Linda M. Young

The Neutral Interstellar Medium in Nearby Dwarf Galaxies. III. Sagittarius DIG, LGS 3, and Phoenix

We present new VLA observations of the H I medium of the Local Group dwarf galaxies Sag DIG, LGS 3, and Phoenix. Sag DIG is a gas-rich, blue dwarf irregular with some known recent star formation, whereas LGS 3 and Phoenix are gas-poor, red galaxies of intermediate irregular/spheroidal type with little recent star formation. These galaxies complete a small sample of Local Group and near-Local Group irregular and elliptical galaxies that have been mapped in H I and, where possible, in CO. We compare the properties and kinematics of the ISM in these different galaxy types in order to gain some insights into the relationship between galaxy properties, star formation, and the ISM. Both Sag DIG and LGS 3 have larger H I extents and higher H I fluxes than previously known, and in both cases the H I extends significantly farther than the stellar component. Neither one shows convincing signs of rotation; both seem to derive a significant amount of their support against gravity from random motions in the gas. The dwarf galaxies of the sample support the idea that there are large variations in the dark/luminous mass ratio at a given luminosity. The high sensitivity and high spectral and spatial resolution of these observations also make it possible to study the physical properties of the H I medium. The H I in Sag DIG is decomposed into broad (σ = 10 km s-1) and narrow (σ = 5 km s-1) components, with the broad component distributed throughout the galaxy and the narrow component concentrated into a small number of prominent clumps of about 8 × 105 M☉. It is argued that these H I components are in fact cold and warm phases of the H I medium, as in Galactic H I and in the dwarf irregular Leo A. LGS 3, on the other hand, shows little sign of such a two-phase H I structure. This new information on the phase structure of the ISM in dwarf galaxies is consistent with theoretical models of the H I medium if the H I line width is greater than purely thermal widths. The lack of a cold H I phase may be a reason for the lack of recent star formation in LGS 3; we suggest that the presence of a cold H I phase serves as a better indicator of conditions appropriate for star formation than measures of total H I content. The Phoenix dwarf and LGS 3 have been interpreted as two dwarf spheroidal galaxies which are unusual in that they contain H I. The presence of H I in LGS 3 is interesting, then, in the context of models that remove the gas from dwarf spheroidals by a burst of star formation. Either LGS 3 has not had a burst of star formation sufficient to remove its gas, or gas removal was not complete. It is not clear whether the Phoenix dwarf has H I. The current observations show emission in the vicinity of the galaxy at +55 km s-1 and -23 km s-1 (heliocentric). However, none of this emission is coincident with the optical galaxy. Until the stellar velocities in Phoenix are known, we cannot distinguish whether any of the detected H I is actually associated with the galaxy or is perhaps associated with the Magellanic Stream.

The Neutral Interstellar Medium in Nearby Dwarf Galaxies. II. NGC 185, NGC 205, and NGC 147

We present new, high-resolution observations of the interstellar medium (ISM) in the two dwarf elliptical galaxies NGC 185 and NGC 205, as well as a new upper limit on the H I content of the dwarf elliptical galaxy NGC 147. The data consist of VLA observations of H I emission at ~80 pc resolution (a factor of 3.5 improvement over older data), CO spectra at ~80 pc resolution in NGC 185 and NGC 205, an image of CO emission in NGC 205 at 40 ? 20 pc resolution from the BIMA array, and H? images of NGC 185 and NGC 205. These observations represent the intersection of two separate fields of inquiry: (1) the distribution, kinematics, and origin of neutral gas in giant ellipticals, which are structurally similar to the dwarf ellipticals but cannot currently be studied at such high linear resolution; and (2) the physical properties of the ISM in the Galaxy and its neighbors, which can be studied at very high linear resolution but which might have very different interstellar media than elliptical galaxies. The new H I images show that the neutral gas and stars of NGC 205 are two distinct dynamical systems, whereas in NGC 185 the gas and stars may be parts of the same dynamical system. The H I distributions in these galaxies are less extended than the optical emission and do not appear to be stable rotating disks, a result that is very different from the pattern established for giant ellipticals. An internal gas origin (mass loss from evolved stars) may be plausible for NGC 185. No H I emission is detected in NGC 147 with an upper limit of 3 ? 103 M? for an unresolved source with a line width of 8 km s-1. Furthermore, the H I distributions in NGC 185 and 205 are extremely clumpy on size and mass scales less than 200 pc and ~104 M?, with H I velocity dispersions ranging from 3 to 15 km s-1. We present evidence that the molecular gas in these galaxies is associated with individual clumps or clouds of atomic gas, similar to many Galactic giant molecular clouds. However, the H I column densities in the dwarf ellipticals are factors of 5-10 less than the 1021 cm-2 thought to be necessary to shield Galactic giant molecular clouds against photodissociation. Images of NGC 185 in H? + [N II] show an extended region of emission 50 pc in diameter near the center of the galaxy and the H I column density peak.

Molecular Disks in the Elliptical Galaxies NGC 83 and NGC 2320

The molecular gas in (some) early-type galaxies holds important clues to the history and the future of these galaxies. In pursuit of these clues, we have used the BIMA millimeter array to map CO emission in the giant elliptical galaxies NGC 83 and NGC 2320 and to search for CO emission from the S0 galaxy NGC 5838. We also present V and R images of NGC 83 and NGC 2320 that trace their dust distributions and enable a search for disky stellar structures. The molecular gas in NGC 83 is well relaxed, but both CO and dust in NGC 2320 show asymmetric structures that may be linked to a recent acquisition of the gas. However, the specific angular momentum distribution of molecular gas in NGC 2320 is consistent with that of the stars. Internal origin of the gas (stellar mass loss) cannot, therefore, be ruled out on angular momentum grounds alone. We also consider the evidence for star formation activity and disk growth in these two elliptical galaxies. Radio continuum and FIR fluxes of NGC 83 suggest star formation activity. NGC 2320 has bright [O III] emission, but its large radio-FIR flux ratio and the mismatch between the kinematics of CO and [O III] suggest that the ionized gas should not be attributed to star formation. The origin and future of these two CO-rich early-type galaxies are thus complex, multifaceted stories.

MOLECULAR CLOUDS IN THE DWARF ELLIPTICAL GALAXY NGC 205

We present observations of CO emission in NGC 205 (a dwarf elliptical companion of M31) obtained with the Berkeley-Illinois-Maryland Association (BIMA) array and the IRAM 30 m telescope. We compare the CO results to VLA observations of the H I emission at the same spatial resolution. On scales of 90 pc, the atomic gas and dust have very similar distributions, and the atomic and molecular gas have similar kinematics. A12CO (2-1)/(1-0) brightness temperature ratio of 0.9 is derived for one molecular cloud in NGC 205. This cloud is resolved by the 40 pc × 20 pc beam of the BIMA array; the cloud is elongated and shows a velocity gradient along its major axis. CO line widths are comparable to those in the Galaxy and the Small Magellanic Cloud (SMC) for clouds of the same size, which suggests that the molecular clouds in NGC 205 have similar virial masses to those in the Galaxy and the SMC.