Star formation in the cluster CLG0218.3-0510 at z = 1.62 and its large-scale environment: the infrared perspective

The galaxy cluster CLG0218.3-0510 at z = 1.62 is one of the most distant galaxy clusters known, with a rich multiwavelength data set that confirms a mature galaxy population already in place. Using very deep, wide-area (20 Mpc × 20 Mpc) imaging by Spitzer MIPS at 24 μm, in conjunction with Herschel five-band imaging from 100 to 500 μm, we investigate the dust-obscured, star formation properties in the cluster and its associated large-scale environment. Our galaxy sample of 693 galaxies at z ∼ 1.62 detected at 24 μm (10 spectroscopic and 683 photo-z) includes both cluster galaxies (i.e. within r < 1 Mpc projected cluster-centric radius) and field galaxies, defined as the region beyond a radius of 3 Mpc. The star formation rates (SFRs) derived from the measured infrared luminosity range from 18 to 2500 M yr−1, with a median of 55 M yr−1, over the entire radial range (10 Mpc). The cluster's brightest far-infrared galaxy, taken as the centre of the galaxy system, is vigorously forming stars at a rate of 256 ± 70 M yr−1, and the total cluster SFR enclosed in a circle of 1 Mpc is 1161 ± 96 M yr−1. We estimate a dust extinction of ∼3 mag by comparing the SFRs derived from [O ii] luminosity with the ones computed from the 24 μm fluxes. We find that the in-falling region (1-3 Mpc) is special: there is a significant decrement (3.5×) of passive relative to star-forming galaxies in this region, and the total SFR of the galaxies located in this region is lower (∼130 M yr−1 Mpc−2) than anywhere in the cluster or field, regardless of their stellar mass. In a complementary approach, we compute the local galaxy density, Σ5, and find no trend between SFR and Σ5. However, we measure an excess of star-forming galaxies in the cluster relative to the field by a factor of 1.7, that lends support to a reversal of SF-density relation in CLG0218.