|Q3: Are ULIGs precursors of QSOs ?|
If one chooses not to believe that ULIGs already harbor a dust enshrouded QSO (i.e. an UV-excess AGN with MB < -22.3), then is there evidence that they will become QSOs ? It is probably a fair summary of the ``Green Tower'' view that at least 20-30% of ULIGs (i.e. the ``warm'' objects) already harbor a bonifide QSO, and that a substantial fraction, if not all, of the cool ULIGs have the potential to eventually become QSOs.
Figure 4. Optical images of infrared-excess, optically selected QSOs, powerful radio galaxies, and infrared selected QSOs (MacKenty & Stockton 1984; Kim 1995; Stockton & Ridgway 1991). The `+' sign indicates the position of putative optical nuclei. Tick marks are at 5" intervals and the scale bar represents 10 kpc. All three objects exhibit strong nuclear concentrations of molecular gas, with typically ~ 1010 M concentrated at galactocentric radii 1 kpc (Sanders et al. 1988c; Mirabel, Sanders, & Kazès 1989; Scoville et al. 1989).
Of course there is no reason to believe that all ULIGs, once unshrouded, will necessarily reach the optical/UV luminosity associated with QSOs - they may already have peaked in Lbol and/or some objects may simply be pure starbursts that for some reason never choose to build/fuel a massive black hole. However, recent studies of the host galaxies of QSOs provide new evidence for a plausible evolutionary connection between the ULIG phase and the optical/UV excess QSO phase. The mean and range of the H-band luminosity of QSO hosts, LH ~ 1-4 L*, reported by McLeod & Rieke (1994) and McLeod, Rieke, & Storrie-Lombardi (1999) are remarkably similar to the H-band luminosities of ULIGs (e.g. review by Sanders & Mirabel 1996). Also, it has been known for some time that QSO hosts often exhibit tidal features indicative of strong interactions/mergers [e.g. Stockton & MacKenty 1983: MacKenty & Stockton 1984 (see the left panel in Figure 4)], and more recent HST images of both radio-loud and radio-quiet QSOs show clear signs of large scale tidal debris, circumnuclear knots, bars and rings (e.g. McLure et al. 1999) similar to the inner structures seen in HST images of ULIGs (e.g. Surace et al. 1998). It is becoming easier to believe that QSO hosts are indeed slightly more evolved stages of ULIG mergers.