arXiv subjects

S. Ettori

Publications and source records attributed to S. Ettori.

204 records · Page 12Linked to original sources

arXiv1999

Is the Butcher-Oemler effect a function of the cluster redshift ?

Using PSPC {\it Rosat} data, we measure x-ray surface brightness profiles, size and luminosity of the Butcher-Oemler (BO) sample of clusters of galaxies. The cluster x-ray size, as measured by the Petrosian r_{η=2} radius, does not change with redshift and is independent from x-ray luminosity. On the other hand, the x-ray luminosity increases with redshift. Considering that fair samples show no-evolution, or negative luminosity evolution, we conclude that the BO sample is not formed from the same class of objects observed at different look-back times. This is in conflict with the usual interpretation of the Butcher-Oemler as an evolutionary (or redshift-dependent) effect, based on the assumption that we are comparing the same class of objects at different redshifts. Other trends present in the BO sample reflect selection criteria rather than differences in look-back time, as independently confirmed by the fact that trends loose strength when we enlarge the sample with x-ray selected sample of clusters. The variety of optical sizes and shapes of the clusters in the Butcher-Oemler sample, and the Malmquist-like bias, are the reasons for these selection effects that mimic the trends usually interpreted as changes due to evolution.

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arXiv1998

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 ($\sim 2.4 h_{50}^{-1}$ 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 $N_{H}$ over the region, and cool components in the core and in the eastern sector. We apply the $β$-model to the profiles from different sectors and present a solution to the, so-called, $β$-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 $\times 10^{43} erg/s$, 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 $β$-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, $r_{200}$, at $2.7 h_{50}^{-1}$ Mpc. Within $2.3 h_{50}^{-1}$ Mpc ($0.85 r_{200}$), the total mass in the Perseus cluster is $1.2 \times 10^{15} M_{\odot}$ and its gas fraction is about 30 per cent.

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arXiv1997

Coulomb interactions in the intracluster medium

In this paper we discuss the effect of Coulomb collisions on the temperature profiles of the intracluster medium in clusters of galaxies, motivated by recent reports of negative temperature gradients in some clusters by Markevitch et al. The timescale for electrons and protons to reach temperature equilibrium can exceed a few billion years beyond radii of a Mpc, if the intracluster gas is assumed to be at the usual cluster virial temperature. If a cluster merger has occurred within that time causing the protons, but not the electrons, to be rapidly heated then a small negative temperature gradient can result. This gradient is larger in clusters with high temperatures and steep density profiles. Applying these considerations to the cluster of galaxies A2163, we conclude that, more plausibly, the observed gradient is due to a lack of hydrostatic equilibrium following a merger.

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arXiv1997

On the mass distribution in the Shapley Supercluster inferred from X-ray observations

We present an analysis of a mosaic of ROSAT PSPC and Einstein Observatory IPC X-ray observations of 14 clusters and 2 groups of galaxies, enclosed in a sky area 15 degrees by 20 degrees centred on A3558 within the Shapley Supercluster region. From the mass of each cluster, extrapolated to a density contrast of 500, we define 4 large structures: (1) the core of the Shapley Supercluster with radius 13 $h_{50}^{-1}$ Mpc; (2) the core plus A1736; (3) the core, A1736 and the western extension (A3528-A3530-A3532) and A3571; and (4), adding the northern cluster pair A1631-A1644 to structure (3), the Supercluster as a whole enclosed within a radius $\sim 90 h_{50}^{-1}$ Mpc. The observed total masses range between 3.5-8.5 $\times 10^{15} M_{\odot}$. The mass values derived from the observed intracluster gas (assuming a baryon fraction consistent with primordial nucleosynthesis) are of the order of few times $10^{16} M_{\odot}$. Given these estimates, the core is a bound structure with a very significant overdensity of at least 5 times the critical density, indicating that is approaching maximum expansion before collapsing. Structure (2) has an overdensity of 1.7 on a scale of $\sim 30 h^{-1}_{50}$ Mpc. The core is then a 2--3.7 sigma fluctuation in an initial Gaussian perturbation field, normalized for galaxies and clusters. The highest value applies if we extrapolate to a density contrast of 200 and assume that $Ω_b < 0.095$. The baryon fraction of the core of the Shapley Supercluster, with A1736, is about 15 per cent over a radius of 28 $h^{-1}_{50}$ Mpc.

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arXiv1996

A Study of the Large-Scale Distribution of Galaxies in the South Galactic Pole Region: II. Further Evidence for a Preferential Clustering Scale?

We analyse a set of new pencil-beam galaxy redshift data in three small regions around the South Galactic Pole (SGP) area. We investigate whether we can find any evidence of the quasi-periodic peaks discovered by Broadhurst et al. (1990) in the distribution of galaxies along the NGP-SGP directions. We use both a power spectrum analysis and a cross-correlation with a sliding comb-like window (the comb-template technique). Despite the data are less deep (~600 h^{-1} Mpc) and certainly not optimal for such an investigation, there is evidence of the same preferential ~130 h^{-1} Mpc scale in two fields displaced respectively 15 and 30 degrees West of the Broadhurst et al. original probe. Taken alone, however, this peak would not be statistically distinguishable from a noise fluctuation. Nevertheless, the statistical significance of the detection raises to 99% when one considers the CONDITIONAL probability of finding a peak around THE SAME scale measured by Broadhurst et al.

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arXiv1995

A Study of the Large--Scale Distribution of Galaxies in the South Galactic Pole Region: I. The Data

We present the data from an extensive, moderately deep (b_J = 19.5) spectroscopic survey of $\sim 600$ galaxies within four regions of sky located near the South Galactic Pole. About 75% of the measured galaxies are in a 3deg x 1.5deg region dominated by the rich cluster Klemola 44 (Abell 4038). The other three smaller areas cover about 1 square degree each. Here we discuss in detail the observing and data reduction strategies, the completeness and errors on the measured redshifts. The data collected are being used for: (1) a study of the large--scale redshift distribution of the galaxies in each field, and (2) a thorough dynamical investigation of Klemola 44. Results from these analyses will be presented in forthcoming papers.

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