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

Abstract

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 Hα 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 Hα + [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.