Evidence for Multiple Mergers among Ultraluminous IR Galaxies (ULIRGs): Remnants of Compact Groups?

In a large sample of ULIRGs imaged with HST, we have identified a significant subsample that shows evidence for multiple mergers. The evidence is seen among two classes of ULIRGs: (1) those with multiple remnant nuclei in their core, sometimes accompanied by a complex system of tidal tails; and (2) those that are in fact dense groupings of interacting (soon-to-merge) galaxies. We conservatively estimate that, in the redshift range 0.05<z<0.20, at least 20 (out of 99) ULIRGs satisfy one or both of these criteria. We present several cases and discuss the possibility that the progenitors of ULIRGs may be the more classical weakly interacting compact groups of galaxies (Hickson 1997). An evolutionary progression is consistent with the results: from compact groups to pairs to ULIRGs to ellipticals. The last step follows the blowout of gas and dust from the ULIRG.

The RMS Survey: Near-IR Spectroscopy of Massive Young Stellar Objects

Near-infrared H- and K-band spectra are presented for 247 objects, selected from the Red MSX Source (RMS) survey as potential young stellar objects (YSOs). 195 (~80%) of the targets are YSOs, of which 131 are massive YSOs (L_BOL > 5x10^3 L_solar), M > 8M_solar. This is the largest spectroscopic study of massive YSOs to date, providing a valuable resource for the study of massive star formation. In this paper we present our exploratory analysis of the data. The YSOs observed have a wide range of embeddedness (2.7 < A_V < 114), demonstrating that this study covers minimally obscured objects right through to very red, dusty sources. Almost all YSOs show some evidence for emission lines, though there is a wide variety of observed properties. The most commonly detected lines are Brgamma, H_2, fluorescent FeII, CO bandhead, [FeII] and HeI 2-1 2^1S-2^1P, in order of frequency of occurrence. In total, ~40% of the YSOs display either fluorescent FeII 1.6878um or CO bandhead emission (or both), indicative of a circumstellar disc; however, no correlation of the strength of these lines with bolometric luminosity was found. We also find that ~60% of the sources exhibit [FeII] or H_2 emission, indicating the presence of an outflow. Three quarters of all sources have Brgamma in emission. A good correlation with bolometric luminosity was observed for both the Brgamma and H_2 emission line strengths, covering 1 L_solar< L_BOL < 3.5x10^5 L_solar. This suggests that the emission mechanism for these lines is the same for low-, intermediate-, and high-mass YSOs, i.e. high-mass YSOs appear to resemble scaled-

Ultraluminous Infrared Galaxies

Ever since their discovery in the 1970s, UltraLuminous InfraRed Galaxies (ULIRGs; classically Lir>10^12Lsun) have fascinated astronomers with their immense luminosities, and frustrated them due to their singularly opaque nature, almost in equal measure. Over the last decade, however, comprehensive observations from the X-ray through to the radio have produced a consensus picture of local ULIRGs, showing that they are mergers between gas rich galaxies, where the interaction triggers some combination of dust-enshrouded starburst and AGN activity, with the starburst usually dominating. Very recent results have thrown ULIRGs even further to the fore. Originally they were thought of as little more than a local oddity, but the latest IR surveys have shown that ULIRGs are vastly more numerous at high redshift, and tantalizing suggestions of physical differences between high and low redshift ULIRGs hint at differences in their formation modes and local environment. In this review we look at recent progress on understanding the physics and evolution of local ULIRGs, the contribution of high redshift ULIRGs to the cosmic infrared background and the global history of star formation, and the role of ULIRGs as diagnostics of the formation of massive galaxies and large-scale structures.

The FIRST Sample of Ultraluminous Infrared Galaxies at High Redshift I. Sample and Near-IR Morphologies

We present a new sample of distant ultraluminous infrared galaxies. The sample was selected from a positional cross--correlation of the IRAS Faint Source Catalog with the FIRST database. Objects from this set were selected for spectroscopy by virtue of following the well-known star-forming galaxy correlation between 1.4 GHz and 60 micron flux, and by being optically faint on the POSS. Optical identification and spectroscopy were obtained for 108 targets at the Lick Observatory 3m telescope. Most objects show spectra typical of starburst galaxies, and do not show the high ionization lines of active galactic nuclei. The redshift distribution covers 0.1 < z < 0.9, with 13 objects at z > 0.5 and an average redshift of 0.31. K-band images were obtained at the IRTF, Lick, and Keck observatories in sub-arcsec seeing of all optically identified targets. About 2/3 of the objects appear to be interacting galaxies, while the other 1/3 appear to be normal. Nearly all the identified objects have far-IR luminosities greater than 10^11 L_sun, and ~25% have L_FIR > 10^12 L_sun.

The extended narrow-line region of two type-I quasi-stellar objects

We investigate the narrow-line region (NLR) of two radio-quiet QSOs, PG1012+008 and PG1307+085, using high signal-to-noise spatially resolved long-slit spectra obtained with FORS1 at the Very Large Telescope. Although the emission is dominated by the point-spread function of the nuclear source, we are able to detect extended NLR emission out to several kpc scales in both QSOs by subtracting the scaled central spectrum from outer spectra. In contrast to the nuclear spectrum, which shows a prominent blue wing and a broad line profile of the [O III] line, the extended emission reveals no clear signs of large scale outflows. Exploiting the wide wavelength range, we determine the radial change of the gas properties in the NLR, i.e., gas temperature, density, and ionization parameter, and compare them with those of Seyfert galaxies and type-II QSOs. The QSOs have higher nuclear temperature and lower electron density than Seyferts, but show no significant difference compared to type-II QSOs, while the ionization parameter decreases with radial distance, similar to the case of Seyfert galaxies. For PG1012+008, we determine the stellar velocity dispersion of the host galaxy. Combined with the black hole mass, we find that the luminous radio-quiet QSO follows the local M_BH-sigma* relation of active galactic nuclei.