# Cosmology with XMM galaxy clusters: the X-CLASS/GROND catalogue and photometric redshifts

## TL;DR

This paper presents the X-CLASS galaxy cluster catalog, including photometric redshifts obtained through a new method combining X-ray and optical data, enabling improved cosmological analyses.

## Contribution

It introduces a novel photometric redshift technique using red sequence colours and X-ray data, applied to a large X-ray-selected galaxy cluster sample.

## Key findings

- Photometric redshifts for 236 clusters with median z=0.39
- Median X-ray luminosity of 7.2×10^43 erg/s
- Good agreement with other X-ray cluster surveys

## Abstract

The XMM Cluster Archive Super Survey (X-CLASS) is a serendipitously-detected X-ray-selected sample of 845 galaxy clusters based on 2774 XMM archival observations and covering approximately 90 deg$^2$ spread across the high-Galactic latitude ($|b|>20$ deg) sky. The primary goal of this survey is to produce a well-selected sample of galaxy clusters on which cosmological analyses can be performed. This article presents the photometric redshift followup of a high signal-to-noise subset of 266 of these clusters with declination $\delta<+20$ deg with GROND, a seven channel ($grizJHK$) simultaneous imager on the MPG 2.2m telescope at the ESO La Silla Observatory. We use a newly developed technique based on the red sequence colour-redshift relation, enhanced with information coming from the X-ray detection to provide photometric redshifts for this sample. We determine photometric redshifts for 236 clusters, finding a median redshift of $z=0.39$ with an accuracy of $\Delta z = 0.02 (1+z)$ when compared to a sample of 76 spectroscopically confirmed clusters. We also compute X-ray luminosities for the entire sample and find a median bolometric luminosity of $7.2\times10^{43} \mathrm{erg\ s^{-1}}$ and a median temperature 2.9 keV. We compare our results to the XMM-XCS and XMM-XXL surveys, finding good agreement in both samples. The X-CLASS catalogue is available online at http://xmm-lss.in2p3.fr:8080/l4sdb/.