Self-similarity of temperature profiles in distant galaxy clusters: the quest for a universal law^⋆

¹ Dipartimento di AstronomiaUniversità di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: alessandro.baldi@oabo.inaf.it
² INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ INFN, Sezione di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
⁴ INAF-IASF, via Bassini 15, 20133 Milan, Italy
⁵ Department of Physics and Astronomy, University of California at Irvine, 4129 Frederick Reines Hall, Irvine, CA 92697–4575, USA

Received: 7 May 2012
Accepted: 27 June 2012

Abstract

Context. We present the XMM-Newton temperature profiles of 12 bright (L_X > 4 × 10⁴⁴ erg s^-1) clusters of galaxies at 0.4 < z < 0.9, having an average temperature in the range 5 ≲ kT ≲ 11 keV.

Aims. The main goal of this paper is to study for the first time the temperature profiles of a sample of high-redshift clusters, to investigate their properties, and to define a universal law to describe the temperature radial profiles in galaxy clusters as a function of both cosmic time and their state of relaxation.

Methods. We performed a spatially resolved spectral analysis, using Cash statistics, to measure the temperature in the intracluster medium at different radii.

Results. We extracted temperature profiles for the clusters in our sample, finding that all profiles are declining toward larger radii. The normalized temperature profiles (normalized by the mean temperature T₅₀₀) are found to be generally self-similar. The sample was subdivided into five cool-core (CC) and seven non cool-core (NCC) clusters by introducing a pseudo-entropy ratio σ = (T_IN/T_OUT) × (EM_IN/EM_OUT)^−1/3 and defining the objects with σ < 0.6 as CC clusters and those with σ ≥ 0.6 as NCC clusters. The profiles of CC and NCC clusters differ mainly in the central regions, with the latter exhibiting a slightly flatter central profile. A significant dependence of the temperature profiles on the pseudo-entropy ratio σ is detected by fitting a function of r and σ, showing an indication that the outer part of the profiles becomes steeper for higher values of σ (i.e. transitioning toward the NCC clusters). No significant evidence of redshift evolution could be found within the redshift range sampled by our clusters (0.4 < z < 0.9). A comparison of our high-z sample with intermediate clusters at 0.1 < z < 0.3 showed how the CC and NCC cluster temperature profiles have experienced some sort of evolution. This can happen because higher z clusters are at a less advanced stage of their formation and did not have enough time to create a relaxed structure, which is characterized by a central temperature dip in CC clusters and by flatter profiles in NCC clusters.

Conclusions. This is the first time that a systematic study of the temperature profiles of galaxy clusters at z > 0.4 has been attempted. We were able to define the closest possible relation to a universal law for the temperature profiles of galaxy clusters at 0.1 < z < 0.9, showing a dependence on both the relaxation state of the clusters and the redshift.