Abstract
We present a cross-correlation of the European Large Area ISO survey (ELAIS) with the ELAIS Deep X-ray Survey of the N1 and N2 fields. There are 7 Chandra point sources with matches in the ELAIS Final Analysis 15um catalogue, out of a total of 28 extragalactic ISO sources present in the Chandra fields. Five of these are consistent with AGN giving an AGN fraction of ~19 per cent in the 15um flux range 0.8-6 mJy. We have co-added the hard X-ray fluxes of the individually-undetected ISO sources and find a low significance detection consistent with star formation in the remaining population. We combine our point source cross-correlation fraction with the XMM-Newton observations of the Lockman Hole and Chandra observations of the Hubble Deep Field North to constrain source count models of the mid-infrared galaxy population. The low dust-enshrouded AGN fraction in ELAIS implied by the number of cross-identifications between the ELAIS mid-infrared sample and the Chandra point sources is encouraging for the use of mid-infrared surveys to constrain the cosmic star formation history, provided there are not further large undetected populations of Compton-thick AGN.
Highlights
Enormous progress has recently been made in resolving the sources that comprise the extragalactic hard X-ray background (e.g. Mushotzky et al 2000; Cowie et al 2002; Moretti et al 2003)
Some inroads were made on the active galactic nuclei (AGN) fraction for brighter midinfrared sources by Alexander et al (2001) using BeppoSAX observations of the European Large Area Infrared Space Observatory (ISO) survey (ELAIS), but the BeppoSAX depth was not sufficient to detect most Compton-thin type 2 Seyfert galaxies in the targeted ELAIS field
Of the sources detected in field surveys by the ISO satellite, by far the largest fraction and largest number of moderate-redshift (z > 0.5) systems are found in mid-infrared ISOCAM surveys, as opposed to the far-infrared (90 –175 μm) surveys conducted by the ISOPHOT instrument
Summary
Enormous progress has recently been made in resolving the sources that comprise the extragalactic hard X-ray background (e.g. Mushotzky et al 2000; Cowie et al 2002; Moretti et al 2003). Some inroads were made on the AGN fraction for brighter midinfrared sources by Alexander et al (2001) using BeppoSAX observations of the European Large Area Infrared Space Observatory (ISO) survey (ELAIS) (described below), but the BeppoSAX depth was not sufficient to detect most Compton-thin type 2 Seyfert galaxies in the targeted ELAIS field. Fadda et al (2002) provided the first study with a reasonably significant number of sources with hard Xray and mid-infrared emission They combined XMM–Newton observations of the Lockman Hole (LH) and Chandra observations of the HDF-N with coincident ISOCAM data.
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