The SRG/eROSITA All-Sky Survey

We selected galaxy cluster candidates from the high-redshift (BEST_Z > 0.9) end of the first SRG (Spectrum Roentgen Gamma)/eROSITA All-Sky Survey (eRASS1) galaxy cluster catalogue, for which we obtained moderately deep J -band imaging data with the OMEGA2000 camera at the 3.5 m telescope of the Calar Alto Observatory. We included J -band data of four additional targets obtained with the three-channel camera at the 2 m Fraunhofer telescope at the Wendelstein Observatory. We complemented the new J -band photometric catalogue with forced photometry in the i and z bands of the tenth data release of the Legacy Survey (LS DR10) to derive the radial colour distribution around the eROSITA/eRASS1 clusters. Without assuming a priori to find a cluster red sequence at a specific colour, we tried to find a radially weighted colour over-density to confirm the presence of high-redshift optical counterparts for the X-ray emission. We compared our confirmation with optical properties derived in earlier works based on LS DR10 data to refine the existing high-redshift cluster confirmation of eROSITA-selected clusters. We attempted to calibrate the colour-redshift relation including the new J -band data by comparing our obtained photometric redshift estimate with the spectroscopic redshift of a confirmed, optically selected, high-redshift galaxy cluster. We find a radial over-density of similar coloured galaxies for 12 out of 18 of the selected galaxy cluster candidates for which we also have i -band data. For 9 of these 12, we provide a photometric redshift estimate. We can report an increase in the relative colour measurement precision from 8% to 4% when including J -band data. In conclusion, our findings indicate a not insignificant spurious contaminant fraction at the high-redshift end (BEST_Z > 0.9) of the eROSITA/eRASS1 galaxy cluster catalogue. This underlines the need for wide and deep near-infrared imaging data for the confirmation and characterisation of high- z galaxy clusters.