2017ApJ...844...43L

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 ∼ 2x 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.