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

We investigate the observed relationship between black hole mass (MBH), bolometric luminosity (Lbol), and Eddington ratio (λEdd) with optical emission line ratios ([N ii] λ6583/Hα, [S ii] λλ6716, 6731/Hα, [O i] λ6300/Hα, [O iii] λ5007/Hβ, [Ne iii] λ3869/Hβ, and He ii λ4686/Hβ) of hard X-ray-selected AGN from the BAT AGN Spectroscopic Survey (BASS). We show that the [N ii] λ6583/Hα ratio exhibits a significant correlation with λEdd (RPear = -0.44, p-value=3 × 10-13, σ = 0.28 dex), and the correlation is not solely driven by MBH or Lbol. The observed correlation between [N ii] λ6583/Hα ratio and MBH is stronger than the correlation with Lbol, but both are weaker than the λEdd 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 [N ii] λ6583/Hα is a useful indicator of Eddington ratio with 0.6 dex of rms scatter, and that it can be used to measure λEdd and thus MBH from the measured Lbol, 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. [ABSTRACT FROM AUTHOR]