The Hi Morphology of Low-Mass Dwarf Galaxies

Abstract

Dwarf irregular galaxies in general show a patchy structure in their neutral hydrogen (Hi) distribution. We present deep optical Calar Alto and high-resolution multiarray VLA data in the 21 cm line of neutral hydrogen of both Holmberg I and M 81 dw A. These members of the nearby M 81 group with total Hi masses of 1.1 × 10⁸ and 8.4 × 10⁶ M _⊙ are at the lower mass range of dwarf irregular galaxies. The Hi in these objects is distributed in a prominent ring-like structure, encompassing the optical counterparts. The radii of the rings are 0.85 kpc for Holmberg I and 0.75 kpc for M 81 dw A, and show Hi fractions of 3/4 and virtually all of the overall Hi content, respectively. The standard model, in which stellar winds and especially supernova explosions are held responsible for the creation of these huge structures, predicts that the shells, if the energy input happened to be recently, are expanding. As our data don’t show clear evidence for expansion, we speculate that break-out and hence the loss of hot gas has occurred which might explain that the shells have stalled.