Gary A. Welch

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.

SCUBA OBSERVATIONS OF EARLY-TYPE GALAXIES

We present SCUBA observations of the emission of 23 E and S0 galaxies at wavelengths of 850 and 450 μm. Four galaxies are detected at 850 μm with signal-to-noise ratios above 3.0, and an additional galaxy, NGC 6524, is detected at both 850 and 450 μm. We use the SCUBA data with published optical and IRAS photometry to construct spectral energy distributions (SEDs) of 15 galaxies extending from 0.36 to 850 μm. Those SEDs are fit by single-temperature dust emission models and by predictions generated by self-consistent Monte Carlo simulations of radiative transfer in three-dimensional distributions of stars and diffuse dust. We discuss implications for the question of cool dust in early-type galaxies, the spatial distribution of diffuse dust, and its ability to generate optical color gradients.

Astronomy: Galaxy formation in action

The tidal forces created by interacting galaxies can literally tear strips from them, producing long tails of matter. These tails often contain stellar clumps that resemble small dwarf galaxies. The idea that these clumps might be recycled to produce new galaxies is supported by evidence for ongoing star formation in these tails.

CCD imagery of the S0 galaxies NGC 3990 and NGC 3998

The structure and colors of NGC 3990 and NGC 3998 are investigated using BR CCD imagery. Fits of bulge-disk models of the galaxies indicate that both disks are somewhat brighter and more compact than typical S0 galaxies in the Virgo and Fornax clusters. Although the two galaxies are separated by only about 3.5 arcmin, none of the obvious signs of gravitational interaction are seen. The colors of both galaxies are normal; the disk of NGC 3998 is somewhat bluer than its bulge. The search has failed to reveal the interstellar dust predicted from the neutral hydrogen observations of NGC 3998. The dust that is seen appears to be mixed with ionized gas which occupies the center of this galaxy and may be the same material seen at longer wavelengths by the IRAS experiment. Its low abundance relative to the neutral gas is consistent with the idea that the ISM was contributed by a gas-rich dwarf galaxy in a destructive merger. 31 refs.