Spectroscopic Confirmation of the Rich z=1.80 Galaxy Cluster JKCS 041 Using the WFC3 Grism: Environmental Trends in the Ages and Structure of Quiescent Galaxies
We present Hubble Space Telescope imaging and grism spectroscopy of the distant galaxy cluster JKCS041. Our survey yields 98 redshifts whose precision is typically ~20 times better than photometric estimates. We confirm that JKCS041 is a rich cluster and derive a redshift z=1.80 via the identification of 19 member galaxies, of which 15 are quiescent. These members are spatially aligned with diffuse X-ray emission seen by Chandra. As JKCS041 is the most distant known cluster with such a large and spectroscopically-confirmed quiescent population, it provides an unique opportunity to study the effect of the environment on galaxy properties at early epochs. We construct composite spectra of the quiescent members that clearly reveal Balmer and metallic absorption lines. From these, we infer that the more massive members (M_*>10^(11)M_☉) have a mean stellar age of 1.4^(+0.3)_(-0.2) Gyr, whereas lower-mass examples (M_*=10^(10.5-11)M_☉) have a younger mean age of 0.9^(+0.2)_(-0.1) Gyr. These ages agree closely with those inferred by Whitaker et al. for similarly-selected quiescent field galaxies, supporting the idea that the cluster environment is more efficient at truncating star formation while not having a strong effect on the mean epoch of quenching. In contrast to field samples, there is no sign of low-level Hβ or [O III] emission in our composite spectra. By comparing the ellipticity distribution of the quiescent members to coeval field galaxies drawn from the CANDELS survey, we find some evidence (90% confidence) for a lower fraction of disk-like quiescent systems in JKCS041. Taking this into account, we do not detect a significant difference between the mass-radius relations of the quiescent JKCS041 members and our z~1.8 field sample. We review claims of environmentally-dependent size growth of z>1 quiescent galaxies, and we demonstrate how differences in morphological mixtures may complicate comparisons of sizes in different environments
