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 $M_*
\geq 10^{10.85} M_{\odot}$, the quiescent fraction in the protocluster is
$f_Q=1.0^{+0.00}_{-0.37}$, consistent with a $\sim 2\times $ enhancement
relative to the field value, $f_Q=0.45^{+0.03}_{-0.03}$. At masses $10^{10.2}
M_{\odot} \leq M_* \leq 10^{10.85} M_{\odot}$, $f_Q$ in the cluster is
$f_Q=0.40^{+0.20}_{-0.18}$, 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 $M_* \leq 10^{10.85} M_{\odot}$. 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 $z\ge 1.5$
