A census of Dark Energy Survey Year 3 redMaPPer galaxy clusters in a high exposure eROSITA All Sky Survey region

Abstract

We present the cross-match of galaxy clusters detected by redMaPPer using the first three years of Dark Energy Survey (DESY3RM) operations, and the first release of the eROSITA-All Sky Survey (eRASS1) taken by eROSITA on board the Spectrum Roentgen Gamma telescope. We define a sky region ($60^{\circ }<R.A.<90^{\circ }$ and $-58^{\circ }<\mathrm{Dec}<-48^{\circ }$) whereby the mean eRASS1 exposure time of clusters falling within this footprint is $\sim$2.5 times longer than the mean exposure time of the full eRASS1--DESY3RM overlap. Limiting to a redshift range $0.2<z<0.65$ and redMaPPer estimated richnesses to $>$50, we obtain a sample of 57 clusters. We match these clusters to the cluster catalogue constructed for the eRASS1, finding 45 clusters with a match and 12 with no match in the catalogue. X-ray properties of the 12 unmatched clusters are estimated via a forced photometry analysis. We investigate the form of the luminosity-richness relation, finding the relation using the full 57 cluster sample is statistically consistent with using only the 45 matched clusters, and will require the deeper data of future eRASS releases (or dedicated X-ray follow-up) to explore further potential differences. Using external data from XMM, ACT and SPT, we find detections of 5 of the 12 unmatched clusters, indicating the presence of virialised gas. Finally, we found the 12 unmatched clusters occupy a higher redshift range and lower richness range (compared to the matched clusters), and find no evidence for an underlying physical reason for the lack of X-ray emission.

arXiv commentAccepted 2026 August 24. Received 2026 August 24; in original form 2026 February 20

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