888 research outputs found
An XMM-Newton study of the sub-structure in M87's halo
The high signal to noise and good point spread function of XMM have allowed
the first detailed study of the interaction between the thermal and radio
emitting plasma in the central regions of M87. We show that the X-ray emitting
structure, previously seen by ROSAT, is thermal in nature and that the east and
southwest extensions in M87's X-ray halo have a significantly lower temperature
(kT= 1.5 keV) than the surrounding ambient medium (kT= 2.3 keV). There is
little or no evidence for non-thermal emission with an upper limit on the
contribution of a power law component of spectral index flatter than 3 being
less than 1% of the flux in the region of the radio lobes.Comment: 6 pages, 8 color figures, to be published in A&A, number 36
A New Radio - X-Ray Probe of Galaxy Cluster Magnetic Fields
Results are presented of a new VLA-ROSAT study that probes the magnetic field
strength and distribution over a sample of 16 ``normal'' low redshift (z < 0.1)
galaxy clusters. The clusters span two orders of magnitude in X-ray luminosity,
and were selected to be free of (unusual) strong radio cluster halos, and
widespread cooling flows. Consistent with these criteria, most clusters show a
relaxed X-ray morphology and little or no evidence for recent merger activity.
Analysis of the rotation measure (RM) data shows cluster-generated Faraday RM
excess out to ~0.5 Mpc from cluster centers. The results, combined with RM
imaging of cluster-embedded sources and ROSAT X-ray profiles indicates that the
hot intergalactic gas within these ``normal'' clusters is permeated with a high
filling factor by magnetic fields at levels of = 5-10 (l/10 kpc)^{-1/2}
microGauss, where l is the field correlation length. These results lead to a
global estimate of the total magnetic energy in clusters, and give new insight
into the ultimate energy origin, which is likely gravitational. These results
also shed some light on the cluster evolutionary conditions that existed at the
onset of cooling flows.Comment: 6 pages, 1 figure, uses emulateapj5.sty, accepted by ApJ
A Systematic Study of X-Ray Substructure of Galaxy Clusters Detected in the ROSAT All-Sky Survey
Results of a systematic study of substructure in X-ray surface brightness distributions of a combined sample of 470 REFLEX+BCS clusters of galaxies are presented. The fully automized morphology analysis is based on data of the 3rd processing of the ROSAT All-Sky survey (RASS-3). After correction for several systematic effects, percent of the REFLEX+BCS clusters are found to be substructured in metric apertures of 1 Mpc radius (). Future simulations will show statistically which mass spectrum of major and minor mergers contributes to this number. Another important result is the discovery of a substructure-density relation, analogous to the morphology-density relation for galaxies. Here, clusters with asymmetric or multi-modal X-ray surface brightness distributions are located preferentially in regions with higher cluster number densities. The substructure analyses techniques are used to compare the X-ray morphology of 53 clusters with radio halos and relics, and 22 cooling flow clusters with the REFLEX+BCS reference sample. After careful equalization of the different `sensitivities' of the subsamples to substructure detection it is found that the halo and relic sample tends to show more often multi-modal and elongated X-ray surface brightness distributions compared to the REFLEX+BCS reference sample. The cooling flow clusters show more often circular symmetric and unimodal distributions compared to the REFLEX+BCS and the halo/relic reference samples. Both findings further support the idea that radio halos and relics are triggered by merger events, and that pre-existing cooling flows might be disrupted by recent major mergers
Cosmic Mass Functions from Gaussian Stochastic Diffusion Processes
Gaussian stochastic diffusion processes are used to derive cosmic mass functions. To get analytic relations previous studies exploited the sharp -space filter assumption yielding zero drift terms in the corresponding Fokker-Planck (Kolmogorov's forward) equation and thus simplifying analytic treatments significantly (excursion set formalism). In the present paper methods are described to derive for given diffusion processes and Gaussian random fields the corresponding mass and filter functions by solving the Kolmogorov's forward and backward equations including nonzero drift terms. This formalism can also be used in cases with non-sharp -space filters and for diffusion processes exhibiting correlations between different mass scales
Quasi-hydrostatic intracluster gas under radiative cooling
Quasi-hydrostatic cooling of the intracluster gas is studied. In the
quasi-hydrostatic model, work done by gravity on the inflow gas with dP \neq 0,
where P is the gas pressure, is taken into account in the thermal balance. The
gas flows in from the outer part so as to compensate the pressure loss of the
gas undergoing radiative cooling, but the mass flow is so moderate and smooth
that the gas is considered to be quasi-hydrostatic. The temperature of the
cooling gas decreases toward the cluster center, but, unlike cooling flows with
dP = 0, approaches a constant temperature of \sim 1/3 the temperature of the
non-cooling ambient gas. This does not mean that gravitational work cancels out
radiative cooling, but means that the temperature of the cooling gas appears to
approach a constant value toward the cluster center if the gas maintains the
quasi-hydrostatic balance. We discuss the mass flow in quasi-hydrostatic
cooling, and compare it with the standard isobaric cooling flow model. We also
discuss the implication of \dot{M} for the standard cooling flow model.Comment: 5 pages, 1 figure, accepted for publication in A&
The XMM/2dF survey III: Comparison between optical and X-ray cluster detection methods
We directly compare X-ray and optical techniques of cluster detection by
combining SDSS photometric data with a wide-field ( deg)
XMM-{\em Newton} survey in the North Galactic Pole region. The optical cluster
detection procedure is based on merging two independent selection methods - a
smoothing+percolation technique, and a Matched Filter Algorithm. The X-ray
cluster detection is based on a wavelet-based algorithm, incorporated in the
SAS v.5.2 package. The final optical sample counts 9 candidate clusters with
richness of more than 20 galaxies, corresponding roughly to APM richness class.
