The bolometric luminosity of type 2 AGN from extinction - corrected [ OIII ] : no evidence for Eddington - limited sources

Context. There have been recent claims that a significant fraction of type 2 AGN accrete close or even above the Eddington limit. In type 2 AGN the bolometric luminosity (Lb) is generally inferred from the [OIII] emission line luminosity (LOIII). The key issue, in order to estimate the bolometric luminosity in these AGN, is therefore to know the bolometric correction to be applied to LOIII. A complication arises from the fact that the observed LOIII is affected by extinction, likely due to dust within the narrow line region. The extinction-corrected [OIII] luminosity (LOIII) is a better estimator of the nuclear luminosity than LOIII. However, so far only the bolometric correction to be applied to the uncorrected LOIII has been evaluated. Aims. This paper is devoted to estimate the bolometric correction COIII=Lb/LOIII in order to derive the Eddington ratios for the type 2 AGN in a sample of SDSS objects. Methods. We have collected from the literature 61 sources with reliable estimate of both LOIII and X-ray luminosities (LX) . To estimate COIII, we combined the observed correlation between LOIII and LX with the X-ray bolometric correction. Results. We found, contrary to previous studies, a linear correlation between LOIII and LX . We estimated COIII using the luminositydependent X-ray bolometric correction of Marconi et al. (2004), and we found a mean value of COIII in the luminosity ranges log LOIII=38-40, 40-42, and 42-44 of 87, 142 and 454 respectively. We used it to calculate the Eddington ratio distribution of type 2 SDSS AGN at 0.3< z <0.4 and we found that these sources are not accreting near their Eddington limit, contrary to previous claims.