Near-infrared spectroscopy of a nitrogen-loud quasar SDSS J1707+6443

Abstract

We present near-infrared spectroscopy of the z ≈ 3.2 quasar SDSS J1707+6443, obtained with MOIRCS on the Subaru Telescope. This quasar is classified as a "nitrogen-loud" quasar because of the fairly strong N III] and N IV] semi-forbidden emission lines from the broad-line region (BLR) observed in its rest-frame UV spectrum. However, our rest-frame optical spectrum from MOIRCS shows strong [O III] emission from the narrow-line region (NLR), suggesting that, at variance with the BLR, NLR gas is not metal-rich. To reconcile these contradictory results, there may be two alternative possibilities: (1) the strong nitrogen lines from the BLR are simply caused by a very high relative abundance of nitrogen and not by a very high BLR metallicity, or (2) the BLR metallicity is not representative of the metallicity of the host galaxy, which is better traced by the NLR. In either case, the strong broad nitrogen lines in the UV spectrum are ot indication of a chemically enriched host galaxy. We estimated the black hole mass and Eddington ratio of this quasar from the velocity width of both C iv and Hβ, which results in log(M BH/M ⊙) = 9.50 and log(L bol/L Edd) =-0.34. The relatively high Eddington ratio is consistent with our earlier result that strong nitrogen emission from BLRs is associated with high Eddington ratios. Finally, we detected significant [Ne III] emission from the NLR, implying a quite high gas density of n e ∼ 10 6 cm -3 and suggesting a strong coupling between quasar activity and dense interstellar clouds in the host galaxy. © 2012 ESO.