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 over-abundance. 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 q₀ ≈ 1/2), implying that QSOs are associated with vigorous star formation. These results are consistent with the starburst model of AGNs.