The Ages of Passive Galaxies in a z=1.62 Protocluster [GA]

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 $z\ge 1.5$

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D. Lee-Brown, G. Rudnick, I. Momcheva, et. al.
Mon, 19 Jun 17

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