The Star-Formation History of BCGs to z = 1.8 from the SpARCS/SWIRE
  Survey: Evidence for significant in-situ star formation at high-redshift
  We present the results of a MIPS-24um study of the Brightest Cluster Galaxies
(BCGs) of 535 high-redshift galaxy clusters. The clusters are drawn from the
Spitzer Adaptation of the Red-Sequence Cluster Survey (SpARCS), which
effectively provides a sample selected on total stellar mass, over 0.2 < z <
1.8 within the Spitzer Wide-Area Infrared Extragalactic (SWIRE) Survey fields.
20%, or 106 clusters have spectroscopically confirmed redshifts, and the rest
have redshifts estimated from the color of their red sequence. A comparison
with the public SWIRE images detects 125 individual BCGs at 24um > 100uJy, or
23%. The luminosity-limited detection rate of BCGs in similar richness clusters
(Ngal> 12) increases rapidly with redshift. Above z ~ 1, an average of ~20\% of
the sample have 24um-inferred infrared luminosities of LIR > 10^12 Lsun, while
the fraction below z ~ 1 exhibiting such luminosities is < 1 \%. The
Spitzer-IRAC colors indicate the bulk of the 24um-detected population is
predominantly powered by star formation, with only 7/125 galaxies lying within
the color region inhabited by Active Galactic Nuclei (AGN). Simple arguments
limit the star-formation activity to several hundred million years and this may
therefore be indicative of the timescale for AGN feedback to halt the star
formation. Below redshift z ~ 1 there is not enough star formation to
significantly contribute to the overall stellar mass of the BCG population, and
therefore BCG growth is likely dominated by dry-mergers. Above z~ 1, however,
the inferred star formation would double the stellar mass of the BCGs and is
comparable to the mass assembly predicted by simulations through dry mergers.
We cannot yet constrain the process driving the star formation for the overall
sample, though a single object studied in detail is consistent with a gas-rich
merger.
