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.

Chemical enrichment and accretion of nitrogen-loud quasars

“…This result, therefore, indicates that NLR metallicities of N-loud quasars are not extremely high relative to normal ones. This result confirms the result obtained by SDSS J1707+6443 in Araki et al (2012). Table 3.…”

“…In this section, we discuss chemical and physical parameters of N-loud quasars by using the measurements as described in Sections 3 and 4. First, we investigate their NLR metallicities with a diagnostic method consisting of equivalent width of [O iii]λ5007 line (Araki et al 2012). Second, we discuss the virial black-hole mass of N-loud quasars estimated based on the Hβ line (see Sec.…”

The mass-metallicity relation of high-z type-2 active galactic nuclei

“…) has found that NLR gas densities of high-z AGNs are relatively higher than those of low-z ones. By investigating rest-frame optical emission lines of quasars, they estimated a typical gas density of ∼ 10 5.5 cm −3 (see also Araki et al 2012).…”

The AGN-Starburst Connection traced by the Nitrogen Abundance

“…Recently, in order to examine whether the N-loud quasars have really extremely high metallicity, we have investigated the narrow-line region (NLR) metallicity of N-loud quasars by using [Oiii]λ5007 emission line; if the NLR metallicity is low, [Oiii] line should be very week due to the low equilibrium temperature of the ionized gas owing to significant metal cooling. Then, we found that the [Oiii] emission line of N-loud quasars is quite strong, suggesting that the NLR in N-loud quasars is not extremely metal rich at all (e.g., Araki et al 2012). Moreover, by investigating the Eddington ratio of N-loud quasars, we found that their Eddington ratio is significantly higher than the average of "normal" SDSS quasars as shown in Figure 1 (Matsuoka et al in prep.).…”