The Ages of Passive Galaxies in a z = 1.62 Protocluster

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_∗ ≥ 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_⊙ ≤ M_∗ ≤ 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_∗ ≤ 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 ≥ 1.5.