Gabriela Canalizo

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 fide 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 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.

3C 48: Stellar Populations and the Kinematics of Stars and Gas in the Host Galaxy*

We present deep Keck LRIS spectroscopy of the host galaxy of 3C 48. Our observations at various slit positions sample the different luminous components near the quasar, including the apparent tidal tail to the northwest and several strong emission line regions. By fitting Bruzual & Charlot population synthesis models to our spectra, we obtain ages for the most recent major episodes of star formation in various parts of the host galaxy covered by our slits. There is vigorous current star formation in regions just northeast and southeast of the quasar and post-starburst regions with ages up to ~108 yr in other parts of the host galaxy, but most of the northwest tidal tail shows no sign of significant recent star formation. We use these model fits, together with the kinematics of the stars and gas, to outline a plausible evolutionary history for the host galaxy, its recent starburst activity, the triggering of the quasar, and the interaction of the radio jet with the ambient gas. There is strong evidence that the 3C 48 host is an ongoing merger, and that it is probably near the peak of its starburst activity. Nevertheless, the quasar itself seems to suffer little extinction, perhaps because we are viewing it along a particularly favorable line of sight.

PG 1700+518 Revisited: Adaptive-Optics Imaging and a Revised Starburst Age for the Companion

We present the results of adaptive-optics imaging of the z=0.2923 QSO PG 1700+518 in the J and H bands. The extension to the north of the QSO is clearly seen to be a discrete companion with a well-defined tidal tail, rather than a feature associated with the host galaxy of PG 1700+518 itself. On the other hand, an extension to the southwest of the QSO (seen best in deeper, but lower resolution, optical images) does likely comprise tidal material from the host galaxy. The spectral energy distribution (SED) derived from images in J, H, and two nonstandard optical bands indicates the presence of dust intermixed with the stellar component. We use our previously reported Keck spectrum of the companion, the SED found from the imaging data, and updated spectral synthesis models to constrain the stellar populations in the companion and to redetermine the age of the starburst. While our best-fit age of 0.085 Gyr is nearly the same as our earlier determination, the fit of the new models is considerably better. This age is found to be remarkably robust with respect to different assumptions about the nature of the older stellar component and the effects of dust.

Searching for mergers in early-type QSO host galaxies and a control sample of inactive ellipticals

We present very deep HST/ACS images of five QSO host galaxies, classified as undisturbed ellipticals in earlier studies. For four of the five objects, our images reveal strong signs of interaction such as tidal tails, shells, and other fine structure, suggesting that a large fraction of QSO host galaxies may have experienced a relatively recent merger event. Our preliminary results for a control sample of inactive elliptical galaxies do not reveal comparable fine structure.