Alan Stockton

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.

Evidence for merger remnants in early-type host galaxies of low-redshift QSOs

We present results from a pilot HSTACS deep imaging study in broadband Vof five low-redshift QSO host galaxies classified in the literature as ellipticals. The aim of our study is to determine whether these early-type hosts formed at high redshift and have since evolved passively, or whether they have undergone relatively recent mergers that may be related to the triggering of the nuclear activity. We perform two-dimensional modeling of the light distributions to analyze the host galaxies’morphology.We find that,while each host galaxy is reasonablywell fitted by a deVaucouleurs profile, the majority of them (4/5) reveal significant fine structure such as shells and tidal tails. These structures contribute between ~5% and 10% to the total V-band luminosity of each host galaxy within a region of r~3r_(eff) and are indicative of merger events that occurred between a few hundred Myr and a Gyr ago. These timescales are comparable to starburst ages in the QSO hosts previously inferred from Keck spectroscopy. Our results thus support a consistent scenario in which most of the QSO host galaxies suffered mergers with accompanying starbursts that likely also triggered the QSO activity in some way, but we are also left with considerable uncertainty on physical mechanisms that might have delayed this triggering for several hundred Myr after the merger.

Extended emission-line regions around QSOs

A narrow-band forbidden O III 5007 A imaging survey of a sample of 47 luminous low-redshift QSOs is reported. The ionized gas is found to be not well correlated with the extended continuum material, as would be expected if it were normal interstellar gas. The distribution of the ionized gas in most of the objects is highly structured over scales of tens of kpc, but this structure is generally quite asymmetric and is clearly not that of gas in an equilibrium configuration. The results are consistent with the assumption that the extended gas has been ionized by the UV continuum from the QSO nucleus. The correlations between the presence of strong extended emission and both the strength of the nuclear narrow-line emission and the radio spectral index generally confirm the results of Boroson and Oke (1984) and Boroson et al. (1985). 67 references.

Two extremely red galaxies

Abstract : This report concerns one of the major observational studies in the ISO Central Programme, the ISO Normal Galaxy Survey. This is a survey of an unbiased sample of spiral and lenticular galaxies selected from the Revised Shapley-Ames Catalog. It is therefore optically-selected, with a brightness limit of blue magnitude = 12, and otherwise randomly chosen. The original sample included 150 galaxies, but this was reduced to 74 when the allocated observing time was expended because the ISO overheads encountered in flight were much larger than predicted.

EXTENDED EMISSION-LINE REGIONS: REMNANTS OF QUASAR SUPERWINDS? *

We give an overview of our recent integral-field-unit spectroscopy of luminous extended emission-line regions (EELRs) around low-redshift quasars, including new observations of five fields. Previous work has shown that the most luminous EELRs are found almost exclusively around steep-spectrum radio-loud quasars, with apparently disordered global velocity fields, and little, if any, morphological correlation with either the host galaxy or the radio structure. Our new observations confirm and expand these results. The EELRs often show some clouds with velocities exceeding 500 km s^(–1), ranging up to 1100 km s^(–1), but the velocity dispersions, with few exceptions, are in the 30-100 km s^(–1) range. Emission-line ratios show that the EELRs are clearly photoionized by the quasars. Masses of the EELRs range up to 10^(10) M☉. Essentially all of the EELRs show relatively low metallicities, and they are associated with quasars that, in contrast to most, show similarly low metallicities in their broad-line regions. The two objects in our sample that do not have classical double-lobed radio morphologies (3C 48, with a compact-steep-spectrum source; Mrk 1014, radio quiet, but with a weak compact-steep-spectrum source) are the only ones that appear to have recent star formation. While some of the less luminous EELRs may have other origins, the most likely explanation for those in our sample is that they are examples of gas swept out of the host galaxy by a large-solid-angle blast wave accompanying the production of the radio jets. The triggering of the quasar activity is almost certainly the result of the merger of a gas-rich galaxy with a massive, gas-poor galaxy hosting the supermassive black hole.

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.

NEAR-INFRARED SPECTROSCOPY OF 0.4 < z < 1.0 CFRS GALAXIES: OXYGEN ABUNDANCES, SFRs, AND DUST

Using new J-band VLT ISAAC and Keck NIRSPEC spectroscopy, we have measured Hα and [N II] λ6584 line fluxes for 0.47 0.1 for most (but not all) of the CFRS galaxies indicate that they lie on the high-metallicity branch of the R23 calibration; (4) about one-third of the 0.47 < z < 0.92 CFRS galaxies in our sample have lower metallicities than local galaxies with similar luminosities and star formation rates; (5) comparison with a chemical evolution model indicates that these low-metallicity galaxies are unlikely to be the progenitors of metal-poor dwarf galaxies at z ~ 0.

Morphologies and Color Gradients of Luminous Evolved Galaxies at z ~ 1.5*

We have examined in detail the morphologies of seven -->z ~ 1.5 passively evolving luminous red galaxies using high-resolution HST NICMOS and ACS imaging data. Almost all of these galaxies appear to be relaxed systems, with smooth morphologies at both rest-frame UV and visible wavelengths. Previous results from spectral synthesis modeling favor a single burst of star formation more than 1 Gyr before the observed epoch. The prevalence of old stellar populations, however, does not correlate exclusively with early-type morphologies as it does in the local universe; the light profiles for some of these galaxies appear to be dominated by massive exponential disks. This evidence for massive old disks, along with the apparent uniformity of stellar age across the disk, suggests formation by a mechanism better described as a form of monolithic collapse than as a hierarchical merger. These galaxies could not have undergone a single major merging event since the bulk of their stars were formed, more than 1 Gyr earlier. There is at least one case, however, that appears to be undergoing a dry merger, which may be an example of the process that converts these unusual galaxies into the familiar spheroids that dominate galaxies comprising old stellar populations at the present epoch.

A Disk Galaxy of Old Stars at z ~ 2.5*

We describe observations of a galaxy in the field of the z = 2.483 radio galaxy 4C 23.56, photometrically selected to have a spectral energy distribution consistent with an old stellar population at the redshift of the radio galaxy. Exploration of redshift?stellar-population?reddening constraints from the photometry indicates that the galaxy is indeed at a redshift close to that of 4C 23.56, that the age of the most recent significant star formation is roughly 2 Gyr, and that reddening is fairly modest, with more reddening required for the younger end of stellar age range. From analysis of a deep adaptive optics image of the galaxy, we find that an r1/4-law profile, common for local spheroidal galaxies, can be excluded quite strongly. On the other hand, a pure exponential profile fits remarkably well, while the best fit is given by a S?rsic profile with index n = 1.49. Reconstruction of the two-dimensional form of the galaxy from the best-fit model is consistent with a disk galaxy with neither a significant bulge component nor gross azimuthal structure. The assembly of roughly 2L* of old stars into such a configuration this early in the history of the universe is not easily explainable by any of the currently popular scenarios for galaxy formation. A galaxy with these properties would seem to require smooth but rapid infall of the large mass of gas involved, followed by a burst of extremely vigorous and efficient star formation in the resulting disk.

A Philosophical Perspective

The paper by Professor Ellis in this volume concludes the contributions from the world of science.