THE SCUBA-2 COSMOLOGY LEGACY SURVEY: ULTRALUMINOUS STAR-FORMING GALAXIES IN A z=1.6 CLUSTER

We analyse new SCUBA-2 submillimeter and archival SPIRE far-infrared imaging of a z =1.62 cluster, Cl 0218.3−0510, which lies in the UKIDSS/UDS field of the SCUBA-2 Cosmology Legacy Survey. Combining these tracers of obscured star formation activity with the extensive photometric and spectroscopic information available for this field, we identify 31 far-infrared/submillimeter-detected probable cluster members with bolometric luminosities ∼ 10 12 L⊙ and show that by virtue of their dust content and activity, these represent some of the reddest and brightest galaxies in this structure. We exploit Cycle-1 ALMA submillimeter continuum imaging which covers one of these sources to confirm the identification of a SCUBA-2-detected ultraluminous star-forming galaxy in this structure. Integrating the total star-formation activity in the central region of the structure, we estimate that it is an order of magnitude higher (in a mass-normalised sense) than clusters at z ∼ 0.5–1. However, we also find that the most active cluster members do not reside in the densest regions of the structure, which instead host a population of passive and massive, red galaxies. We suggest that while the passive and active populations have comparable near-infrared luminosities at z =1.6, MH ∼ −23, the subsequent stronger fading of the more active galaxies means that they will evolve into passive systems at the present-day which are less luminous than the descendants of those galaxies which were already passive at z ∼ 1.6 (MH ∼ −20.5 and MH ∼ −21.5 respectively at z ∼ 0). We conclude that the massive galaxy population in the dense cores of present-day clusters were already in place at z =1.6 and that in Cl 0218.3−0510 we are seeing continuing infall of less extreme, but still ultraluminous, star-forming galaxies onto a pre-existing structure. Subject headings: cosmology: observations — galaxies: evolution — galaxies: formation — galaxies: clusters, individual (Cl 0218−0510)