THE CHEMICAL ENRICHMENT OF GAS IN BROAD ABSORPTION LINE QSOs: RAPID STAR FORMATION IN THE EARLY HISTORY OF GALAXIES

摘要

Utilizing spectra from ground-based and Hubble Space Telescope observations, Korista et al. identified at least 12 broad absorption lines in the broad absorption line (BAL) QSO 0226 — 1024 (z_(em) = 2.256), many for the first time. In the present paper we use seven of their measured column densities to investigate the BAL gas ionization equilibrium and metallicity. We assume that the metal abundances follow the rapid star formation (RSF) models of Hamann & Ferland or are simply scaled from solar ratios to (1) limit the free parameters in constraining the overall metallicity and (2) test the applicability of such a scenarios to the enrichment of BAL gas. Photoionization calculations which consider separately the highly ionized gas (C Ⅳ, N Ⅴ, and O Ⅵ) and the doubly ionized metal gas (C Ⅲ, N Ⅲ, and O Ⅲ), as well as those which consider the full set of ionic column densities, all indicate significant enhancements in the metal abundances over solar, up to ～ 10 Z_☉. The requirement for greater than solar metallicities is shown to be independent of the large range of ionizing continuum shapes and of the adopted enrichment scenario, and even larger metallicities could be derived if the metals are allowed to vary independently in a non-RSF enrichment scenario. Because the BALs in 0226 — 1024 that are measurable from the ground have column densities typical in BAL QSOs, the main result, Z > Z_☉, should apply generally to the BAL QSO phenomenon. This result supports an enrichment scenario expected in the cores of massive galaxies in which early rapid star formation produces metal enriched gas. Metallicities enhanced over solar will increase the opacity of the outflowing BAL gas without significantly increasing the mass load of whatever is driving the outflow. This should have a significant impact on radiative acceleration models of BAL outflows which have, up to now, assumed solar metallicity.