Chemical abundances in broad emission line regions: The "nitrogen-loud" quasi-stellar object Q0353-383

The intensity of the strong N v lambda1240 line relative to C IV lambda1549 or to He II lambda1640 has been proposed as an indicator of the metallicity of quasi-stellar object (QSO) broad emission line regions, allowing abundance measurements in a large number of QSOs out to the highest redshifts. Previously, it had been shown that the (normally) much weaker lines N III] lambda1750 and N IV] lambda1486 could be used in the same way. The redshift 1.96 QSO 0353-383 has long been known to have N III] and N IV] lines that are far stronger relative to Lyalpha or C IV than in any other QSO. Because in this particular case these intercombination lines can be easily measured, this unusual object provides an ideal opportunity for testing whether the N v line is a valid abundance indicator. Using new observations of Q0353-383 made both with the Hubble Space Telescope in the ultraviolet and from the ground in the visible passband, we have carefully remeasured the emission lines and reanalyzed their strengths using the latest models of the QSO broad emission line region. We find that intensity ratios involving the strengths of N V N IV, and N III relative to lines of He, C, and O all indicate that nitrogen is over-abundant relative to oxygen in Q0353-383 by a factor of similar to15 compared to solar ratios. This agreement among the diagnostics supports the use of these lines for measuring broad emission line region chemical abundances. If nitrogen behaves like a secondary element, such that N/O proportional to O = H, then the extreme nitrogen enhancement in Q0353-383 implies a metallicity of similar to15 times the solar value. Even if Q0353-383 represents an extreme outlier in the N/O proportional to O = H relation, the overall metallicity should still be at least 5 times solar. Unusually high metallicities in Q0353-383 might imply that we caught this object just as the gas-phase metallicity in the central part of its host galaxy has peaked, at a time when the interstellar gas supply is nearly exhausted and hence the fuel source for the central QSO is ready to shut off.