Obscured star formation in clusters at z = 1.6-2.0: massive galaxy formation and the reversal of the star formation-density relation

Abstract

Clusters of galaxies at z $\mathrel {\gtrsim }$ 1 are expected to be increasingly active sites of star formation. To test this, an 850 $\mu$m survey was undertaken of eight clusters at z = 1.6-2.0 using SCUBA-2 on the James Clerk Maxwell Telescope. Mid-infrared properties were used to identify 53 probable counterparts to 45 SCUBA-2 sources with colours that suggested they were cluster members. This uncovered a modest overdensity of 850 $\mu$m sources, with far-infrared luminosities of L_IR ≥ 10¹² L_⊙ (SFR $\mathrel {\gtrsim }$ 100 M_⊙ yr^-1) and colours consistent with being cluster members, of a factor of 4 ± 1 within the central 1 Mpc radius of the clusters. The submillimetre photometry of these galaxies was used to estimate the total cluster star formation rates. These showed that the mass-normalized rates in the clusters are two orders of magnitude higher than in local systems, evolving as (1 + z)^{5.5 ± 0.6}. This rapid evolution means that the mass-normalized star formation rates in these clusters matched that of average haloes in the field at z ~ 1.8 ± 0.2 marking the epoch where the local star formation-density relation reverses in massive haloes. The estimated stellar masses of the cluster submillimetre galaxies suggests that their descendants will be amongst the most massive galaxies in z ~ 0 clusters. This reinforces the suggestion that the majority of the massive early-type galaxy population in z ~ 0 clusters were likely to have formed at z $\mathrel {\gtrsim }$ 1.5-2 through very active, but dust-obscured, starburst events.