Letter to the Editor

LoCuSS: Probing galaxy transformation physics with Herschel ^*,**

¹ School of Physics and Astronomy, University of Birmingham, Edgbaston B15 2TT, UK e-mail: gps@star.sr.bham.ac.uk
² Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ85721, USA
³ Department of Physics and Astronomy, The Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
⁴ Rensselaer Polytechnic Institute (RPI) 110 Eighth Street, Troy, NY 12180, USA
⁵ Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, BC, Canada
⁶ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁷ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁸ Center for Space Science Technology, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁹ Academia Sinica Institute of Astronomy and Astrophysics, PO Box 23-141, 10617 Taipei, Taiwan
¹⁰ Institute of Computational Cosmology, University of Durham, South Road, Durham, DH1 3LE, UK
¹¹ Institute for Physics & Mathematics of the Universe, University of Tokyo, 5-1-5 Kashiwa-no-Ha, Kashiwa City 277-8582, Japan

Received: 31 March 2010
Accepted: 19 May 2010

Abstract

We present an early broad-brush analysis of Herschel/PACS observations of star-forming galaxies in 8 galaxy clusters drawn from our survey of 30 clusters at z 0.2. We define a complete sample of 192 spectroscopically confirmed cluster members down to L_TIR > 3 × 10¹⁰  and L_K > 0.25 . The average K-band and bolometric infrared luminosities of these galaxies both fade by a factor of ~2 from clustercentric radii of ~2 r₂₀₀ to ~0.5 r₂₀₀, indicating that as galaxies enter the clusters ongoing star-formation stops first in the most massive galaxies, and that the specific star-formation rate (SSFR) is conserved. On smaller scales the average SSFR jumps by ~25%, suggesting that in cluster cores processes including ram pressure stripping may trigger a final episode of star-formation that presumably exhausts the remaining gas. This picture is consistent with our comparison of the Herschel-detected cluster members with the cluster mass distributions, as measured in our previous weak-lensing study of these clusters. For example, the spatial distribution of the Herschel sources is positively correlated with the structures in the weak-lensing mass maps at ~5σ significance, with the strongest signal seen at intermediate group-like densities. The strong dependence of the total cluster IR luminosity on cluster mass – L_TIR M_virial² – is also consistent with accretion of galaxies and groups of galaxies (i.e. the substructure mass function) driving the cluster IR luminosity. The most surprising result is that roughly half of the Herschel-detected cluster members have redder S₁₀₀/S₂₄ flux ratios than expected, based on the Rieke et al. models. On average cluster members are redder than non-members, and the fraction of red galaxies increases towards the cluster centers, both of which indicate that these colors are not attributable to systematic photometric errors. Our future goals include to intepret physically these red galaxies, and to exploit this unique large sample of clusters with unprecedented multi-wavelength observations to measure the cluster-cluster scatter in S0 progenitor populations, and to intepret that scatter in the context of the hierarchical assembly of clusters.