The cool-core bias in X-ray galaxy cluster samples

Abstract

Aims. When selecting flux-limited cluster samples, the detection efficiency of X-ray instruments is not the same for centrally-peaked and flat objects, which introduces a bias in flux-limited cluster samples. We quantify this effect in the case of a well-known cluster sample, HIFLUGCS. Methods. We simulate a population of X-ray clusters with various surface-brightness profiles, and use the instrumental characteristics of the ROSAT All-Sky Survey (RASS) to select flux-limited samples similar to the HIFLUGCS sample and predict the expected bias. For comparison, we also estimate observationally the bias in the HIFLUGCS sample using XMM-Newton and ROSAT data. Results. We find that the selection of X-ray cluster samples is significantly biased (∼29%) in favor of the peaked, cool-core (CC) objects, with respect to non-cool-core (NCC) systems. Interestingly, we find that the bias affects the low-mass, nearby objects (groups, poor clusters) much more than the more luminous objects (i.e massive clusters). We also note a moderate increase of the bias for the more distant systems. Conclusions. Observationally, we propose to select the objects according to their flux in a well-defined physical range excluding the cores, 0.2r 500 −r 500 , to get rid of the bias. From the fluxes in this range, we reject 13 clusters out of the 64 in the HIFLUGCS sample, none of which appears to be NCC. As a result, we estimate that less than half (35-37%) of the galaxy clusters in the local Universe are strong CC. In the paradigm where the CC objects trace relaxed clusters as opposed to unrelaxed, merging objects, this implies that to the present day the majority of the objects are not in a relaxed state. From this result, we estimate a rate of heating events of ∼1/3 Gyr −1 per dark-matter halo.

Lofar, Vla, and Chandra Observations of the Toothbrush Galaxy Cluster

“…We used PROFFIT (Eckert et al 2011) to extract and fit the surface brightness profiles. The areas covered by the compact sources were excluded from the fitting (see Section 3.4).…”

Sardinia Radio Telescope observations of Abell 194

“…The surface brightness profile has been computed with 30 arcsec bins centered on the centroid of the image (RA=01 25 54; DEC=-01 21 05) out to 50 arcmin. The surface brightness profile was fitted with a single β-model (Cavaliere & Fusco-Femiano 1976) plus a constant (to take into account the sky background composed by Galactic foregrounds and residual CXB), with the software PROFFIT v1.3 (Eckert et al 2011). The best fitting model has a core radius r c = 11.5 ± 1.2 arcmin (248.4 ± 26 kpc) and β = 0.67 ± 0.06 (1σ) for a χ 2 /dof=1.77.…”

The gas distribution in the outer regions of galaxy clusters

“…On the observational side, the density profiles in this radial range are well-fitted by the β-model (see Eq. (4)), and several independent works converge to the canonical value of β ∼ 0.7 (e.g., Mohr et al 1999;Ettori & Fabian 1999;Vikhlinin et al 1999;Croston et al 2008;Ettori & Balestra 2009;Eckert et al 2011b). As a benchmark, we computed the values of β for our average density profile and the various sets of simulations, fixing the core radius to 0.12r 200 (e.g., Mohr et al 1999).…”