2009MNRAS.397.1326R

We have combined the well-studied CLASXS Chandra survey in Lockman with the Spitzer SWIRE survey data to study the X-ray-infrared connection for active galactic nuclei (AGN). The sample consists of 401 X-ray-sources, of which 306 are detected by Spitzer, and a further 257 AGN candidates detected through their dust torus, but not by Chandra. We have used spectroscopic redshifts and classifications from the literature, where available, and photometric redshifts for the remainder. For X-ray sources, the X-ray hardness ratio has been modelled in terms of a power law (Γ = 1.9) with absorption N(H). The optical and infrared data have been modelled in terms of our well-established optical galaxy and quasi-stellar object templates, and infrared templates based on radiative transfer models. This type of analysis gives better insight into the infrared spectral energy distributions, and a better separation of the contribution of starbursts and AGN dust tori, than a simple comparison of 24 µm to optical or X-ray fluxes. We also believe this gives more insight than using a library of fixed ultraviolet-infrared templates.

Our estimate of the N(H) distribution is consistent with other studies, but we do find a higher proportion of low absorption objects at z < 0.5 than at z > 0.5. While we find only one X-ray AGN with N(H) > 10²⁴/cm2, we argue that 10 objects with torus luminosity apparently exceeding the bolometric X-ray to 3µm luminosity are strong candidates for being heavily absorbed in X-rays. We also estimate that at least half of the infrared-detected AGN dust tori, which are undetected in X-rays, are likely to be Compton thick. Our estimate of the total number of Compton-thick objects in the 0.4deg² area is ≥130, corresponding to ≥20 per cent of the combined SWIRE-CLASXS sample (and with an upper limit of 39 per cent).

We find no evidence for AGN with no dust tori, and none with a covering factor <1 per cent but there are clear examples of AGN with covering factors of only a few percent and these, though rare, do not fit easily with a unified picture for AGN. The range of dust covering factors is 1-100 per cent, with a mean of 40 per cent, that is a Type 2 fraction of 40 per cent. Measured by the ratio of dust torus luminosity to X-ray or (for Type 1 objects) optical luminosity, the covering factor appears to decrease towards intermediate AGN luminosity, in contradiction to estimates based on ratios of narrow-line and broad-line spectra, but may increase again at low AGN luminosity.

We find 7-10 candidate X-ray starbursts in the SWIRE-CLASXS sample, with X-ray luminosities ranging up to L_Xh = 10⁴⁴erg/s. This is a considerable extension of the luminosity range of X-ray starbursts previously reported, but is consistent with the an extrapolation of the X-ray-infrared relation for starbursts into the realm of hyperluminous infrared galaxies.