We provide constraints on the AGN contribution to the mid-IR extragalactic background light from a correlation analysis of deep X-ray and mid-IR observations in two regions centred on the Lockman Hole and Hubble Deep Field North ( HDFN ) .
The Lockman region , of more than 200 square arcminutes , was observed by ISOCAM and XMM–Newton to a depth of 0.3 mJy at 15 \mu m ( resolving more than 30 % of the mid-IR background ) .
In the same area XMM–Newton reached flux limits of 1.4 \times 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } in the 2–10 keV energy band and 2.4 \times 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } in the 5–10 keV energy band , resolving about 80 % of the 2–10 keV and 60 % of the 5–10 keV backgrounds ( the deepest observation in this hard band to date ) .
Among the 76 galaxies detected by XMM–Newton , 24 show mid-IR emission , but the relative percentage of X-ray sources with mid-IR counterparts increases with the band energy : from 30 % of the 0.5–2 keV sources up to 63 % of the 5–10 keV sources .
In contrast , only a small fraction of the mid-IR sources ( around 10 % ) show X-ray emission within the sensitivity limits of XMM–Newton observations .
The region centred on the HDFN has been observed by ISOCAM ( 24 square arcminutes ) to a depth of 0.05 mJy ( more than 50 % of the mid-IR background is resolved at this limit ) and covered with a 1 Msec exposure by Chandra .
In this case , 25 % of the mid-IR sources are detected in the X-ray , while 30–40 % of the X-ray sources show mid-IR emission .
Under the conservative assumption that all XMM sources except stars or galaxy clusters are AGN–dominated , AGNs contribute ( 15 \pm 5 ) \% of the total mid-IR flux in the Lockman Hole .
For the HDFN we have assumed that AGN–dominated sources are luminous X-ray sources and sources with SEDs from radio to X-ray wavelengths typical of local AGNs , in which case we find that ( 18 \pm 7 ) \% of the mid-IR flux are due to AGN emission .
If we put together all the existing information from the deepest HDFN data to the bright large-area sample in the ELAIS S1 region observed with BeppoSAX ( for a total of 50 X-ray–mid-IR matched sources ) using the median mid-IR to X-ray spectral indices as a function of the X-ray flux , we find an AGN contribution to the 15 \mu m background of ( 17 \pm 2 ) \% .
This figure should be taken as an upper limit to the AGN contribution to the CIRB energy density , assuming standard SEDs for IR sources .
We conclude that the population of IR luminous galaxies detected in the ISOCAM deep surveys , and the CIRB sources themselves , are mainly constituted by dust-obscured starbursts .