ALMA Pinpoints a Strong Overdensity of U/LIRGs in the Massive Cluster XCS J2215 at z = 1.46

Abstract

We surveyed the core regions of the z  = 1.46 cluster XCS J2215.9−1738 with the Atacama Large Millimeter Array (ALMA) and the MUSE-GALACSI spectrograph on the Very Large Telescope (VLT). We obtained high spatial resolution observations with ALMA of the 1.2 mm dust continuum and molecular gas emission in the central regions of the cluster. These observations detect 14 significant millimeter sources in a region with a projected diameter of just ∼500 kpc (∼1′). For six of these galaxies, we also obtain 12 CO(2–1) and 12 CO(5–4) line detections, confirming them as cluster members, and a further five of our millimeter galaxies have archival 12 CO(2–1) detections, which also place them in the cluster. An additional two millimeter galaxies have photometric redshifts consistent with cluster membership, although neither show strong line emission in the MUSE spectra. This suggests that the bulk (≥11/14, ∼80%) of the submillimeter sources in the field are in fact luminous infrared galaxies lying within this young cluster. We then use our sensitive new observations to constrain the dust-obscured star formation activity and cold molecular gas within this cluster. We find hints that the cooler dust and gas components within these galaxies may have been influenced by their environment, reducing the gas reservoir available for their subsequent star formation. We also find that these actively star-forming galaxies have dynamical masses and stellar population ages expected for the progenitors of massive, early-type galaxies in local clusters, potentially linking these populations.