Ultraviolet Morphology and Unobscured Uv Star Formation Rates of Clash Brightest Cluster Galaxies

Abstract

Brightest cluster galaxies (BCGs) are usually quiescent, but many exhibit star formation. Here we exploit the opportunity provided by rest-frame UV imaging of galaxy clusters in the CLASH (Cluster Lensing and Supernovae with Hubble) Multi-Cycle Treasury Project to reveal the diversity of UV morphologies in BCGs and to compare them with recent simulations of the cool, star-forming gas structures produced by precipitation-driven feedback. All of the CLASH BCGs are detected in the rest-frame UV (280 nm), regardless of their star-formation activity, because evolved stellar populations produce a modest amount of UV light that traces the relatively smooth, symmetric, and centrally peaked stellar distribution seen in the near infrared. Ultraviolet morphologies among the BCGs with strong UV excesses exhibit distinctive knots, multiple elongated clumps, and extended filaments of emission that distinctly differ from the smooth profiles of the UV-quiet BCGs. These structures, which are similar to those seen in the few star-forming BCGs observed in the UV at low redshift, are suggestive of bi-polar streams of clumpy star formation, but not of spiral arms or large, kpc-scale disks. Based on the number of streams and lack of culprit companion galaxies, these streams are unlikely to have arisen from multiple collisions with gas-rich galaxies. These star-forming UV structures are morphologically similar to the cold-gas structures produced in simulations of precipitation-driven AGN feedback in which jets uplift low-entropy gas to greater altitudes, causing it to condense. Unobscured star-formation rates estimated from CLASH UV images using the Kennicutt relation range up to 80 M yr −1 in the most extended and highly structured systems. The circumgalactic gas-entropy threshold for star formation in CLASH BCGs at z ∼ 0.2-0.5 is indistinguishable from that for clusters at z < 0.2.

Interplay Among Cooling, Agn Feedback, and Anisotropic Conduction in the Cool Cores of Galaxy Clusters

“…The snapshots are chosen to represent the diverse morphology of the cold gas, including compact, filamentary, linear, and dispersed distributions (from left to right columns, respectively). Such a wide range of structures are also found in observations of cold gas (e.g., Donahue et al 2000;McDonald et al 2011;Werner et al 2014) and stellar populations (e.g., Donahue et al 2015;Tremblay et al 2015) in CC clusters. In particular, the long, nearly isotropic filaments extending tens of kiloparsecs in the Perseus cluster (Conselice et al 2001) are reproduced (second panel in the bottom row).…”

STAR-FORMING BRIGHTEST CLUSTER GALAXIES AT 0.25 < z < 1.25: A TRANSITIONING FUEL SUPPLY

“…Two of the BCGs shown in Figure 8 (SPT-CLJ0000-5748, SPT-CLJ2043-5035) appear to be in the midst of major mergers, based on both the near-IR and near-UV imaging, despite the fact that these two BCGs reside in the two most relaxed high-z clusters in our sample. None of the starforming BCGs in this high-z, relaxed cluster subsample show evidence for extended, asymmetric filaments of star formation, which are commonly found in analogous low-z systems (e.g., O'Dea et al 2010;McDonald et al 2011b;Donahue et al 2015, Tremblay et al 2015. Instead, the UV emission is concentrated in the BCG center for four out of five BCGs, perhaps indicating that these systems are experiencing either nuclear starbursts or are AGN misidentified as star-forming galaxies.…”

“…These surveys include both optically selected (McDonald 2011;M. McDonald et al, in preparation) and X-ray-selected (Donahue et al 1992Hoffer et al 2012;Rawle et al 2012;Fraser-McKelvie et al 2014;Donahue et al 2015) clusters, and are being compared here to an SZ-selected sample. For the ACCEPT (Cavagnolo et al 2009) and CLASH ) samples, we apply a small correction because both of these cluster samples are biased toward cool core clusters.…”

“…Figure 5 demonstrates that, at 0.5z1, BCGs are evolving similarly to the cluster member galaxies, suggesting a common fueling mechanism. At low-z (z0.5), the evolution of BCGs is less rapid than in the cluster environment, suggesting that the quenching processes acting on member Donahue et al 1992Donahue et al , 2010Donahue et al , 2015McDonald 2011;Hoffer et al 2012;Rawle et al 2012;Fraser-McKelvie et al 2014). Samples marked with an asterisk in the legend have had a bias correction applied, assuming that non-cool cores have passively evolving BCGs and that the true underlying fraction of cool cores is 30% (see Section 3.2).…”

“…Early work focused on searching for multiphase gas and star formation in brightest cluster galaxies (BCGs). Numerous studies have found evidence for ultraviolet (UV) and infrared (IR) continuum (e.g., McNamara & O'Connell 1989;Hicks & Mushotzky 2005;O'Dea et al 2008;McDonald et al 2011b;Hoffer et al 2012;Rawle et al 2012;Fraser-McKelvie et al 2014;Donahue et al 2015), warm, ionized gas (e.g., Hu et al 1985;Johnstone et al 1987;Heckman et al 1989;Crawford et al 1999;Edwards et al 2007;Hatch et al 2007;McDonald et al 2010McDonald et al , 2011a, and both warm and cold molecular gas (e.g., Jaffe & Bremer 1997;Donahue et al 2000;Edge 2001;Edge et al 2002;Edge & Frayer 2003;Salomé & Combes 2003;Hatch et al 2005;Jaffe et al 2005;Johnstone et al 2007;Oonk et al 2010;McDonald et al 2012c)-all of which are indicative of ongoing or recent star formation. Star-forming BCGs were found preferentially in galaxy clusters with "cool cores," as identified by a central density enhancement in the ICM (e.g., Vikhlinin et al 2007;Santos et al 2008;Hudson et al 2010) or low central entropy/cooling time (e.g., Cavagnolo et al 2008Cavagnolo et al , 2009Hudson et al 2010).…”