An ISOCAM survey through gravitationally lensing galaxy clusters

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Abstract
ESA's Infrared Space Observatory (ISO) was used to perform a deep survey with ISOCAM through three massive gravitationally lensing clusters of galaxies. The total area surveyed depends on source flux, with nearly seventy square arcminutes covered for the brighter flux levels in maps centred on the three clusters Abell 370, Abell 2218 and Abell 2390. We present maps and photometry at 6.7 μm (hereafter 7 μm) and 14.3 μm (hereafter 15 μm), showing a total of 145 mid-infrared sources and the associated source counts. At 15 μm these counts reach the faintest level yet recorded. Almost all of the sources have been confirmed on more than one infrared map and all are identified with counterparts in the optical or near-infrared. Detailed models of the three clusters have been used to correct for the effects of gravitational lensing on the background source population. Lensing by the clusters increases the sensitivity of the survey, and the weakest sources have lensing corrected fluxes of 5 and 18 μJy at 7 and 15 μm, respectively. Roughly 70% of the 15 μm sources are lensed background galaxies. Of sources detected only at 7 μm, 95% are cluster galaxies for this sample. Of fifteen SCUBA sources within the mapped regions of the three clusters seven were detected at 15 microns. The redshifts for five of these sources lie in the range 0.23 to 2.8, with a median value of 0.9. Flux selected subsets of the field sources above the 80% and 50% completeness limits were used to derive source counts to a lensing corrected sensitivity level of 30 μJy at 15 μm, and 14 μJy at 7 μm. The source counts, corrected for the effects of completeness, contamination by cluster sources and lensing, confirm and extend earlier findings of an excess by a factor of ten in the 15 μm population with respect to source models with no evolution. The observed mid-infrared field sources occur mostly at redshifts between 0.4 and 1.5. For the counts at 7 μm, integrating in the range 14 μJy to 460 μJy, we resolve W m-2 sr-1 of the infrared background light into discrete sources. At 15 μm we include the counts from other extensive ISOCAM surveys to integrate over the range 30 μJy to 50 mJy, reaching two to three times deeper than the unlensed surveys to resolve W m-2 sr-1 of the infrared background light. These values correspond to 10% and 55%, respectively, of the upper limit to the infrared background light, derived from photon-photon pair production of the high energy gamma rays from BL-Lac sources on the infrared background photons. However, the recent detections of TeV gamma rays from the BL Lac H1426+428 suggest that the value for the 15 μm background reported here is already sufficient to imply substantial absorption of TeV gamma rays from that source.