Three, of our optically detected clusters are also detected in our X-ray
survey. The most probable cause of the small number of optical cluster
candidates detected in our X-ray survey is that they are relatively poor
clusters, fainter than the X-ray flux limit (for extended sources) of our
survey .Comment: 9 pages, 5 figures, MNRAS accepte
The Shapley super-cluster. New X-ray detections and mass distribution
The largest and the deepest super-structure known today is the Shapley
super-cluster. This is the sky area with the highest over-density of galaxy
clusters and therefore also an ideal region to test the effects of a high
density environment on galaxies and on clusters. We performed an X-ray survey
of a wide region surrounding the Shapley super-structure. Additionally to
previously known super-cluster X-ray members, we identified diffuse X-ray
emission from 35 cluster candidates without previous X-ray detection. 21 of
them were previously known, optically selected super-cluster members, while the
other candidates had not been previously detected in any wavelength range.
Optical follow-up observations revealed that at least four of these new
candidates also have optical cluster counterparts. The super-cluster shows a
slightly flattened and elongated morphology. Clusters outside the central dense
core are preferentially located in four perpendicular filaments in a similar
way to what is seen in simulations of Large Scale Structure. We measure the
cluster number density in the region to be more than one order of magnitude
higher than the mean density of rich Abell clusters previously observed at
similar Galactic latitudes; this over-density, in the super-cluster outskirts,
is mainly due to an excess of low X-ray luminous clusters (with respect to an
average population), which leads us to think that the whole region is still
accreting low luminosity, small objects from the outskirts. Pushing our total
X-ray mass estimate to fainter clusters would drastically increase the total
super-cluster mass measure, because of the presence of the rich X-ray low
luminosity population.Comment: 27 pages; accepted for publication in A&A. A version of the paper
with higher resolution images can be downloaded at:
http://people.na.infn.it/~betty/publications_files/Shapley.ps.g
Detection of an X-Ray Hot Region in the Virgo Cluster of Galaxies with ASCA
Based on mapping observations with ASCA, an unusual hot region with a spatial
extent of 1 square degree was discovered between M87 and M49 at a center
coordinate of R. A. = 12h 27m 36s and Dec. = (J2000). The X-ray
emission from the region has a 2-10 keV flux of ergs
s cm and a temperature of keV, which is
significantly higher than that in the surrounding medium of keV. The
internal thermal energy in the hot region is estimated to be ergs with a gas density of cm. A power-law
spectrum with a photon index is also allowed by the data. The hot
region suggests there is an energy input due to a shock which is probably
caused by the motion of the gas associated with M49, infalling toward the M87
cluster with a velocity km s.Comment: 12 pages, 3 figures, accepted to ApJ
Feedback under the microscope: thermodynamic structure and AGN driven shocks in M87
(abridged) Using a deep Chandra exposure (574 ks), we present high-resolution
thermodynamic maps created from the spectra of 16,000 independent
regions, each with 1,000 net counts. The excellent spatial resolution of
the thermodynamic maps reveals the dramatic and complex temperature, pressure,
entropy and metallicity structure of the system. Excluding the 'X-ray arms',
the diffuse cluster gas at a given radius is strikingly isothermal. This
suggests either that the ambient cluster gas, beyond the arms, remains
relatively undisturbed by AGN uplift, or that conduction in the intracluster
medium (ICM) is efficient along azimuthal directions. We confirm the presence
of a thick (40 arcsec or 3 kpc) ring of high pressure gas at a
radius of 180 arcsec (14 kpc) from the central AGN. We verify that
this feature is associated with a classical shock front, with an average Mach
number M = 1.25. Another, younger shock-like feature is observed at a radius of
40 arcsec (3 kpc) surrounding the central AGN, with an estimated
Mach number M > 1.2. As shown previously, if repeated shocks occur every
10 Myrs, as suggested by these observations, then AGN driven weak shocks
could produce enough energy to offset radiative cooling of the ICM. A high
significance enhancement of Fe abundance is observed at radii 350 - 400 arcsec
(27 - 31 kpc). This ridge is likely formed in the wake of the rising bubbles
filled with radio-emitting plasma that drag cool, metal-rich gas out of the
central galaxy. We estimate that at least solar masses of
Fe has been lifted and deposited at a radius of 350-400 arcsec; approximately
the same mass of Fe is measured in the X-ray bright arms, suggesting that a
single generation of buoyant radio bubbles may be responsible for the observed
Fe excess at 350 - 400 arcsec.Comment: 18 pages, 16 figures. Accepted to 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
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