Low-Ionization BAL QSOs in Ultraluminous Infrared Systems

Low-ionization broad absorption line (BAL) QSOs present properties that cannot generally be explained by simple orientation effects. We have conducted a deep spectroscopic and imaging study of the host galaxies of the only four BAL QSOs that are currently known at z < 0.4, and found that all four objects reside in dusty, starburst or poststarburst, merging systems. The starburst ages derived from modeling the stellar populations are in every case a few hundred million years or younger. There is strong evidence that the ongoing mergers triggered both the starbursts and the nuclear activity, thus indicating that the QSOs have been recently triggered or rejuvenated. The low-ionization BAL phenomenon then appears to be directly related to young systems, and it may represent a short-lived stage in the early life of a large fraction of QSOs.