# The cool core state of Planck SZ-selected clusters versus X-ray selected samples: evidence for cool core bias

## TL;DR

This study compares the cool core fractions in SZ-selected and X-ray selected galaxy cluster samples, revealing a bias in X-ray surveys towards cool core clusters and exploring the implications for cluster population studies.

## Contribution

It provides the first direct comparison of cool core fractions between Planck SZ-selected and X-ray selected samples, quantifying the cool core bias in X-ray surveys.

## Key findings

- X-ray selected clusters have a higher cool core fraction (59%) than SZ-selected clusters (29%).
- Cool core bias significantly affects the observed differences between the samples.
- SZ surveys may include more non cool core, X-ray under-luminous clusters.

## Abstract

We characterized the population of galaxy clusters detected with the SZ effect with Planck, by measuring the cool core state of the objects in a well-defined subsample of the Planck catalogue. We used as indicator the concentration parameter Santos et al. (2008). The fraction of cool core clusters is $29 \pm 4 \%$ and does not show significant indications of evolution in the redshift range covered by our sample. We compare the distribution of the concentration parameter in the Planck sample with the one of the X-ray selected sample MACS (Mann & Ebeling, 2011): the distributions are significantly different and the cool core fraction in MACS is much higher ($59 \pm 5 \%$). Since X-ray selected samples are known to be biased towards cool cores due to the presence of their prominent surface brightness peak, we simulated the impact of the "cool core bias" following Eckert et al. (2011). We found that it plays a large role in the difference between the fractions of cool cores in the two samples. We examined other selection effects that could in principle bias SZ-surveys against cool cores but we found that their impact is not sufficient to explain the difference between Planck and MACS. The population of X-ray under-luminous objects, which are found in SZ-surveys but missing in X-ray samples (Planck Collaboration 2016), could possibly contribute to the difference, as we found most of them to be non cool cores, but this hypothesis deserves further investigation.