  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.
