Abstract

Red but not dead: Unveiling the Star-forming Far-infrared Spectral Energy Distribution of SpARCS Brightest Cluster Galaxies

Nina Bonaventura (McGill University)

Tracy Webb (McGill University), Adam Muzzin (York University), Allison Noble (University of Toronto), Chris Lidman (Australian Astronomical Observatory), Gillian Wilson (UC Riverside), Howard Yee (University of Toronto), James Geach (University of Hertfordshire), Yashar Hezaveh (Stanford University), David Shupe (Caltech), Jason Surace (Caltech)

We present the results of a comprehensive Spitzer/Herschel infrared analysis of the largest (716) and highest-redshift sample of Brightest Cluster Galaxies (BCGs) from massive (10^14 solar masses) galaxy clusters in the Spitzer Adaptation of the Red-Sequence Cluster Survey (SpARCS), which reveals their unexpected star-forming nature through the identification of their dominant source of infrared energy output (Bonaventura et al. 2016, submitted). We compare the stacked, infrared, broadband spectral energy distributions (SEDs) of the BCGs to a variety of model templates in the literature, in multiple redshift bins between z=0 and z=1.8, from which we derive various physical parameters to serve as star-formation diagnostics. What we uncover is a star-forming, as opposed to a "red and dead" BCG population at z < 2, vigorously producing hundreds of solar masses per year with high efficiency at z > 1, and up to tens of solar masses per year at lower redshifts; follow-up optical spectroscopy of a subset of the BCG sample confirms these findings. This discovery conflicts with the 'dissipationless merger' hypothesis commonly invoked to explain BCG stellar-mass growth over the last 10 Gyr, but is in agreement with the improved semi-analytic model of hierarchical structure formation of Tonini et al. (2012), which predicts star-forming BCGs throughout the epoch considered in our study.

We preliminarily attribute the star formation inferred from our SEDs to both major and minor 'wet' mergers, based on a lack of key signatures (to date) of cooling-flow-induced star formation, as well as a number of observational and simulation-based studies which support this scenario. Furthermore, we consider the lack of a discernible contribution from an Active Galactic Nucleus to the mid-infrared SEDs of the Spitzer 24μm-bright BCGs, comprising 24.4% of the full sample, to reveal a fundamental difference between the BCGs selected from optical/infrared versus X-ray cluster surveys.

Mode of presentation: poster