Gas entropy in a representative sample of nearby X-ray galaxy clusters (REXCESS): relationship to gas mass fraction

By G. Pratt, M. Arnaud, R. Piffaretti, H. Böhringer, Trevor J. Ponman, Judith Croston, G. Mark Voit, Stefano Borgani, Richard G. Bower. 2009

243 citations recorded by the source corpus.

We examine the radial entropy distribution and its scaling using 31 nearby galaxy clusters from the Representative XMM-Newton Cluster Structure Survey (REXCESS), a sample in the temperature range 2-9 keV selected in X-ray luminosity only, with no bias toward any particular morphological type. The entropy profi les are robustly measured at least out to R1000 in all systems and out to R500 in thirteen systems. Compared to theoretical expectations from non-radiative cosmological simulations, the observed distributions show a radial and mass-dependent excess entropy, such that the excess is greater and extends to larger radii in lower mass systems. At R500, the mass dependence and entropy excess are both negligible within the large observational and theoretical uncertaint ies. Mirroring this behaviour, the scaling of gas entropy is shal lower than self-similar in the inner regions, but steepens w ith radius, becoming consistent with self-similar at R500. There is a large dispersion in scaled entropy in the inner re gions, apparently linked to the presence of cool cores and dynamical activity; at larger radii the dispersion decreases by approximately a factor of two to 30 per cent, and the dichotomy between subsamples disappears. There are two peaks in the distribution of both inner slope and, after parameterising the profiles with a power law plus const ant model, in central entropy K0. However, we are unable to distinguish between a bimodal or a left-skewed distribution of K0 with the present data. The distribution of outer slopes is un imodal with a median value of 0.98, and there is a clear correlation of outer slope with temperature. Renormalising the dimensionless entropy profiles b y the gas mass fraction profile fgas(< R), leads to a remarkable reduction in the scatter, implying t hat gas mass fraction variations with radius and mass are the cause of the observed entropy structural and scaling properties. The results are consistent with the picture of a cluster population in which entropy modification is cent rally concentrated and extends to larger radii at lower mass, leading to both a radial and a mass-dependence in the gas mass fraction, but which is increasingly self-similar at large radius. T he observed normalisation, however, would suggest entropy modificatio n at least up to R1000, and even beyond, in all but the most massive systems. We discuss a tentative scenario to explain the observed behaviour of the entropy and gas mass fraction in the REXCESS sample, in which a combination of extra heating and merger mixing maintains an elevated central entropy level in the majority of the population, and a smaller fraction of systems is able to develop a cool core.

Subfields: Astronomy and Astrophysics

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The MCXC: a meta-catalogue of x-ray detected clusters of galaxies (2010, background): “…satellitesXMM-Newton, Chandra andSuzaku, have provided statistical samples for cosmological studies (e.g., Vikhlinin et al. 2009; Mantz et al. 2009) and detailed information on the structural properties of the cluster population (e.g., Vikhlinin et al. 2006; Pratt et al. 2010; Arnaud et al. 2010).”
Planck intermediate results: V. Pressure profiles of galaxy clusters from the Sunyaev-Zeldovich effect (2012, unknown): “Y X relation of A10 (see also Pratt et al. 2010), assuming standard evolution (see Eq.”
The cool-core bias in X-ray galaxy cluster samples - I. Method and application to HIFLUGCS (2010, background): “However, other studies found no evidence for bimodality in the distribution of clusters (e.g., Pratt et al. 2010), and the recent identification of regions reminiscent of cool cores in merging clusters (“cool-core remnants”, Rossetti & Molendi (2010)), probably associated with ancient cool cores…”
Thermal instability and the feedback regulation of hot haloes in clusters, groups and galaxies (2011, background): “This physics can quantitatively account for the observed excess of clusters with central entropies around 10 keV cm2 (e.g., Cavagnolo et al. 2008; Pratt et al. 2010).”
JOINT ANALYSIS OF CLUSTER OBSERVATIONS. II. CHANDRA/XMM-NEWTON X-RAY AND WEAK LENSING SCALING RELATIONS FOR A SAMPLE OF 50 RICH CLUSTERS OF GALAXIES (2012, result): “Similarly to previous studies (e.g., Morandi et al. 2007; Pratt et al. 2010; Mittal et al. 2011), we find that the luminosity–temperature (LX–TX) relationship exhibits a significant scatter of ≈50% when the core of the cluster is included—a scatter which is diminished considerably, to 36%, when the…”
Gas expulsion by quasar-driven winds as a solution to the overcooling problem in galaxy groups and clusters (2010, background): “…have improved the quality of the measurements and extended both the range of the masses and radii over which the entropy has been measured (e.g. Finoguenov et al. 2002; Ponman, Sanderson & Finoguenov 2003; Johnson, Ponman & Finoguenov 2009; Sun et al. 2009; Giodini et al. 2010; Pratt et al. 2010).”