A Kinematic Approach To Assessing Environmental Effects: Star-Forming
  Galaxies in a z~0.9 SpARCS cluster using Spitzer 24um Observations
  We present an infrared study of a z=0.872 cluster, SpARCS J161314+564930,
with the primary aim of distinguishing the dynamical histories of
spectroscopically confirmed star-forming members to assess the role of cluster
environment. We utilize deep MIPS imaging and a mass-limited sample of 85
spectroscopic members to identify 16 24um-bright sources within the cluster,
and measure their 24um star formation rates (SFRs) down to ~6 Msolar/year.
Based on their line-of-sight velocities and stellar ages, MIPS cluster members
appear to be an infalling population that was recently accreted from the field
with minimal environmental dependency on their star formation. However, we
identify a double-sequenced distribution of star-forming galaxies amongst the
members, with one branch exhibiting declining specific SFRs with mass. The
members along this sub-main sequence contain spectral features suggestive of
passive galaxies. Using caustic diagrams, we kinematically identify these
galaxies as a virialized and/or backsplash population. Moreover, we find a mix
of dynamical histories at all projected radii, indicating that standard
definitions of environment (i.e., radius and density) are contaminated with
recently accreted interlopers, which could contribute to a lack of
environmental trends for star-forming galaxies. A cleaner narrative of their
dynamical past begins to unfold when using a proxy for accretion histories
through profiles of constant (r/r_200)x(Delta v/sigma_v); galaxies accreted at
earlier times possess lower values of (r/r_200)x(Delta v/sigma_v) with minimal
contamination from the distinct infalling population. Therefore, adopting a
time-averaged definition for density (as traced by accretion histories) rather
than an instantaneous density yields a depressed specific SFR within the
dynamical cluster core.
