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

A. Lamastra, S. Bianchi+4 more|arXiv (Cornell University)|May 27, 2009

Astrophysical Phenomena and Observations42 references86 citations

TL;DR

This study recalibrates the bolometric luminosity of type 2 AGN using extinction-corrected [OIII] line luminosities, finding a linear correlation between corrected [OIII] and X-ray luminosities. It derives bolometric corrections (C_OIII) of 87, 142, and 454 for luminosity ranges log L_OIII = 38–40, 40–42, and 42–44, respectively, and concludes that type 2 AGN in the SDSS sample at 0.3 < z < 0.4 do not accrete near the Eddington limit, contradicting prior claims of Eddington-limited accretion.

ABSTRACT

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 (L_b) is generally inferred from the [OIII] emission line luminosity (L_OIII). The key issue, in order to estimate the bolometric luminosity in these AGN, is therefore to know the bolometric correction to be applied to L_OIII. A complication arises from the fact that the observed L_OIII is affected by extinction, likely due to dust within the narrow line region. The extinction-corrected [OIII] luminosity (L^c_OIII) is a better estimator of the nuclear luminosity than L_OIII. However, so far only the bolometric correction to be applied to the uncorrected L_OIII has been evaluated. This paper is devoted to estimate the bolometric correction C_OIII=L_b/L^c_OIII in order to derive the Eddington ratios for the type 2 AGN in a sample of SDSS objects. We have collected from the literature 61 sources with reliable estimate of both L^c_OIII and X-ray luminosities (L_X). To estimate C_OIII, we combined the observed correlation between L^c_OIII and L_X with the X-ray bolometric correction. We found, contrary to previous studies, a linear correlation between L^c_OIII and L_X. We estimated C_OIII using the luminosity-dependent X-ray bolometric correction of Marconi et al. (2004), and we found a mean value of C_OIII in the luminosity ranges log L_OIII=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

Motivation & Objective

• To re-evaluate the bolometric correction factor C_OIII = L_bol / L_OIII^c for type 2 AGN, correcting for dust extinction in the narrow line region.
• To resolve discrepancies in previous estimates of Eddington ratios in type 2 AGN, which claimed a significant fraction accrete near or above the Eddington limit.
• To use X-ray luminosities as a reliable proxy for bolometric luminosity, combined with luminosity-dependent X-ray bolometric corrections.
• To test whether the observed high Eddington ratios in previous studies stem from uncorrected [OIII] luminosities affected by dust extinction.

Proposed method

• Collected a sample of 61 type 2 AGN from the SDSS with reliable measurements of both extinction-corrected [OIII] luminosity (L_OIII^c) and X-ray luminosity (L_X).
• Used the Balmer decrement to estimate and correct for dust extinction in the narrow line region, ensuring L_OIII^c reflects intrinsic nuclear luminosity.
• Established a linear correlation between L_X and L_OIII^c across three luminosity bins, contradicting previous non-linear relations.
• Combined the observed L_X / L_OIII^c ratio with the luminosity-dependent X-ray bolometric correction from Marconi et al. (2004) to derive C_OIII.
• Calculated Eddington ratios using the derived C_OIII and black hole masses from the M_BH – σ* relation in the B06 sample.

Experimental results

Research questions

• RQ1 What is the correct bolometric correction factor C_OIII = L_bol / L_OIII^c for type 2 AGN, accounting for dust extinction in the narrow line region?
• RQ2 Does the observed correlation between [OIII] luminosity and X-ray luminosity support a linear or non-linear relationship, and how does this affect bolometric corrections?
• RQ3 Are the high Eddington ratios reported in previous studies for type 2 AGN due to uncorrected extinction in the [OIII] line luminosity?
• RQ4 What is the true Eddington ratio distribution for local type 2 AGN when bolometric luminosity is accurately estimated from extinction-corrected [OIII]?
• RQ5 Does the Iwasawa-Taniguchi effect (anti-correlation between Fe Kα EW and X-ray luminosity) imply a luminosity-dependent torus covering factor that affects [OIII]–X-ray correlations?

Key findings

• A linear correlation was found between extinction-corrected [OIII] luminosity (L_OIII^c) and X-ray luminosity (L_X), contradicting earlier claims of non-linear relations.
• The bolometric correction factor C_OIII was estimated at 87, 142, and 454 for luminosity ranges log L_OIII = 38–40, 40–42, and 42–44, respectively.
• The mean Eddington ratio ⟨λ⟩ for type 2 SDSS AGN at 0.3 < z < 0.4 was found to be approximately 0.1, not 1.0 as previously reported.
• The overestimation of bolometric luminosity in earlier studies is attributed to uncorrected dust extinction in the [OIII] line, which inflates L_OIII by ~0.6 dex on average.
• The results reject the claim that a significant fraction of type 2 AGN are accreting at or above the Eddington limit, aligning them with the typical Eddington ratio distribution seen in other AGN samples.
• The study demonstrates that extinction correction of [OIII] luminosity is essential for accurate bolometric luminosity and Eddington ratio estimation in type 2 AGN.