Leslie J. Sage

The Cool Interstellar Medium in S0 Galaxies. I. A Survey of Molecular Gas

Lenticular galaxies remain remarkably mysterious as a class. Observations to date have not led to any broad consensus about their origins, properties, and evolution, although they are often thought to have formed in one big burst of star formation early in the history of the universe and to have evolved relatively passively since then. In that picture, current theory predicts that stellar evolution returns substantial quantities of gas to the interstellar medium; most is ejected from the galaxy, but significant amounts of cool gas might be retained. Past searches for that material, though, have provided unclear results. We present results from a survey of molecular gas in a volume-limited sample of field S0 galaxies selected from the Nearby Galaxies Catalog. CO emission is detected from 78% of the sample galaxies. We find that the molecular gas is almost always located inside the central few kiloparsecs of a lenticular galaxy, meaning that in general it is more centrally concentrated than in spirals. We combine our data with H I observations from the literature to determine the total masses of cool and cold gas. Curiously, we find that, across a wide range of luminosity, the most gas-rich galaxies have ~10% of the total amount of gas ever returned by their stars. That result is difficult to understand within the context of either monolithic or hierarchical models of evolution of the interstellar medium.

THE COOL ISM IN ELLIPTICAL GALAXIES. I. A SURVEY OF MOLECULAR GAS

We present preliminary results from a survey of CO emission from members of a volume-limited sample of noncluster elliptical galaxies. Our intent is to compare the gas properties of these ellipticals to a sample of lenticulars selected using similar criteria. The data, although still sparse, suggest that the cool gas in ellipticals shows the same puzzling upper mass cutoff found in the lenticular galaxies. We find, however, significantly lower detection rates and possibly much lower H2/Himass ratios in the ellipticals. The detection rate is higher among the lower mass galaxies, ashasbeenfoundpreviously.Thisseemspuzzlinggiventhat thedeeperpotentialwells of thelargergalaxies oughtto make gas retention easier, but perhaps that effect is overwhelmed by feedback from the central supermassive black hole. As we have observed � 40% of our full sample, the conclusions are necessarily tentative at this time. Subject headingg galaxies: elliptical and lenticular, cD — galaxies: evolution — galaxies: ISM

The Cool ISM in S0 Galaxies. II. A Survey of Atomic Gas

The place of lenticular galaxies within the range of types of galaxies remains unclear. We previously reported the mass of molecular hydrogen for a volume-limited sample of lenticular galaxies, where we saw that the amount of gas was less than that predicted by the return of stellar mass to the interstellar medium. Here we report observations of atomic hydrogen (H I) for the same sample. Detections in several galaxies make more compelling the case presented in our earlier paper that the mass of cool gas in S0 galaxies cuts off at ~10% of what is expected from current models of gas return from stellar evolution. The molecular and atomic phases of the gas in our sample galaxies appear to be separate and distinct, both spatially and in velocity space. We propose that the molecular gas arises mostly from the stellar mass returned to the galaxy, while the atomic hydrogen is mainly accumulated from external sources (infall, captured dwarfs, etc.). While this proposal fits most of the observations, it makes the presence of the upper mass cutoff even more mysterious.

THE COOL INTERSTELLAR MEDIUM IN ELLIPTICAL GALAXIES. II. GAS CONTENT IN THE VOLUME-LIMITED SAMPLE AND RESULTS FROM THE COMBINED ELLIPTICAL AND LENTICULAR SURVEYS

We report new observations of atomic and molecular gas in a volume-limited sample of elliptical galaxies. Combining the elliptical sample with an earlier and similar lenticular one, we show that cool gas detection rates are very similar among low-luminosity E and S0 galaxies but are much higher among luminous S0s. Using the combined sample we revisit the correlation between cool gas mass and blue luminosity which emerged from our lenticular survey, finding strong support for previous claims that the molecular gas in ellipticals and lenticulars has different origins. Unexpectedly, however, and contrary to earlier claims, the same is not true for atomic gas. We speculate that both the active galactic nucleus feedback and merger paradigms might offer explanations for differences in detection rates, and might also point toward an understanding of why the two gas phases could follow different evolutionary paths in Es and S0s. Finally, we present a new and puzzling discovery concerning the global mix of atomic and molecular gas in early-type galaxies. Atomic gas comprises a greater fraction of the cool interstellar medium in more gas-rich galaxies, a trend which can be plausibly explained. The puzzle is that galaxies tend to cluster around molecular-to-atomic gas mass ratios near either 0.05 or 0.5.

