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

Abstract

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 profiles are robustly measured at least out to R 1000 in all systems and out to R 500 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 R 500 , the mass dependence and entropy excess are both negligible within the large observational and theoretical uncertainties. Mirroring this behaviour, the scaling of gas entropy is shallower than self-similar in the inner regions, but steepens with radius, becoming consistent with self-similar at R 500 . There is a large dispersion in scaled entropy in the inner regions, 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 constant model, in central entropy K 0 . However, we are unable to distinguish between a bimodal or a left-skewed distribution of K 0 with the present data. The distribution of outer slopes is unimodal with a median value of 0.98, and there is a clear correlation of outer slope with temperature. Renormalising the dimensionless entropy profiles by the gas mass fraction profile f gas (<R), leads to a remarkable reduction in the scatter, implying that 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 centrally 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. The observed normalisation, however, would suggest entropy modification at least up to R 1000 , 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.

Planckintermediate results

“…The preliminary X-ray data analysis follows that described in Pratt et al (2007), Pratt et al (2010), andPlanck Collaboration (2011f). In brief, surface brightness profiles centred on the X-ray emission peak were extracted in the 0.3-2 keV band and used to derive the regularised gas density profiles, n e (r), using the nonparametric deprojection and PSF-correction method of Croston et al (2006).…”

“…In brief, surface brightness profiles centred on the X-ray emission peak were extracted in the 0.3-2 keV band and used to derive the regularised gas density profiles, n e (r), using the nonparametric deprojection and PSF-correction method of Croston et al (2006). The projected temperature was measured in annuli as described in Pratt et al (2010). The 3D temperature profiles, T (r), were calculated by convolving a suitable parametric model with a response matrix that takes into account projection and PSF effects, projecting this model accounting for the bias introduced by fitting isothermal models to multi-temperature plasma emission (Mazzotta et al 2004;Vikhlinin 2006), and fitting to the projected annular profile.…”

A pan-chromatic view of the galaxy cluster XMMU J1230.3+1339 atz= 0.975

“…For the temperature determination of the cluster we followed the general procedure described e.g. in Pratt et al (2010). As for the flux determination, we focused the spectral analysis on field 0112552101, since the effects of the broadened XMM-Newton PSF at larger off-axis angle outweigh the gain of additional source photons.…”

“…For this reason, the X-ray luminosity is of crucial importance for most largescale X-ray surveys and provides the link to other ICM properties and the total cluster mass (e.g. Reiprich & Böhringer 2002) via L X scaling relations.…”

The XMM cluster outskirts project (X‐COP)

“…Such departures have been observed for a long time in cluster cores, where gas cooling and feedback from supernovae and active galactic nuclei are important (e.g. David et al 1996;Ponman et al 1999;Pratt et al 2010). More recently, several works reported a deficit of entropy in massive clusters around the virial radius (see Reiprich et al 2013, for a review), which has been interpreted as a lack of thermalization of the ICM induced, e.g., by an incomplete virialization of the gas (e.g.…”