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 $Z=0.2Z_\odot$. We cannot derive accurate gradients for these quantities with our data, but there is strong evidence that the temperature is lower ($\sim 2.8 keV$) and the metallicity much higher (Z $\sim 0.8 Z_\odot$) 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 $M_{\rm Dyn}\sim (2.1-2.9)10^{14} 1/h_50 M$_{\odot}$,$M_{gas}/M_{Dyn}\sim 0.18 h_{50}^{-1.5}$. The stellar mass is$6.7\ 10^{12}M_{\odot}$, giving a mass to light ratio of$M/L_{V}\sim 300$. 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

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 ($z = 0.17$). 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 $\sim 4$ Gyr agoComment: ApJ accepted, 20 pages, 11 figures; Figure 1 fixe

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

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