The Ages of Passive Galaxies in a z = 1.62 Protocluster

Abstract

We present a study of the relation between galaxy stellar age and mass for 14 members of the z = 1.62 protocluster IRC 0218, using multiband imaging and HST G102 and G141 grism spectroscopy. Using UVJ colors to separate galaxies into star-forming and quiescent populations, we find that, at stellar masses {M}_* ≥slant {10}^10.85 {M}_⊙ , the quiescent fraction in the protocluster is {f}_Q={1.0}_-0.37^+0.00, consistent with a ∼ 2× enhancement relative to the field value, {f}_Q={0.45}_-0.03^+0.03. At masses {10}^10.2 {M}_⊙ ≤slant {M}_* ≤slant {10}^10.85 {M}_⊙ , f _Q in the cluster is {f}_Q={0.40}_-0.18^+0.20, consistent with the field value of {f}_Q={0.28}_-0.02^+0.02. Using galaxy {D}_n(4000) values derived from the G102 spectroscopy, we find no relation between galaxy stellar age and mass. These results may reflect the impact of merger-driven mass redistribution—which is plausible, as this cluster is known to host many dry mergers. Alternately, they may imply that the trend in f _Q in IRC 0218 was imprinted over a short timescale in the protocluster’s assembly history. Comparing our results with those of other high-redshift studies and studies of clusters at z∼ 1, we determine that our observed relation between f _Q and stellar mass only mildly evolves between z∼ 1.6 and z∼ 1, and only at stellar masses {M}_* ≤slant {10}^10.85 {M}_⊙ . Both the z∼ 1 and z∼ 1.6 results are in agreement that the red sequence in dense environments was already populated at high redshift, z≳ 3, placing constraints on the mechanism(s) responsible for quenching in dense environments at z≥slant 1.5.