Red but not dead : Unveiling the Star-forming Far-infrared Spectral
  Energy Distribution of SpARCS Brightest Cluster Galaxies at 0 < z < 1.8
  We present the results of a Spitzer/Herschel infrared photometric analysis of
the largest (716) and highest-redshift (z=1.8) sample of Brightest Cluster
Galaxies (BCGs), those from the Spitzer Adaptation of the Red-Sequence Cluster
Survey (SpARCS). Given the tension that exists between model predictions and
recent observations of BCGs at z<2, we aim to uncover the dominant physical
mechanism(s) guiding the stellar-mass buildup of this special class of
galaxies, the most massive in the Universe uniquely residing at the centres of
galaxy clusters. Through a comparison of their stacked, broadband, infrared
spectral energy distributions (SEDs) to a variety of SED model templates in the
literature, we identify the major sources of their infrared energy output, in
multiple redshift bins between 0 < z < 1.8. We derive estimates of various BCG
physical parameters from the stacked {\nu}L{\nu} SEDs, from which we infer a
star-forming, as opposed to a 'red and dead' population of galaxies, producing
tens to hundreds of solar masses per year down to z=0.5. This discovery
challenges the accepted belief that BCGs should only passively evolve through a
series of gas-poor, minor mergers since z~4 (De Lucia & Blaizot 2007), but
agrees with the improved semi-analytic model of hierarchical structure
formation of Tonini et al. (2012), which predicts star-forming BCGs throughout
the epoch considered. We attribute the star formation inferred from the stacked
infrared SEDs to both major and minor 'wet' (gas-rich) mergers, based on a lack
of key signatures (to date) of the cluster cooling flows to which BCG star
formation is typically attributed, as well as a number of observational and
simulation-based studies that support this scenario.
