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 $M_{\ast }\ge 10^{10.85}{M}_{\odot }$, the quiescent fraction in the protocluster is $f_Q=1.0_{-0.37}^{+0.00}$, consistent with a $\sim 2\times$ enhancement relative to the field value, $f_Q=0.45_{-0.03}^{+0.03}$. At masses $10^{10.2}{M}_{\odot }\le M_{\ast }\le 10^{10.85}{M}_{\odot }$, $f_Q$ 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 $z\sim 1$, we determine that our observed relation between $f_Q$ and stellar mass only mildly evolves between $z\sim 1.6$ and $z\sim 1$, and only at stellar masses $M_{\ast }\le 10^{10.85}{M}_{\odot }$. Both the $z\sim 1$ and $z\sim 1.6$ results are in agreement that the red sequence in dense environments was already populated at high redshift, $z\ge 3$, placing constraints on the mechanism(s) responsible for quenching in dense environments at $z\ge 1.5$

17 pages, 8 figures, 3 tables. Accepted for publication in ApJ

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