Quasars with Super Metal Rich Emission Line Regions

Abstract

We study the degree of chemical enrichment in the Broad Emission Line Regions (BELRs) of two QSOs with unusually strong nitrogen emission lines. The N V 1240/ C IV 1549 intensity ratio is often used as a metallicity indicator for QSOs. The validity of this approach can be tested by studying objects in which the N IV] and N III] lines, in addition to N V, are unusually strong and easily measurable. If all of these ionization states of nitrogen point to the same metallicity, it implies that the large N V strengths observed in most QSOs are not due to some peculiarity of the N V 1240 line. This test had previously been applied to Q0353-383, a QSO long known to have extremely strong N III] and N IV] lines, with the result supporting high metallicity in that object. Here we make the same check in two other QSOs with very strong nitrogen lines, as a step towards using such QSOs to better probe the early chemical enrichment histories of their host galaxies. J1254+0241 has a metallicity of about 10x solar, with good agreement between the abundance results from different line ratios. J1546+5253 has a more moderate metallicity, about 5x solar, but the abundances determined from different line ratios show a much wider scatter than they do for J1254+0241 or Q0353-383. This QSO also has an unusual low-ionization emission line spectrum similar to some low-ionization BAL QSOs and to the unusual AGN I Zw 1. We attribute the peculiarities in its spectrum to some combination of unusual structure and/or unusual physical conditions in its BELR. Our results further affirm the validity of the N V/C IV ratio as an abundance indicator in QSOs.