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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 464; no. 2; p. 1466
• Main Authors: Oh, Kyuseok, Schawinski, Kevin, Koss, Michael, Trakhtenbrot, Benny, Lamperti, Isabella, Ricci, Claudio, Mushotzky, Richard, Veilleux, Sylvain, Berney, Simon, Crenshaw, D Michael, Gehrels, Neil, Harrison, Fiona, Masetti, Nicola, Soto, Kurt T, Stern, Daniel, Treister, Ezequiel, Ueda, Yoshihiro
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 11-01-2017
• Subjects: Active galactic nuclei; Astrophysics; Black holes; Correlation; Correlation analysis; Galaxies; H alpha line; Luminosity; Pears; Spectroscopy; Spectrum analysis; Stars & galaxies; X-rays
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stw2467

We investigate the observed relationship between black hole mass (M sub( BH)), bolometric luminosity (L sub( bol)) and Eddington ratio ( lambda sub( Edd)) with optical emission-line ratios ([NII] lambda 6583/H alpha , [SII] lambda lambda 6716, 6731/H alpha , [OI] lambda 6300/H alpha , [OIII] lambda 5007/H beta , [NeIII] lambda 3869/H beta and He II lambda 4686/H beta ) of hard X-ray-selected active galactic nuclei (AGN) from the BAT AGN Spectroscopic Survey. We show that the [NII] lambda 6583/H alpha ratio exhibits a significant correlation with lambda sub( Edd) (R sub( Pear) = -0.44, p-value = 3 x 10 super( -13), sigma = 0.28 dex), and the correlation is not solely driven by M sub( BH) or L sub( bol). The observed correlation between [NII] lambda 6583/H alpha ratio and M sub( BH) is stronger than the correlation with L sub( bol), but both are weaker than the lambda sub( 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 [NII] lambda 6583/H alpha is a useful indicator of Eddington ratio with 0.6 dex of rms scatter, and that it can be used to measure lambda sub( Edd) and thus M sub( BH) from the measured L sub( 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.