Multi-Wavelength Data


Galaxies emit radiation across the full extent of the electromagnetic spectrum. It turns out that to obtain robust measures of their fundamental properties such as their stellar masses, star formation rates or the gas and dust contents that together make up their interstellar medium (ISM), astronomers need to observe galaxies not just in the narrow optical wavelength range visible to the naked eye, but extend into the infrared, ultraviolet, radio or X-rays. This has required the building of space telescopes capable of observing in the wavelength ranges that are unavailable to ground-based telescopes, and the development of multi-wavelength surveys combining datasets from optical, infrared and ultraviolet regimes.

The centrepiece of the survey is the LoCuSS Herschel Key Programme to map all 30 clusters with far-infrared imaging at five wavebands from 100-500um with Herschel’s PACS and SPIRE instruments, covering not just the cluster cores, but extending out to the virial radius and beyond, to detect the thermal emission from dust-obscured star-formation in galaxies falling into the clusters for the first time. In conjunction our existing 24um imaging from the MIPS instrument on the Spitzer Space Telescope, we will have full spectral coverage of the mid- and far-infrared thermal emission from dust heated by star formation and the general interstellar radiation field. Combining these data we are able to derive robust measures of the total obscured star formation rate in the cluster galaxies, as well as gain a better understanding of the dust contents and interstellar medium of cluster galaxies, constraining the dust masses and temperatures of a large sample of cluster galaxies for the first time.

We have also obtained wide-field near-infrared data for each of the 30 clusters from the 4m telescope at the Kitt Peak National Observatory in Arizona and the United Kingdom Infra-Red Telescope (UKIRT) on Mauna Kea, vital to identify probable cluster members for ACReS and to derive robust stellar mass estimates of the galaxies. For most of these clusters there is existing optical data from the Sloan Digital Sky Survey and/or our deep Subaru imaging, while we have a GALEX programme which has obtained deep ultraviolet imaging sensitive to the component of star formation not obscured by dust, providing a natural complement to our Spitzer mid-infrared data. Our X-ray imaging from Chandra has allowed us to not only map the hot baryonic content of the cluster, but identify those cluster galaxies hosting X-ray AGN.

Map showing the spatial coverages provided by the various multi-wavelength datasets we have assembled for Abell 1835 at z=0.252, one of the clusters in our sample: including GALEX ultraviolet imaging (magenta circle); MMT/Hectospec fiber-fed spectroscopy (black circle); UKIRT J,K-band near-infrared imaging (larger red square); Subaru V,i optical imaging (brown box) used for the weak lensing analysis of Okabe et al. (2010); Spitzer/MIPS 24um mid-infrared imaging (green box); Herschel PACS+SPIRE 100-500um far-infrared imaging (blue box). The spectroscopically-confirmed members of Abell 1835 are indicated as small filled symbols (scaled by their K-band luminosity) colored green if detected by Spitzer, blue if also detected by Herschel, and red if not detected by either infrared space telescopes. For comparison, the r_200 radius of the cluster is shown by the dashed black circle, demonstrating that our infrared data cover the virialized regions of the cluster, while our spectroscopic, UV and near-infrared data extend well into the infall regions.



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