to convert the shear into a
mass estimate. Using non-parametric methods which combine weak and strong
lensing data, the projected masses within R200 can be constrained with a
precision of ~10%. De-projection of lensing masses increases the scatter around
the true masses by more than a factor of two due to cluster triaxiality. X-ray
mass measurements have much smaller scatter (about a factor of two smaller than
the lensing masses) but they are generally biased low by 5-20%. This bias is
ascribable to bulk motions in the gas of our simulated clusters. Using the
lensing and the X-ray masses as proxies for the true and the hydrostatic
equilibrium masses of the simulated clusters and averaging over the cluster
sample we are able to measure the lack of hydrostatic equilibrium in the
systems we have investigated.Comment: 27 pages, 21 figures, accepted for publication on A&A. Version with
full resolution images can be found at
http://pico.bo.astro.it/~massimo/Public/Papers/massComp.pd
Apparent high metallicity in 3-4 keV galaxy clusters: the inverse iron-bias in action in the case of the merging cluster Abell 2028
Recent work based on a global measurement of the ICM properties find evidence
for an increase of the iron abundance in galaxy clusters with temperature
around 2-4 keV up to a value about 3 times larger than that typical of very hot
clusters. We have started a study of the metal distribution in these objects
from the sample of Baumgartner et al. (2005), aiming at resolving spatially the
metal content of the ICM. We report here on a 42ks XMM observation of the first
object of the sample, the cluster Abell 2028. The XMM observation reveals a
complex structure of the cluster over scale of 300 kpc, showing an interaction
between two sub-clusters in cometary-like configurations. At the leading edges
of the two substructures cold fronts have been detected. The core of the main
subcluster is likely hosting a cool corona. We show that a one-component fit
for this region returns a biased high metallicity. This inverse iron bias is
due to the behavior of the fitting code in shaping the Fe-L complex. In
presence of a multi-temperature structure of the ICM, the best-fit metallicity
is artificially higher when the projected spectrum is modeled with a single
temperature component and it is not related to the presence of both Fe-L and
Fe-K emission lines in the spectrum. After accounting for the bias, the overall
abundance of the cluster is consistent with the one typical of hotter, more
massive clusters. We caution the interpretation of high abundances inferred
when fitting a single thermal component to spectra derived from relatively
large apertures in 3-4 keV clusters, because the inverse iron bias can be
present. Most of the inferences trying to relate high abundances in 3-4 keV
clusters to fundamental physical processes will likely have to be revised.Comment: 13 pages, 8 figures.Accepted for publication in Astronomy and
Astrophysycs. Minor changes to match published versio
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