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 approximately 2-9 in the high-redshift (z greater-than-or-similar-to 2) sources. Chemical enrichment models indicate that highly evolved gas, with metallicities reaching approximately 3-14 times solar, is needed to produce the N over-abundance. Initial mass functions favoring high-mass stars are also required. The chemical evolution must occur in less than approximately 0.5 Gyr for the z greater-than-or-similar-to 4 objects (if q0 almost-equal-to 1/2), implying that QSOs are associated with vigorous star formation. These results are consistent with the starburst model of AGNs.