The Ages of Passive Galaxies in a z=1.62 Protocluster

Abstract

Understanding how galaxies stop forming stars is a principle pursuit of extragalactic astrophysics. Here, I present a study of the relation between galaxy stellar age and mass in the $z=1.62$ protocluster IRC 0218. After separating star forming and quiescent galaxies on the basis of their $UVJ$ colors, we find that at stellar masses $M_* \ge 10^{10.8} M_{\odot}$, the quiescent fraction in IRC 0218 is $f_Q=1.0^{+0.00}_{-0.36}$, approximately $2-3 \times$ higher than the field value. At lower masses, $f_Q$ is consistent with the field. Using galaxy $D_{n}(4000)$ values as measures of stellar age, we find no relation between age and mass. This may indicate that the mass dependence of $f_Q$ was imprinted early in IRC 0218's history. Alternatively, the lack of a relation may be due to mass redistribution through gasless merging. Ultimately, our results place constraints on the mechanism(s) responsible for quenching in dense environments at $z\ge 1.5$.