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

N. Araki (1), T. Nagao (2,3), K. Matsuoka (1,3), A. Marconi (4), R. Maiolino (5,6), H. Ikeda (1,3), T. Hashimoto (7), Y. Taniguchi (8), and T. Murayama (9) ((1) Graduate School of Science and Engineering, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan; (2) The Hakubi Center for Advanced Research, Kyoto University, Yoshida-Ushinomiya-cho, Sakyo-ku, Kyoto 606-8302, Japan; (3) Department of Astronomy, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan; (4) Dipartimento di Fisica e Astronomia, Universit`a degli Studi di Firenze, Largo E. Fermi 2, 50125, Firenze, Italy; (5) INAF – Osservatorio Astrofisico di Roma, Via di Frascati 33, 00040 Monte Porzio Catone, Italy; (6) Cavendish Laboratory, Univerisy of Cambridge, 19 J. J. Thomson Ave., Cambridge CB3 0HE, UK; (7) Optical and Infrared Astronomy Division, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka 181-8588, Japan; (8) Research Center for Space and Cosmic Evolution, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan; (9) Astronomical Institute, Graduate School of Science, Tohoku University, Aramaki, Aoba, Sendai 980-8578, Japan)

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 NIII] and NIV] 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 [OIII] emission from the narrow-line region (NLR) suggesting that, at variance with the BLR, NLR gas is not metal-rich. In order to reconcile these contradictory results, there may be two alternative possibilities; (1) the strong nitrogen lines from the BLR are simply due to a very high relative abundance of nitrogen rather than to a very high BLR metallicity, or (2) the BLR metallicity is not representative of the metallicity of the host galaxy, better traced by the NLR. In either case, the strong broad nitrogen lines in the UV spectrum are not 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 CIV and H_beta, that results in log(M_BH/M_sun) = 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 [NeIII] emission from the NLR, implying a quite high gas density of n~10^6 cm^-3 and suggesting a strong coupling between quasar activity and dense interstellar clouds in the host galaxy.

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