The X-ray/SZ view of the virial region
I. Thermodynamic properties⋆
1 Astronomical Observatory of the University of Geneva, ch. d’Ecogia 16, 1290 Versoix, Switzerland
2 INAF – IASF-Milano, via E. Bassini 15, 20133 Milano, Italy
3 Hamburg Observatory, Gojansbergsweg 112, 21029 Hamburg, Germany
4 INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
5 INFN, Sezione di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
Received: 18 September 2012
Accepted: 20 December 2012
Aims. We measure the thermodynamic properties of cluster outer regions to provide constraints on the processes that rule the formation of large-scale structures.
Methods. We derived the thermodynamic properties of the intracluster gas (temperature, entropy) by combining the Sunyaev-Zel’dovich thermal pressure from Planck and the X-ray gas density from ROSAT. This method allowed us to reconstruct for the first time temperature and entropy profiles out to the virial radius and beyond in a large sample of objects.
Results. At variance with several recent Suzaku studies, we find that the entropy rises steadily with radius, albeit at at a somewhat lower rate than predicted by self-similar expectations. We note significant differences between relaxed, cool-core systems and unrelaxed clusters in the outer regions. Relaxed systems appear to follow the self-similar expectations more closely than perturbed objects.
Conclusions. Our results indicate that the well-known entropy excess observed in cluster cores extends well beyond the central regions. When correcting for the gas depletion, the observed entropy profiles agree with the prediction from gravitational collapse only, especially for cool-core clusters.
Key words: X-rays: galaxies: clusters / galaxies: clusters: general / galaxies: clusters: intracluster medium / cosmology: observations
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© ESO, 2013