The SRG/eROSITA All-Sky Survey $J$-Band Follow-Up Observations for Selected High-Redshift Galaxy Cluster Candidates

We select galaxy cluster candidates from the high-redshift (BEST_Z > 0.9) end of the first SRG/eROSITA All-Sky Survey (eRASS1) galaxy cluster catalogue, for which we obtain moderately deep J-band imaging data with the OMEGA2000 camera at the 3.5m telescope of the Calar Alto Observatory. We include J-band data of four additional targets obtained with the three-channel camera at the 2m Fraunhofer telescope at the Wendelstein Observatory. We complement the new J-band photometric catalogue with forced photometry in the i- and z-bands of the tenth data release of the Legacy Survey (LSDR10) to derive the radial colour distribution around the eRASS1 clusters. Without assuming a priori to find a cluster red sequence at a specific colour, we try to find a radially weighted colour over-density to confirm the presence of high-redshift optical counterparts for the X-ray emission. We compare our confirmation with optical properties derived in earlier works based on LSDR10 data to refine the existing high-redshift cluster confirmation of eROSITA-selected clusters. We attempt 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 confirm 9 out of 18 of the selected galaxy cluster candidates with a radial over-density of similar coloured galaxies for which 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, as well as it underlines the necessity for wide and deep near infrared imaging data for confirmation and characterisation of high-$z$ galaxy clusters.

💡 Research Summary

The paper presents a targeted near‑infrared (J‑band) follow‑up of high‑redshift galaxy cluster candidates identified in the first eROSITA All‑Sky Survey (eRASS1). From the eRASS1 catalogue the authors selected 18 candidates with a “best” redshift estimate (BEST_Z) greater than 0.9, an X‑ray extent likelihood (EXT_LIKE) above 5, and declinations suitable for observations from the northern hemisphere. These objects lie at the extreme redshift tail of the eROSITA sample, where the 4000 Å break has moved beyond the optical griz bands, making photometric confirmation with only optical data unreliable.

To overcome this limitation the team obtained moderately deep J‑band imaging for all targets using two facilities: the OMEGA2000 camera on the 3.5 m telescope at Calar Alto (CAHA) and the three‑channel 3KK camera on the 2 m Fraunhofer telescope at the Wendelstein Observatory. CAHA observations consisted of 45 × 60 s exposures per field (total 2700 s, with one field receiving 3840 s) split into 6 × 10 s sub‑exposures to avoid background saturation; a standard dither pattern was applied. Data reduction employed the THELI pipeline, including overscan correction, dark subtraction, two‑pass background modelling, astrometric calibration against 2MASS, and co‑addition with SWarp. Wendelstein data were reduced with the pipeline described in Obermeier et al. (2020), featuring bias/dark subtraction, flat‑fielding, hot‑pixel masking, crosstalk and non‑linearity corrections, and sky subtraction via a night‑sky‑flat built from the science frames themselves. Photometric zero‑points were tied to 2MASS J‑band magnitudes.

In addition to the new J‑band catalogues, the authors performed forced photometry on the i‑ and z‑band images from the tenth data release of the Legacy Survey (LS DR10). Rather than assuming a pre‑defined red‑sequence colour, they devised a radial colour‑overdensity method: for each X‑ray centre they measured the distribution of i–J and z–J colours within concentric annuli, applied a radial weighting, and compared the resulting colour histogram to that of random background regions. An overdensity exceeding 3σ was taken as evidence for a genuine cluster red‑sequence, even if its exact colour was not known a priori.

Applying this technique, nine of the eighteen candidates displayed a statistically significant colour overdensity, leading to their confirmation as high‑redshift clusters. For these confirmed systems the authors derived photometric redshifts using the three‑band (i, z, J) colours and calibrated the colour‑redshift relation against a spectroscopically confirmed cluster at z = 1.02. Inclusion of the J‑band reduced the relative colour measurement uncertainty from about 8 % (optical‑only) to roughly 4 %, effectively halving the redshift error budget at z ≈ 1.

The remaining nine candidates showed no significant colour overdensity, indicating a substantial spurious contamination fraction (≈ 50 %) among eRASS1 high‑z entries when optical data alone are used. This underscores the limitation of X‑ray extent plus optical colour information for confirming clusters beyond z ≈ 1, and highlights the necessity of deep NIR imaging. The authors discuss how forthcoming wide‑field NIR surveys such as Euclid will eventually provide homogeneous J‑band (and longer‑wavelength) coverage, but until then targeted follow‑up remains essential.

Overall, the study demonstrates (1) that J‑band imaging effectively captures the redshifted 4000 Å break, enabling reliable colour‑based confirmation of z > 1 clusters; (2) a radial colour‑overdensity approach can identify clusters without a priori red‑sequence colour assumptions; (3) the addition of a single NIR band can improve colour precision by a factor of two; and (4) the high‑redshift tail of the eROSITA catalogue contains a non‑negligible fraction of false positives. These findings provide practical guidance for the design of multi‑wavelength follow‑up strategies in the era of large X‑ray and NIR surveys.