The HELLAS2XMM survey

XIII. Multi-component analysis of the spectral energy distribution of obscured AGN

¹ Dipartimento di Astronomia, Università degli Studi di Bologna, via Ranzani 1, 40127 Bologna, Italy e-mail: f.pozzi@unibo.it
² INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Instituto de Estructura de la Materia (IEM/CSIC), C/ Serrano 121, 28006 Madrid, Spain
⁴ Sterrenkundig Observatorium, Vakgroep Fysica en Sterrenkunde, Universeit Gent, Krijgslaan 281, S9 9000 Gent, Belgium
⁵ INAF - Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Roma, Italy
⁶ Max Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching bei München, Germany

Received: 31 July 2009
Accepted: 23 March 2010

Abstract

Aims. We combine near-to-mid-IR Spitzer data with shorter wavelength observations (optical to X-rays) to get insight into the properties of a sample of luminous, obscured active galactic nuclei (AGN). We aim at modelling their broad-band spectral energy distributions (SEDs) in order to estimate the main parameters related to the dusty torus that is assumed to be responsible for the reprocessed IR emission. Our final goal is to estimate the intrinsic nuclear luminosities and the Eddington ratios for our luminous, obscured AGN.

Methods. The sample comprises 16 obscured high-redshift (0.9 z 2.1), X-ray luminous quasars (L_{2-10 keV} ~ 10⁴⁴ erg s^-1) selected from the HELLAS2XMM survey in the 2–10 keV band. The optical-IR SEDs are described by a multi-component model that includes a stellar component to account for the optical and near-IR emission, an AGN component that dominates in the mid-IR (mainly emission from a dusty torus heated by nuclear radiation), and a starburst to reproduce the far-IR bump. A radiative transfer code to compute the spectrum and intensity of dust reprocessed emission was extensively tested against our multiwavelength data. While the torus parameters and the BH accretion luminosities are a direct output of the SED-fitting procedure, the BH masses were estimated indirectly by means of the local M_bulge–M_BH relation. 


Results. The majority (~80%) of the sources show moderate optical depth (τ_{9.7 µm} ≤ 3), and the derived column densities N_H are consistent with the X-ray inferred values (10²² N_H 3 × 10²³ cm^-2) for most of the objects, confirming that the sources are moderately obscured Compton-thin AGN. Accretion luminosities in the range 5 × 10⁴⁴ L_bol 4 × 10⁴⁶ erg s^-1 are inferred from the multiwavelength fitting procedure. We compare model luminosities with those obtained by integrating the observed SED, finding that the latter are lower by a factor of ~2 in the median. The discrepancy can be as high as an order of magnitude for models with high optical depth (τ_{9.7 µm} = 10). The ratio between the luminosities obtained by the fitting procedure and from the observed SED suggest that, at least for type 2 AGN, observed bolometric luminosities are likely to underestimate intrinsic ones and the effect is more severe for highly obscured sources.
Bolometric corrections from the hard X-ray band are computed and have a median value of k_{2-10 keV} ~ 20. The obscured AGN in our sample are characterized by relatively low Eddington ratios (median λ_Edd ~ 0.08). On average, they are consistent with the Eddington ratio increasing at increasing bolometric correction.