509 research outputs found
Chandra constraints on the thermal conduction in the intracluster plasma of A2142
In this Letter, we use the recent Chandra observation of A2142 reported by
Markevitch et al. to put constraints on thermal conduction in the intracluster
plasma. We show that the observed sharp temperature gradient requires that
classical conductivity has to be reduced at least by a factor of between 250
and 2500. The result provides a direct constraint on an important physical
process relevant to the gas in the cores of clusters of galaxies.Comment: 3 pages. To appear in MNRA
Chandra detection of reflected X-ray emission from the type 2 QSO in IRAS 09104+4109
We present X-ray imaging spectroscopy of the extremely luminous infrared
galaxy IRAS 09104+4109 (z=0.442) obtained with the Chandra X-ray Observatory.
With the arcsec resolution of Chandra, an unresolved source at the nucleus is
separated from the surrounding cluster emission. A strong iron K line at 6.4
keV on a very hard continuum is detected from the nuclear source, rendering
IRAS 09104+4109 the most distant reflection-dominated X-ray source known.
Combined with the BeppoSAX detection of the excess hard X-ray emission, it
provides further strong support to the presence of a hidden X-ray source of
quasar luminosity in this infrared galaxy. Also seen is a faint linear
structure to the North, which coincides with the main radio jet. An X-ray
deficit in the corresponding region suggests an interaction between the cluster
medium and the jet driven by the active nucleus.Comment: 5 pages, accepted for publication as a Letter in MNRA
Magnetorotational instability in cool cores of galaxy clusters
Clusters of galaxies are embedded in halos of optically thin, gravitationally
stratified, weakly magnetized plasma at the system's virial temperature. Due to
radiative cooling and anisotropic heat conduction, such intracluster medium
(ICM) is subject to local instabilities, which are combinations of the thermal,
magnetothermal and heat-flux-driven buoyancy instabilities. If the ICM rotates
significantly, its stability properties are substantially modified and, in
particular, also the magnetorotational instability (MRI) can play an important
role. We study simple models of rotating cool-core clusters and we demonstrate
that the MRI can be the dominant instability over significant portions of the
clusters, with possible implications for the dynamics and evolution of the cool
cores. Our results give further motivation for measuring the rotation of the
ICM with future X-ray missions such as ASTRO-H and ATHENA.Comment: 17 pages, 10 figures, accepted for publication in Journal of Plasma
Physics, Special Issue "Complex Plasma Phenomena in the Laboratory and in the
Universe
Can giant radio halos probe the merging rate of galaxy clusters?
Radio and X-ray observations of galaxy clusters probe a direct link between
cluster mergers and giant radio halos (RH), suggesting that these sources can
be used as probes of the cluster merging rate with cosmic time. In this paper
we carry out an explorative study that combines the observed fractions of
merging clusters (fm) and RH (fRH) with the merging rate predicted by
cosmological simulations and attempt to infer constraints on merger properties
of clusters that appear disturbed in X-rays and of clusters with RH. We use
morphological parameters to identify merging systems and analyze the currently
largest sample of clusters with radio and X-ray data (M500>6d14 Msun, and
0.2<z<0.33, from the Planck SZ cluster catalogue). We found that in this sample
fm~62-67% while fRH~44-51%. The comparison of the theoretical f_m with the
observed one allows to constrain the combination (xi_m,tau_m), where xi_m and
tau_m are the minimum merger mass ratio and the timescale of merger-induced
disturbance. Assuming tau_m~ 2-3 Gyr, as constrained by simulations, we find
that the observed f_m matches the theoretical one for xi_m~0.1-0.18. This is
consistent with optical and near-IR observations of clusters in the sample
(xi_m~0.14-0.16). The fact that RH are found only in a fraction of merging
clusters may suggest that merger events generating RH are characterized by
larger mass ratio; this seems supported by optical/near-IR observations of RH
clusters in the sample (xi_min~0.2-0.25). Alternatively, RH may be generated in
all mergers but their lifetime is shorter than \tau_m (by ~ fRH/fm). This is an
explorative study, however it suggests that follow up studies using the
forthcoming radio surveys and adequate numerical simulations have the potential
to derive quantitative constraints on the link between cluster merging rate and
RH at different cosmic epochs and for different cluster masses.Comment: 10 pages, 3 figures, accepted for publication in A&
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
A deep Chandra observation of the cluster environment of the z=1.786 radio galaxy 3C294
We report the results from a 200 ks Chandra observation of the z=1.786 radio
galaxy 3C294 and its cluster environment, increasing by tenfold our earlier
observation. The diffuse emission, extending about 100 kpc around the nucleus,
has a roughly hourglass shape in the N-S direction with surprisingly sharp
edges to the N and S. The spectrum of the diffuse emission is well fitted by
either a thermal model of temperature 3.5 keV and abundance <0.9 solar
(2-sigma), or a power-law with photon index 2.3. If the emission is due to hot
gas then the sharp edges mean that it is probably not in hydrostatic
equilibrium. Much of the emission is plausibly due to inverse Compton
scattering of the Cosmic Microwave Background (CMB) by nonthermal electrons
produced earlier by the radio source. The required relativistic electrons would
be of much lower energy and older than those responsible for the present radio
lobes. This could account for the lack of detailed spatial correspondence
between the X-rays and the radio emission, the axis of which is at a position
angle of about 45 deg. Hot gas would still be required to confine the
relativistic plasma; the situation could parallel that of the radio bubbles
seen as holes in nearby clusters, except that in 3C294 the bubbles are bright
in X-rays owing to the extreme power in the source and the sixty fold increase
in the energy density of the CMB. The X-ray spectrum of the radio nucleus is
hard, showing a reflection spectrum and iron line. The source is therefore an
obscured radio-loud quasar.Comment: In press (MNRAS), 10 pages, 12 figures (2 colour
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
Evolution in the iron abundance of the ICM
We present a Chandra analysis of the X-ray spectra of 56 clusters of galaxies
at , which cover a temperature range of keV. Our analysis
is aimed at measuring the iron abundance in the ICM out to the highest redshift
probed to date. We find that the emission-weighted iron abundance measured
within in clusters below 5 keV is, on average, a factor of
higher than in hotter clusters, following , which confirms the trend seen in local samples. We made use of
combined spectral analysis performed over five redshift bins at
to estimate the average emission weighted iron abundance. We find a constant
average iron abundance as a function of redshift,
but only for clusters at . The emission-weighted iron abundance is
significantly higher () in the redshift range
, approaching the value measured locally in the inner radii for a mix of cool-core and non cool-core clusters in the
redshift range . The decrease in with can be
parametrized by a power law of the form . The observed
evolution implies that the average iron content of the ICM at the present epoch
is a factor of larger than at . We confirm that the ICM is
already significantly enriched () at a look-back time
of 9 Gyr. Our data provide significant constraints on the time scales and
physical processes that drive the chemical enrichment of the ICM.Comment: 4 pages, 4 figures, to appear in the Proceedings of "The Extreme
Universe in the Suzaku Era", Dicember 2006, Kyoto (Japan
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