Quasi-Stellar Objects, Ultraluminous IR Galaxies, and Mergers

We test the hypothesis that QSOs are formed via strong tidal interactions or mergers, initially going through an ultraluminous infrared phase, by looking for traces to this phase in the host galaxies of QSOs. We present a systematic spectroscopic and imaging study of a sample of QSOs that may be in a transitionary stage. By modeling the spectra, we obtain ages for the recent starburst events in the host galaxies and interacting companions. We have discussed in detail the first 5 objects in the sample in previous publications; here we present results for the remaining 4 objects, and discuss the sample as a whole. We find that all 9 transition QSOs are undergoing tidal interactions and that 8 are major mergers. Every object also shows strong recent star-forming activity, and in at least eight 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 of the QSOs in the sample are also 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. (abridged)