Star Formation in High-Redshift Cluster Ellipticals
  We measure the star formation rates (SFRs) of massive
($M_{\star}>10^{10.1}M_{\odot}$) early-type galaxies (ETGs) in a sample of 11
high-redshift ($1.0 < z < 1.5$) galaxy clusters drawn from the IRAC Shallow
Cluster Survey (ISCS). We identify ETGs visually from Hubble Space Telescope
imaging and select likely cluster members as having either an appropriate
spectroscopic redshift or red sequence color. Mid-infrared SFRs are measured
using Spitzer 24 $\mu$m data for isolated cluster galaxies for which
contamination by neighbors, and active galactic nuclei, can be ruled out.
Cluster ETGs show enhanced specific star formation rates (sSFRs) compared to
cluster galaxies in the local Universe, but have sSFRs more than four times
lower than that of field ETGs at $1 < z < 1.5$. Relative to the late-type
cluster population, isolated ETGs show substantially quenched mean SFRs, yet
still contribute 12% of the overall star formation activity measured in $1 < z
< 1.5$ clusters. We find that new ETGs are likely being formed in ISCS
clusters; the fraction of cluster galaxies identified as ETGs increases from
34% to 56% from $z \sim 1.5 \rightarrow 1.25$. While the fraction of cluster
ETGs that are highly star-forming ($\textrm{SFR}\geq26\ M_{\odot}$ yr$^{-1}$)
drops from 27% to 10% over the same period, their sSFRs are roughly constant.
All these factors taken together suggest that, particularly at $z\gtrsim1.25$,
the events that created these distant cluster ETGs$-$likely mergers, at least
among the most massive$-$were both recent and gas-rich.
