BAT AGN Spectroscopic Survey-III. An observed link between AGN Eddington ratio and narrow emission line ratios

Abstract

We investigate the observed relationship between black hole mass ($M_{\mathrm{BH}}$), bolometric luminosity ($L_{\mathrm{bol}}$), and Eddington ratio ($\hat{I}{»}_{\mathrm{Edd}}$) with optical emission line ratios ([NII] λ6583/Hα, [SII] λλ6716,6731/Hα, [OI] λ6300/Hα, [OIII] λ5007/Hβ, [NeIII] λ3869/Hβ, and HeII λ4686/Hβ) of hard X-ray-selected AGN from the BAT AGN Spectroscopic Survey (BASS). We show that the [NII] λ6583/Hα ratio exhibits a significant correlation with $\hat{I}{»}_{\mathrm{Edd}}$ ($R_{\mathrm{Pear}}$ = -0.44, $p$-value=$3\times 10^{-13}$, σ = 0.28 dex), and the correlation is not solely driven by $M_{\mathrm{BH}}$ or $L_{\mathrm{bol}}$. The observed correlation between [NII] λ6583/Hα ratio and $M_{\mathrm{BH}}$ is stronger than the correlation with $L_{\mathrm{bol}}$, but both are weaker than the $\hat{I}{»}_{\mathrm{Edd}}$ correlation. This implies that the large-scale narrow lines of AGN host galaxies carry information about the accretion state of the AGN central engine. We propose that the [NII] λ6583/Hα is a useful indicator of Eddington ratio with 0.6 dex of rms scatter, and that it can be used to measure $\hat{I}{»}_{\mathrm{Edd}}$ and thus $M_{\mathrm{BH}}$ from the measured $L_{\mathrm{bol}}$, even for high redshift obscured AGN. We briefly discuss possible physical mechanisms behind this correlation, such as the mass-metallicity relation, X-ray heating, and radiatively driven outflows.

Accepted for publication in MNRAS, 9 pages, 5 figures, 1 table