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 in the field of the distant galaxy cluster JKCS 041 using the Wide Field Camera 3. We confirm that JKCS 041 is a rich cluster and derive a redshift z=1.80 via the spectroscopic identification of 19 member galaxies, of which 15 are quiescent. These are centered upon diffuse X-ray emission seen by the Chandra observatory. As JKCS 041 is the most distant known cluster with such a large, spectroscopically confirmed quiescent population, it provides a unique opportunity to study the effect of the environment on galaxy properties at early epochs. We construct high-quality composite spectra of the quiescent cluster members that reveal prominent Balmer and metallic absorption lines. Using these, we measure the mean stellar ages in two bins of stellar mass. The quiescent cluster members' ages agree remarkably closely with that inferred by Whitaker et al. for similarly selected samples in the field, 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. We find some evidence (90% confidence) for a lower fraction of disk-like quiescent systems in JKCS 041 compared to a sample of coeval field galaxies drawn from the CANDELS survey. Taking this into account, we do not detect a significant difference between the mass-radius relations of the quiescent JKCS 041 members and our z~1.8 field sample. Finally, we demonstrate how differences in the morphological mixture of quenched systems can complicate measures of the environmental dependence of size growth.