Spectroscopic confirmation of the galaxy clusters CARLA J0950+2743 at

Galaxy clusters are the largest gravitationally bound structures in the Universe and therefore are a powerful tool for studying mass assembly at different epochs. At z > 2, they provide the unique opportunity to place solid constraints not only on the growth of the dark matter halo, but also on the mechanisms of galaxy quenching and morphological transformation when the Universe was younger than 3.3 Gyr. However, the currently available sample of confirmed z > 2 clusters remains very limited. We present the spectroscopic confirmation of the galaxy cluster CARLA J0950+2743 at z = 2.363 ± 0.005 and a new serendipitously discovered cluster, CARLA-Ser J0950+2743 at z = 2.243 ± 0.008, in the same region. We confirm eight star-forming galaxies in the first and five in the second cluster by detecting [O<ns0:sc>II</ns0:sc>], [O<ns0:sc>III</ns0:sc>], and Hα emission lines. The analysis of an archival X-ray Chandra dataset that covers the cluster position revealed a counterpart with a total luminosity of L0.5−5keV = 2.9 ± 0.6 × 1045 erg s−1. Because the depth of the X-ray observations is limited, we cannot distinguish the 1D profile of the source from a point spread function model, but our statistical analysis of the 2D profile favors an extended component that might be associated with a thermal contribution from the intracluster medium. If the extended X-ray emission is due to the hot intracluster medium, the total combined dark matter mass for the two clusters would be $ M_{200} \approx 3.0^{+0.20}_{-0.23 (\mathrm{stat})} $ $ ^{+1.13}_{-0.85 (\mathrm{sys})} \times10^{14}\,M_{\odot} $, assuming a ∼30% contribution from the active galactic nucleus. Our two clusters are therefore interesting targets for studies of the structure growth in the cosmological context. However, future investigation will require deeper high-resolution X-ray and spectroscopic observations to rule out the hypotheses that the emission is entirely due to the active galactic nucleus or that it originates from other contaminating radio galaxies and structures.