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

Publisher Website
Google Scholar
Abstract
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.