The HELLAS2XMM survey - X. The bolometric output of luminous obscured quasars: The Spitzer perspective

Aims. We aim at estimating the spectral energy distributions (SEDs) and the physical parameters related to the black holes harbored in eight high X-ray-to-optical (F-X/F-R > 10) obscured quasars at z > 0.9 selected in the 2-10 keV band from the HELLAS2XMM survey. Methods. We use IRAC and MIPS 24 mu m observations, along with optical and K-s-band photometry, to obtain the SEDs of the sources. The observed SEDs are modeled using a combination of an elliptical template and torus emission (using the phenomenological templates of Silva et al. 2004) for six sources associated with passive galaxies; for two point-like sources, the empirical SEDs of red quasars are adopted. The bolometric luminosities and the M-BH - L-K relation are used to provide an estimate of the masses and Eddington ratios of the black holes residing in these AGN. Results. All of our sources are detected in the IRAC and MIPS (at 24 mu m) bands. The SED modeling described above is in good agreement with the observed near- and mid-infrared data. The derived bolometric luminosities are in the range approximate to 10(45)-10(47) erg s(-1), and the median 2-10 keV bolometric correction is approximate to 25, consistent with the widely adopted value derived by Elvis et al. (1994). For the objects with elliptical-like profiles in the K-s band, we derive high stellar masses (0.8-6.2) x 10(11) M., black hole masses in the range (0.2-2.5) x 10(9) M., and Eddington ratios L/L-Edd < 0.1, suggesting a low-accretion phase.