Chronos and Kairos: Mosfire Observations of Post-Starburst Galaxies in
  z~1 Clusters and Groups
  In this study we present the exploration of $\sim$500 spectroscopically
confirmed galaxies in and around two large scale structures at $z\sim1$ drawn
from the ORELSE survey. A sub-sample of these galaxies ($\sim$150) were
targeted for the initial phases of a near-infrared MOSFIRE spectroscopic
campaign investigating the differences in selections of galaxies which had
recently ended a burst of star formation or had rapidly quenched (i.e.,
post-starburst or K+A galaxies). Selection with MOSFIRE resulted in a
post-starburst sample more than double that selected by traditional $z\sim1$
(observed-frame optical) methods even after the removal of the relatively large
fraction of dusty starburst galaxies selected through traditional methods.
While the traditional post-starburst fraction increased with increased global
density, the MOSFIRE-selected post-starburst fraction was found to be constant
in field, group, and cluster environments. However, this fraction relative to
the number of galaxies with ongoing star formation was observed to elevate in
the cluster environment. Post-starbursts selected with MOSFIRE were
predominantly found to exhibit moderately strong [OII] emission originating
from activity other than star formation. Such galaxies, termed K+A with
ImposteR [OII]-derived Star formation (KAIROS) galaxies, were found to be
considerably younger than traditionally-selected post-starbursts and likely
undergoing some form of feedback absent or diminished in traditional
post-starbursts. A comparison between the environments of the two types of
post-starbursts suggests a picture in which the evolution of a post-starburst
galaxy is considerably different in cluster environments than in the more
rarefied environments of a group or the field.
