Measuring the dynamical state of Planck SZ-selected clusters: X-ray peak - BCG offset

TL;DR

This study compares the dynamical states of galaxy clusters selected by SZ effect and X-ray surveys, revealing that SZ-selected clusters are generally less relaxed, likely due to different selection biases.

Contribution

It introduces a method to quantify cluster dynamical states using X-ray peak-BCG offsets and compares SZ-selected clusters with X-ray-selected ones, highlighting selection effects.

Findings

SZ-selected clusters are less relaxed than X-ray-selected clusters.

The fraction of relaxed clusters is about 52% in SZ samples versus 74% in X-ray samples.

Different selection biases influence the observed dynamical states.

Abstract

We want to characterize the dynamical state of galaxy clusters detected with the Sunyaev-Zeldovich (SZ) effect by Planck and compare them with the dynamical state of clusters selected in X-rays survey. We analyzed a representative subsample of the Planck SZ catalogue, containing the 132 clusters with the highest signal to noise ratio and characterize their dynamical state using as indicator the projected offset between the peak of the X-ray emission and the position of the Brightest cluster galaxy. We study the distribution of our indicator in our sample and compare it to its distribution in X-ray selected samples (HIFLUGCS, MACS and REXCESS). The distributions are significantly different and the fraction of relaxed objects is smaller in the Planck sample ($52 \pm 4 \%$) than in X-ray samples ($\simeq 74\%$) We interpret this result as an indication of different selection effects affecting X-rays (e.g. "cool core bias") and SZ surveys of galaxy clusters.