The HELLAS2XMM survey

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 ∼ 10 44 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 (10 22 < ∼ N H < ∼ 3 × 10 23 cm −2 ) for most of the objects, confirming that the sources are moderately obscured Compton-thin AGN. Accretion luminosities in the range 5 × 10 44 < ∼ L bol < ∼ 4 × 10 46 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.

TheMBH − Mstarrelation of obscured AGNs at high redshift

“…In contrast, X-ray selected, obscured AGNs (at least in our ERO subsample) may be the descendants of SMGs, observed at a later stage when, at the end of the whole interaction/merging phase, galaxies have already settled onto the local M BH /M star relation. Our targets have in general elliptical-like profiles (Mignoli et al 2004) and an SED typical of quiescent galaxies (Pozzi et al 2010), therefore supporting the latter scenario. However, we that a few objects in the parent sample have strong far-IR and submm emission indicative of vigorous star-forming activity Pozzi et al 2010).…”

“…5 and elliptical-like profiles, implying that the K-band light is generally dominated by an early type host. The latter result is also confirmed in most objects by a SED analysis (Pozzi et al 2007(Pozzi et al , 2010.…”

“…The stellar masses were inferred by exploiting the multiwavelength photometric points available for these sources and the SED fitting code (Fritz et al 2006) used in Pozzi et al (2010) and Vignali et al (2009) for a subsample of obscured AGN in the HELLAS2XMM sample. This code combines synthetic stellar libraries, AGN dusty torii models (Fritz et al 2006) including the intrinsic AGN component (if visible), and starburst IR templates to reproduce the observed SED.…”

“…This code combines synthetic stellar libraries, AGN dusty torii models (Fritz et al 2006) including the intrinsic AGN component (if visible), and starburst IR templates to reproduce the observed SED. We refer to Pozzi et al (2010) for a detailed description of the method. Here we only The red diamonds with errorbars show the location of the three X-ray selected obscured AGNs presented in this paper.…”

Hidden starbursts and active galactic nuclei at 0 < z < 4 from theHerschel-VVDS-CFHTLS-D1 field: Inferences on coevolution and feedback

“…While attempts at quantifying relative contributions have been made using photoionization models (e.g., Kewley et al 2001), much ambiguity still remains. In the absence of spectra, separating, or quantifying the relative strengths of AGN and starbursts have been attempted through spectral energy distribution (SED) fitting (e.g., Marshall et al 2007;Lonsdale et al 2009;Pozzi et al 2010;Johnson et al 2013). This method, however, rapidly loses its effectiveness for galaxies whose SEDs are not dominated by one of the two phenomena, a difficulty which limits its usefulness for investigations on the connection between AGN and starbursts.…”