Chemical Abundances in Broad Emission Line Regions: The “Nitrogen‐loud” Quasi‐Stellar Object Q0353−383

The intensity of the strong N V 1240 line relative to C IV 1549 or to He II1640 has been proposed as an indicator of the metallicity of QSO broad emissionline regions, allowing abundance measurements in a large number of QSOs out tothe highest redshifts. Previously, it had been shown that the (normally) muchweaker lines N III] 1750 and N IV] 1486 could be used in the same way. Theredshift 1.96 QSO 0353-383 has long been known to have N III] and N IV] linesthat are far stronger relative to Ly-alpha or C IV than in any other QSO.Because in this particular case these intercombination lines can be easilymeasured, this unusual object provides an ideal opportunity for testing whetherthe N V line is a valid abundance indicator. Using new observations ofQ0353-383 made both with HST in the ultraviolet and from the ground in thevisible passband, 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 isoverabundant relative to oxygen in Q0353-383 by a factor of ~15 compared tosolar ratios. This agreement among the diagnostics supports the use of theselines for measuring BLR chemical abundances. If nitrogen behaves like asecondary element, such that N/O is proportional to O/H, then the extremenitrogen enhancement in Q0353-383 implies a metallicity of ~15 times the solarvalue. Even if Q0353-383 represents an extreme outlier in the N/O proportionalto O/H relation, the overall metallicity should still be at least five timessolar. Unusually high metallicities in Q0353-383 might imply that we caughtthis object just as the gas-phase metallicity in the central part of its hostgalaxy has peaked, at a time when the interstellar gas supply is nearlyexhausted and hence the fuel source for the central QSO is ready to shut off.Comment: 21 pages, including 3 tables and 7 figures. Accepted for publication in Ap