The AGN content in luminous infrared galaxies at z∼2 from a global SED analysis including Herschel data

We use Herschel-PACS far-infrared (IR) data, combined with previous multiband information and mid-IR spectra, to properly account for the presence of an active nucleus and constrain its energetic contribution in luminous IR sources at z similar to 2. The sample is composed of 24 sources in the GOODS-South field, with typical IR luminosity of 1012 L?. Data from the 4 Ms Chandra X-ray imaging in this field are also used to identify and characterize active galactic nucleus (AGN) emission. We reproduce the observed spectral energy distribution (SED), decomposed into a host-galaxy and an AGN component. A smooth-torus model for circumnuclear dust is used to account for the direct and reprocessed contribution from the AGN. We confirm that galaxies with typical at z similar to 2 are powered predominantly by star formation. An AGN component is present in nine objects similar to 35 per cent of the sample) at the 3s confidence level, but its contribution to the 81000 m emission accounts for only similar to 5 per cent of the energy budget. The AGN contribution rises to similar to 23 per cent over the 530 m range (in agreement with Spitzer IRS results) and to similar to 60 per cent over the narrow 26 m range. The presence of an AGN is confirmed by X-ray data for three (out of nine) sources, with X-ray spectral analysis indicating the presence of significant absorption, i.e. NH similar to 10231024 cm-2. An additional source shows indications of obscured AGN emission from X-ray data. The comparison between the mid-IR-derived X-ray luminosities and those obtained from X-ray data suggests that obscuration is likely present also in the remaining six sources that harbour an AGN according to the SED-fitting analysis.