30 research outputs found
BeppoSAX observation of the rich cluster of galaxies Abell 85
We report the observation of the Intra-Cluster Medium (ICM) of Abell 85 by the X-ray satellite BeppoSAX. We have both analysed the spectrum obtained in the central 8 arcmin circular region centred on the Very Steep Spectrum Radio Source (VSSRS) and the spectra from a number of sub-regions. Analysis of the spectra allowed us to independently obtain new estimates of the temperature, metallicity and line-of-sight hydrogen density column, both globally (T=6.6\pm0.3 keV, Z = 0.38\pm0.06 Z_\odot and N_H = 5.5^+0.9_-0.7 10^20 cm^-2) and locally. These measures are in good agreement with previous measures based on ROSAT and ASCA data. In the region of the VSRSS, we have tried to disentangle the thermal from the non-thermal X-ray emission. Although we could not do this unambiguously, we have nonetheless estimated the extended magnetic field using the radio spectrum available for this region. We obtain a lower limit intensity of 0.9 \mu G, consistent with our previous estimate. We also derive alpha-elements/iron abundance ratios that turn out to be higher than 1. Such a result tends to support the burst model for elliptical galaxies, where a strong galactic wind develops early in the galaxy history and type II supernovae (SN) may have the main role in the enrichment of the ICM. A two-temperature ICM model was fitted in the central region yielding a main component with roughly the mean cluster temperature and a cooler component with temperature less than 0.1 keV
The Brera Multi-scale Wavelet (BMW) ROSAT HRI source catalog. I: the algorithm
We present a new detection algorithm based on the wavelet transform for the
analysis of high energy astronomical images. The wavelet transform, due to its
multi-scale structure, is suited for the optimal detection of point-like as
well as extended sources, regardless of any loss of resolution with the
off-axis angle. Sources are detected as significant enhancements in the wavelet
space, after the subtraction of the non-flat components of the background.
Detection thresholds are computed through Monte Carlo simulations in order to
establish the expected number of spurious sources per field. The source
characterization is performed through a multi-source fitting in the wavelet
space. The procedure is designed to correctly deal with very crowded fields,
allowing for the simultaneous characterization of nearby sources. To obtain a
fast and reliable estimate of the source parameters and related errors, we
apply a novel decimation technique which, taking into account the correlation
properties of the wavelet transform, extracts a subset of almost independent
coefficients. We test the performance of this algorithm on synthetic fields,
analyzing with particular care the characterization of sources in poor
background situations, where the assumption of Gaussian statistics does not
hold. For these cases, where standard wavelet algorithms generally provide
underestimated errors, we infer errors through a procedure which relies on
robust basic statistics. Our algorithm is well suited for the analysis of
images taken with the new generation of X-ray instruments equipped with CCD
technology which will produce images with very low background and/or high
source density.Comment: 8 pages, 6 figures, ApJ in pres
The rich cluster of galaxies ABCG 85. II. X-ray analysis using the ROSAT HRI
We present a new X-ray analysis mainly based on ROSAT HRI data. The HRI
spatial resolution combined with an improved wavelet analysis method and with
complementary radio and optical data provides new results compared to a
previous paper based on ROSAT PSPC data (Pislar et al. 1997). We use also
redshift data in order to identify galaxies dynamically belonging to the main
body of the cluster and/or to superimposed substructures. Various kinds of
emission are superimposed on a mean thermal X-ray emission due to the
intra-cluster gas: a) an X-ray flux excess in the centre; b) a south blob,
partially generated by individual galaxies. The mean velocity and velocity
dispersion of the galaxies located in this region are the same as those of the
cluster as a whole: it therefore does not seem to be a bound subgroup; c) West
emission due to a foreground group with self-emission from a Seyfert galaxy
located at the north-west; d) emission in the south-west due to inverse Compton
emission associated to a very steep radio source (the remnant of an active
galactic nucleus). We have examined the possibility for the central peak to be
an "unusual" galaxy, as assumed for the central galaxy of J2310-43 (Tananbaum
et al. 1997). We conclude on the existence of a cooling flow region, in which
the presence of at least three small features certainly related to cooler blobs
is revealed by the wavelet analysis. We have performed a pixel-to-pixel
modelling of the double X-ray emission. The large scale emission component is
comparable to those derived from by the PSPC data and the small scale one is
interpreted as a cooling-flow. A multiphase gas model analysis leads to a mass
deposit of 50-150 M_\odot/yr.Comment: 11 pages, 6 figures, 3 tables, LaTeX Accepted for publication in
Astronomy & Astrophysics main journa
The rich cluster of galaxies ABCG 85.I. X-ray analysis
We present an X-ray analysis of the rich cluster ABCG 85 based on ROSAT PSPC
data. By applying an improved wavelet analysis, we show that our view of this
cluster is notably changed from what was previously believed (a main region and
a south blob). The main emission comes from the central part of the main body
of the cluster on which is superimposed that of a foreground group of galaxies.
