Chemical enrichment and accretion of nitrogen-loud quasars

Abstract

We present rest-frame optical spectra of 12 "nitrogen-loud" quasars at z - 2:2, whose rest-frame ultraviolet (UV) spectra show strong nitrogen broad emission lines. To investigate their narrow-line region (NLR) metallicities, we measure the equivalent width (EW) of the [O iii]λ5007 emission line: if the NLR metallicity is remarkably high, as suggested by the strong UV nitrogen lines, the [O iii]λ5007 line flux should be very weak due to the low equilibrium temperature of the ionized gas owing to significant metal cooling. In the result we found that our spectra show moderate EW of the [O iii]λ5007 line similar to general quasars. This indicates that nitrogen-loud quasars do not have extremely metal-rich gas clouds in NLRs. This suggests that strong nitrogen lines from broadline regions (BLRs) originate from exceptionally high abundances of nitrogen relative to oxygen without very high BLR metallicities. This result indicates that broad emission lines of nitrogen are not good indicators of the BLR metallicity in some cases. On the other hand, we also investigate virial black hole masses and Eddington ratios by using the Hβ and Civλ1549 lines for our sample. As a result, we found that black hole masses and Eddington ratios of nitrogen-loud quasars tend to be low and high relative to normal quasars, suggesting that nitrogen-loud quasars seem to be in a rapidly accreting phase. This can be explained in terms of a positive correlation between Eddington ratios and nitrogen abundances of quasars, which is probably caused by the connection between the mass accretion onto black holes and nuclear star formation.