Quasi-stellar objects, ultraluminous infrared galaxies, and mergers

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 of this phase in the host galaxies of QSOs. We select a sample of low-redshift bona Dde QSOs that may be in a transitionary stage between ultraluminous infrared galaxies (ULIGs) and QSOs. These objects, which we 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 the recent starburst events in the host galaxies and interacting companions. We have discussed the first five objects in the sample in detail in previous publications; here we present results for the remaining four objects, and discuss the sample as a whole. We find that all nine transition QSOs are undergoing tidal interactions and that eight 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 poststarburst ages less than or similar to 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 less than or similar to 300 Myr; or (2) at least some QSOs are born under the same conditions as ULIGs, and their lifetime as QSOs lasts less than or similar to 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.