The foreground group and the main cluster are separated (if redshifts are
cosmological) by 46 1/h_50 Mpc. The southern blob is clearly not a group: it is
resolved into X-ray emitting galaxies (in particular the second more luminous
galaxy of the main cluster). Several X-ray features are identified with bright
galaxies. We performed a spectral analysis and derived the temperature (T),
metallicity (Z) and hydrogen column density NH. The global quantities are:
T=4keV (in agreement with the velocity dispersion of 760km/s) and
. We cannot derive accurate gradients for these quantities with
our data, but there is strong evidence that the temperature is lower () and the metallicity much higher (Z ) in the very centre
(within about 50 1/h_50 kpc). We present a pixel by pixel method to model the
physical properties of the X-ray gas and derive its density distribution. We
apply classical methods to estimate the dynamical, gas and stellar masses, as
well as the cooling time and cooling flow characteristics. At the limiting
radius of the image (1.4 1/h_50 Mpc), we find _{\odot}M_{gas}/M_{Dyn}\sim 0.18 h_{50}^{-1.5}6.7\ 10^{12}M_{\odot}M/L_{V}\sim 300$.
The cooling time is estimated for different models, leading to a cooling radius
of 30-80 kpc depending on theComment: 14 pages incl 16 postscript figures available, 4 tables, corrected
stellar mass. Accepted for publication in Astronomy & Astrophysic
Metallicity Gradients in the Intracluster Gas of Abell 496
Analysis of spatially resolved ASCA spectra of the intracluster gas in Abell
496 confirms there are mild metal abundance enhancements near the center, as
previously found by White et al. (1994) in a joint analysis of Ginga LAC and
Einstein SSS spectra. Simultaneous analysis of spectra from all ASCA
instruments (SIS + GIS) shows that the iron abundance is 0.36 +- 0.03 solar
3-12' from the center of the cluster and rises ~50% to 0.53 +- 0.04 solar
within the central 2'. The F-test shows that this abundance gradient is
significant at the >99.99% level. Nickel and sulfur abundances are also
centrally enhanced. We use a variety of elemental abundance ratios to assess
the relative contribution of SN Ia and SN II to the metal enrichment of the
intracluster gas. We find spatial gradients in several abundance ratios,
indicating that the fraction of iron from SN Ia increases toward the cluster
center, with SN Ia accounting for ~50% of the iron mass 3-12' from the center
and ~70% within 2'. The increased proportion of SN Ia ejecta at the center is
such that the central iron abundance enhancement can be attributed wholly to SN
Ia; we find no significant gradient in SN II ejecta. These spatial gradients in
the proportion of SN Ia/II ejecta imply that the dominant metal enrichment
mechanism near the center is different than in the outer parts of the cluster.