The Missing Interstellar Medium of NGC 147 and M32

We present the results of a search for CO 1 ? 0 emission from NGC 147 and M32, two of the four dwarf elliptical companions of M31. Return of gas from evolved stars to the interstellar medium of these galaxies should have resulted in detections, but we find instead upper limits of 4100 and 5100 M? of H2 for NGC 147 and M32, respectively. Including and earlier H I limit, we find that the gaseous interstellar medium (ISM) of NGC 147 comprises less than 2% of what is expected. The large published H I mass limit for M32 prevents us from reaching a similarly extreme conclusion for this galaxy. These results stand in stark contrast to what is seen in NGC 185 and NGC 205, where the observed gas is approximately what is expected from stellar mass return, although some of the gas in NGC 205 probably had an external origin. There are no obvious differences in masses or luminosities that would explain the results. The proposal that differences may be related to the recent interaction histories of the galaxies with M31 is rendered moot by the lack of orbital information but does not seem to be viable for NGC 147 and NGC 185. We can offer no convincing explanation for these puzzling results, although they may point to a fundamental gap in our understanding of galaxy evolution.

The Puzzling Features of the Interstellar Medium in NGC 205

We present CO 1 → 0 and 2 → 1 observations of NGC 205, which is a dwarf elliptical galaxy and a companion to M31. It has long been suggested that NGC 205 has interacted with M31 in the past. The total mass of gas (molecular, atomic, and X-ray emitting) associated with NGC 205 is ~106 M☉, which is 1 order of magnitude less than the mass that should have been returned to its ISM by evolved stars during its lifetime. A burst of star formation began in NGC 205 ~5 × 108 yr ago, ending a few million yr ago; while some of the mass that was returned to the ISM would be used up by the starburst, star formation is a notoriously inefficient process. Moreover, the gas in NGC 205 is rotating, while the stars are not, so any gas returned to the ISM should have zero net angular momentum. We suggest that the angular momentum was added when the preexisting gas interacted with an external gas cloud, either in the disk of M31 or, perhaps less likely, in the region surrounding it. The recent starburst could have been triggered by the interaction, and the blast waves from ensuing supernovae would then have removed most of the remaining gas. Sufficient time has passed since these events for planetary nebulae to have contributed most of the gas we see in this galaxy.

Detailed Radio View on Two Stellar Explosions and Their Host Galaxy: XRF 080109/SN 2008D and SN 2007uy in NGC 2770

The galaxy NGC 2770 hosted two core-collapse supernova (SN) explosions, SN 2008D and SN 2007uy, within 10 days of each other and 9 years after the first SN of the same type, SN 1999eh, was found in that galaxy. In particular, SN 2008D attracted a lot of attention due to the detection of an X-ray outburst, which has been hypothesized to be caused by either a (mildly) relativistic jet or the SN shock breakout. We present an extensive study of the radio emission from SN 2008D and SN 2007uy: flux measurements with the Westerbork Synthesis Radio Telescope and the Giant Metrewave Radio Telescope, covering ~600 days with observing frequencies ranging from 325 MHz to 8.4 GHz. The results of two epochs of global Very Long Baseline Interferometry observations are also discussed. We have examined the molecular gas in the host galaxy NGC 2770 with the Arizona Radio Observatory 12 m telescope, and present the implications of our observations for the star formation and seemingly high SN rate in this galaxy. Furthermore, we discuss the near-future observing possibilities of the two SNe and their host galaxy at low radio frequencies with the Low Frequency Array.

The Nature of Nearby Counterparts to Intermediate-Redshift Luminous Compact Blue Galaxies. III. Interferometric Observations of Neutral Atomic and Molecular Gas

We present the results of a Very Large Array (VLA) and Owens Valley Radio Observatory Millimeter Wavelength Array (OVRO MMA) follow-up to our single-dish surveys of the neutral atomic and molecular gas in a sample of nearby luminous compact blue galaxies (LCBGs). These luminous, blue, high surface brightness, starbursting galaxies were selected using criteria similar to those used to define LCBGs at higher redshifts. The surveys were undertaken to study the nature and evolutionary possibilities of LCBGs, using dynamical masses and gas depletion timescales as constraints. Here we present nearly resolved VLA H I maps of four LCBGs, as well as results from the literature for a fifth LCBG. In addition, we present OVRO MMA maps of CO(J = 1-0) in two of these LCBGs. We have used the resolved H I maps to separate the H I emission from target galaxies and their companions to improve the accuracy of our gas and dynamical mass estimates. For this subsample of LCBGs, we find that the dynamical masses measured with the single-dish telescope and interferometer are in agreement. However, we find that we have overestimated the mass of H I in two galaxies by a significant amount, possibly as much as 75%, when compared to the single-dish estimates. These two galaxies have companions within a few arcminutes; we find that our single-dish and interferometric measurements of H I masses are in reasonable agreement for galaxies with more distant companions. Our CO(J = 1-0) maps, despite long integration times, were faint and barely resolved, making analysis difficult, except to verify the central concentration of the molecular gas. The H I velocity fields indicate that all five galaxies are clearly rotating, yet distorted, likely due to recent interactions. Our measurements of the gas and dynamical masses of LCBGs point toward evolution into low-mass galaxies such as dwarf ellipticals, irregulars, and low-mass spirals, consistent with studies of LCBGs at higher redshifts.