Age dating of interaction-induced starbursts in QSO host galaxies and companions

In this dissertation, we test the hypothesis that QSOs are formed via strong tidal interactions or mergers, initially going through an ultraluminous infrared phase. Our approach is to look for traces to this phase in the host galaxies of QSOs. We select a sample of low-redshift bona fide QSOs that may be in a transitionary stage between ULIGs and QSOs. These objects, which we shall call transition QSOs, have an intermediate position in the far infrared color-color diagram between the regions occupied by the two classes of objects. We carry out a systematic spectroscopic and imaging study of these objects in order to determine their interaction and star-forming histories. By modeling the spectra, we obtain ages for recent starburst events in the host galaxies and interacting companions. We find that 9 out of 9 transition QSOs are undergoing tidal interactions, and most are major mergers where at least one parent galaxy had a disk and a bulge. Every object also shows strong recent star-forming activity, and in at least 8 out of 9 cases this activity is directly related to the tidal interaction. The ages we derive for the starburst populations range from currently active star formation in some objects, to post-starburst ages ≲ 300 Myr in others. There is also a clear connection between interactions, starbursts, and QSO activity. Seven out of eight bona fide QSOs in the sample are also bona fide ULIGs. Statistical considerations show that the two phenomena are necessarily physically related in these objects. Our results imply one of two scenarios: (1) at least some ULIGs evolve to become classical QSOs, and the transition stage lasts ≲ 300 Myr, or (2) at least some QSOs are born under the same conditions as ULIGs, and their lifetime as QSOs lasts ≲ 300 Myr. We discuss other properties and trends found in the sample, and propose a model that accounts for all of them, as well as the youth of these systems.