LoCuSS: Probing galaxy transformation physics withHerschel

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 × 10 10 L and L K > 0.25 L K . The average K-band and bolometric infrared luminosities of these galaxies both fade by a factor of ∼2 from clustercentric radii of ∼2 r 200 to ∼0.5 r 200 , 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 2 virial -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 S 100 /S 24 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.

TheHerschelLensing Survey (HLS): Overview

“…These galaxies therefore are likely to have a larger amount of hotter dust and a more pronounced infrared SED peak compared to the local counterparts with similar infrared luminosities. Similar galaxies were also found in other clusters (Pereira et al 2010;Smith et al 2010), which possibly suggests that this type of SEDs may be specific to the cluster environment.…”

“…The majority of our targets come from the sample of the Local Cluster Substructure Survey (LoCuSS) (e.g., Smith et al 2010), which adopts the following selection criteria: (1) L X > 2 × 10 44 erg/s, (2) 0.15 < z < 0.3, (3) N HI < 7 × 10 20 cm −2 , and (4) −70 < δ < 70. In addition, some number of clusters with spectacular lensed systems were included in the sample.…”

“…The Herschel Lensing Survey is also closely related to two other Herschel open-time key programs: "LoCuSS: A Legacy Survey of Galaxy Clusters at z = 0.2 (Smith et al 2010) and "Constraining the Cold Gas and Dust in Cluster Cooling Flows" (Edge et al 2010a,b). The former will obtain wide (30 × 30 ) and shallow PACS 100/160 μm maps of ∼30 massive galaxy clusters at z ∼ 0.2, many of which are also targeted by the HLS.…”

LoCuSS: weak-lensing mass calibration of galaxy clusters

“…Luminous Infrared Galaxies (LIRGs) and Ultra-Luminous Infrared Galaxies (ULIRGs). On the latter point, the COS-MOS photometry extends to 8µm, whereas we have observed half of the cluster sample discussed in this article with Spitzer/MIPS at 24µm and with Herschel/PACS and SPIRE at 100 − 500µm (Smith et al 2010), albeit only to a depth corresponding to a bolometric infrared luminosity of LIR ≃ 5 × 10 10 L⊙ at z ≃ 0.2. The spectroscopic completeness of follow-up observations with Hectospec (Fabricant et al 2005) is 96 per cent for objects detected with Spitzer down to 0.5mJy and and ∼ 80 per cent down to i = 20 (Mulroy et al 2014;Haines et al 2015).…”

LoCuSS: AHerschelview of obscured star formation in Abell 1835

“…All the SED templates are poor representations of these galaxies, which appear too red in their 100 μm/24 μm colors. The CE01 templates appear to fit the colors of ∼1/2 of the sample, but with templates corresponding to total infrared luminosities that are one to two orders of magnitude fainter than the actual measured luminosities -see Smith et al (2010) and Rawle et al (2010) for a more extended discussion of this 100 μm excess. We estimate total infrared luminosities, L TIR , for these galaxies in two ways: first, we fit the CE01 templates (redshifted to the cluster rest-frame) to the 24, 100 and 160 μm emission and integrate the best-fit spectrum, normalized to the observed fluxes, from rest-frame 3-1100 μm.…”