We show that the central abundance enhancement is unlikely to be due to ram
pressure stripping of gas from cluster galaxies, or to secularly accumulated
stellar mass loss within the central cD. We suggest that the additional SN Ia
ejecta near the center is the vestige of a secondary SN Ia-driven wind from the
cD (following a more energetic protogalactic SN II-driven wind phase), which
was partially smothered in the cD due to its location at the cluster center.Comment: 25 pages AASTeX; 6 encapsulated PostScript figures; accepted for
publication in ApJ. Replaced with revised versio
Gemini and Chandra observations of Abell 586, a relaxed strong-lensing cluster
We analyze the mass content of the massive strong-lensing cluster Abell 586
(). We use optical data (imaging and spectroscopy) obtained with the
Gemini Multi-Object Spectrograph (GMOS) mounted on the 8-m Gemini-North
telescope, together with publicly available X-ray data taken with the
\textit{Chandra} space telescope. Employing different techniques -- velocity
distribution of galaxies, weak gravitational lensing, and X-ray spatially
resolved spectroscopy -- we derive mass and velocity dispersion estimates from
each of them. All estimates agree well with each other, within a 68% confidence
level, indicating a velocity dispersion of 1000 -- 1250 \kms. The projected
mass distributions obtained through weak-lensing and X-ray emission are
strikingly similar, having nearly circular geometry. We suggest that Abell 586
is probably a truly relaxed cluster, whose last major merger occurred more than
Gyr agoComment: ApJ accepted, 20 pages, 11 figures; Figure 1 fixe
Gravitational lensing in low-redshift clusters of galaxies: the arc-like object in Abell 3408 and its lensing interpretation
We analyze the seldomly discussed lensing effects expected in low-z clusters
(z = 0.05-0.15), using as an example the bright arc (z=0.073) discovered by
Campusano and Hardy(1996) near the dominant cD galaxy of the cluster Abell 3408
(z=0.042). We present photometric and spectroscopic observations for both the
dominant galaxy and the arc. The mass distribution in A3408 is modeled by
scaled versions of the representative distributions derived from studies of
clusters at higher redshifts. The two gravitational potentials considered are:
i) a ``minimum'' mass case where the mass distribution follows the light
profile of the central elliptical galaxy and, ii) a ``maximum'' mass case where
a typical massive dark halo is added to the previous case. The observed arc is
well reproduced by both models, but rather small magnifications of the source
galaxy are implied. The source galaxy is tentatively identified in both the
lensing and non-lensing scenarios as being a spiral. The smaller lensed spiral
(14.6 h_50^{-1} kpc, M_B=-18.2) predicted by the dark halo model appears to fit
the observations marginally better. Furthermore, we found that only the dark
halo model predicts a measurable amount of weak shear in the images of faint
background galaxies. We conclude that observations, under very good seeing
conditions, of week shear in faint background galaxies in the direction of
low-redshift galaxy clusters are possible. When the latter are combined with
X-ray data, a powerful tool to probe the mass distribution in the very central
region of galaxy clusters emerges.Comment: 7 pages, 3 figures, ApJ Letters in press (accepted December 5, 1997
Radial Temperature Profiles of X-Ray--Emitting Gas Within Clusters of Galaxies
Previous analyses of ASCA data of clusters of galaxies have found conflicting
results regarding the slope of the temperature profile of the hot X-ray gas
within clusters, mainly because of the large, energy-dependent point spread
function (PSF) of the ASCA mirrors. We present a summary of all ASCA-determined
cluster temperature profiles found in the literature, and find a discrepancy in
the radial temperature trend of clusters based on which PSF-correction routine
is used. This uncertainty in the cluster temperature profile in turn can lead
to large uncertainties in the amount of dark matter in clusters. In this study,
we have used ROSAT PSPC data to obtain independent relative temperature
profiles for 26 clusters, most of which have had their temperature profiles
determined by ASCA. Our aim is not to measure the actual temperature values of
the clusters, but to use X-ray color profiles to search for a hardening or
softening of the spectra with radius for comparison to ASCA-derived profiles.
The radial color profiles indicate that outside of the cooling flow region, the
temperature profiles of clusters are in general constant. Within 35% of the
virial radius, we find a temperature drop of 20% at 10 keV and 12% at 5 keV can
be ruled out at the 99% confidence level. A subsample of non-cooling flow
clusters shows that the condition of isothermality applies at very small radii
too, although cooling gas complicates this determination in the cooling flow
subsample. The colors predicted from the temperature profiles of a series of
hydrodynamical cluster simulations match the data very well, although they
cannot be used to discriminate among different cosmologies. An additional
result is that the color profiles show evidence for a central peak in
metallicity in low temperature clusters.Comment: 39 pages, 15 embedded Postscript figures, uses aaspp4.sty, accepted
for publication in Astrophysical Journa