The age and chemical evolution of high-redshift QSOs

We use observed N V/C IV line ratios in QSOs to estimate the N/C abundances, infer the metallicities, and constrain the chemical history of the broad-line gas. Spectral synthesis calculations suggest that nitrogen is overabundant by factors of ~2-9 in the high-redshift (z >~ 2) sources. Chemical enrichment models indicate that highly evolved gas, with metallicities reaching ~3-14 times solar, is needed to produce the N overabundance. Initial mass functions favoring high-mass stars are also required. The chemical evolution must occur in less than ~0.5 Gyr for the z >~ 4 objects (if q_0_ ~1/2), implying that QSOs are associated with vigorous star formation. These results are consistent with the starburst model of AGNs.