Searching for cold gas traced by MgII quasar absorbers in massive X-ray-selected galaxy clusters

Context. Almost 50% of galaxies in the local Universe are in clusters or groups coexisting with both hot and cold gas components. In the present study, we observationally probed the cold-gas content of X-ray-selected massive galaxy clusters with spectroscopic redshift measured from the SDSS/SPIDERS survey. This paper focuses on the most massive structures: galaxy clusters with a mean mass of M_500c = 2.7 × 10¹⁴ M_⊙.
Aims: We used a large number of background quasar optical spectra from SDSS DR16 to probe the diffuse T = 10⁴ K gas in their intracluster medium.
Methods: We first analysed a sample of spectra with known MgII absorbers, and then blindly stacked about 16 000 archival spectra at the redshifts of the foreground galaxy clusters.
Results: We tentatively (3.7σ significance) detect MgII in the clusters with an equivalent width EW(MgII λ2796) of 0.056 ± 0.015 Å, corresponding to a column density of log [N(MgII)/cm⁻²] = 12.12 ± 0.1. We tested our methodology by generating 22 000 mock SDSS spectra with MgII absorbers from Illustris-TNG50 cosmological magnetohydrodynamical simulations, combining photo-ionisation modelling and ray tracing. We also performed bootstrapping stacking at different cluster redshifts and stacked quasar spectra with no intervening clusters in the line of sight to measure the significance of our detection.
Conclusions: These results are in line with the findings of recent, similar observational studies but challenge predictions from Illustris-TNG simulations. Together, our findings indicate that large amounts of cold gas may be found in the most massive structures of the Universe.

All the data used in this work are publicly available.