8,027 research outputs found
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
A textbook example of ram-pressure stripping in the Hydra A/A780 cluster
In the current epoch, one of the main mechanisms driving the growth of galaxy clusters is the continuous accretion of group-scale halos. In this process, the ram pressure applied by the hot intracluster medium on the gas content of the infalling group is responsible for stripping the gas from its dark-matter halo, which gradually leads to the virialization of the infalling gas in the potential well of the main cluster. Using deep wide-field observations of the poor cluster Hydra A/A780 with XMM-Newton and Suzaku, we report the discovery of an infalling galaxy group 1.1 Mpc south of the cluster core. The presence of a substructure is confirmed by a dynamical study of the galaxies in this region. A wake of stripped gas is trailing behind the group over a projected scale of 760 kpc. The temperature of the gas along the wake is constant at kT ~ 1.3 keV, which is about a factor of two less than the temperature of the surrounding plasma. We observe a cold front pointing westwards compared to the peak of the group, which indicates that the group is currently not moving in the direction of the main cluster, but is moving along an almost circular orbit. The overall morphology of the group bears remarkable similarities with high-resolution numerical simulations of such structures, which greatly strengthens our understanding of the ram-pressure stripping process
Spatial Wavelet Statistics of SAR Backscatter for Characterizing Degraded Forest:A Case Study From Cameroon
A Soft X-ray Component in the Abell 754 Cluster
We have analyzed the Chandra, BeppoSax, and ROSAT observations of Abell 754
and report evidence of a soft, diffuse X-ray component. The emission is peaked
in the cluster center and is detected out to 8' from the X-ray center. Fitting
a thermal model to the combined BeppoSax and PSPC spectra show excess emission
below 1 keV in the PSPC and above 100 keV in the BeppoSax PDS. The source 26W20
is in the field of view of the PDS. The addition of a powerlaw with the
spectral parameters measured by Silverman et al. (1998) for 26W20 successfully
models the hard component in the PDS. The remaining excess soft emission can be
modeled by either a low temperature, 0.75 - 1.03 keV component, or by a
powerlaw with a steep spectral index, 2.3. Addition of a second thermal
component model provides a much better fit to the data than does the addition
of a non-thermal component. The Chandra temperature map does not show any
region cooler than 6.9 keV within the region where the cool component was
detected. Simulations of the emission from embedded groups were performed and
compared with the Chandra temperature map which show groups are a plausible
source of ~1 keV emission. The cool component is centrally peaked in the
cluster and the gas density and temperature are relatively high arguing against
the WHIM as the source of the X-ray emission. X-ray emission from elliptical
galaxies is not high enough to provide the total cool component luminosity,
7.0x10^43 ergs s^-1. The peak of the cool component is located between the low
frequency radio halos arguing against a non-thermal interpretation for the
emission. We conclude that emission from embedded groups is the most likely
origin of the cool component in Abell 754.Comment: Submitted to Ap
Iron Abundance Profiles of 12 Clusters of Galaxies Observed With BeppoSAX
We have derived azimuthally-averaged radial iron abundance profiles of the
X-ray gas contained within 12 clusters of galaxies with redshift 0.03 < z < 0.2
observed with BeppoSAX. We find evidence for a negative metal abundance
gradient in most of the clusters, particularly significant in clusters that
possess cooling flows. The composite profile from the 12 clusters resembles
that of cluster simulations of Metzler & Evrard (1997). This abundance gradient
could be the result of the spatial distribution of gas-losing galaxies within
the cluster being more centrally condensed than the primordial hot gas. Both
inside and outside the core region, we find a higher abundance in cooling flow
clusters than in non-cooling flow clusters. Outside of the cooling region this
difference cannot be the result of more efficient sputtering of metals into the
gaseous phase in cooling flow clusters, but might be the result of the mixing
of low metallicity gas from the outer regions of the cluster during a merger.Comment: 8 pages, 2 embedded Postscript figures, accepted by Astrophysical
Journa
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