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

Abstract

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∼ 2. The sample is composed of 24 sources in the GOODS-South field, with typical IR luminosity of 10^(12) 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 L_(8-1000µm) ~ 10^(12) L_☉ at z∼ 2 are powered predominantly by star formation. An AGN component is present in nine objects (∼35 per cent of the sample) at the 3σ confidence level, but its contribution to the 8–1000 µm emission accounts for only ∼5 per cent of the energy budget. The AGN contribution rises to ∼23 per cent over the 5–30 µm range (in agreement with Spitzer IRS results) and to ∼60 per cent over the narrow 2–6 µm 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. N_H ∼ 10^(23)–10^(24) 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.

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