DDO 88: A Galaxy-sized Hole in the Interstellar Medium

Abstract

We present an HI and optical study of the gas-rich dwarf irregular galaxy DDO 88. Although DDO 88's global optical and HI parameters are normal for its morphological type, it hosts a large (3 kpc diameter) and unusually complete ring of enhanced HI emission. The gas ring is located at approximately one-third of the total HI radius and one-half the optically-defined Holmberg radius, and contains 30% of the total HI of the galaxy. The ring surrounds a central depression in the HI distribution, so it may be a shell formed by a starburst episode. However, the UBV colors in the HI hole are not bluer than the rest of the galaxy as would be expected if an unusual star-forming event had taken place there recently, but there is an old (~1-3 Gyr), red cluster near the center of the hole that is massive enough to have produced the hole in the HI. An age estimate for the ring, however, is uncertain because it is not observed to be expanding. An expansion model produces a lower estimate of 0.5 Gyr, but the presence of faint star formation regions associated with the ring indicate a much younger age. We also estimate that the ring could have dispersed by now if it is older than 0.5 Gyr. This implies that the ring is younger than 0.5 Gyr. A younger age would indicate that the red cluster did not produce the hole and ring. If this ring and the depression in the gas which it surrounds were not formed by stellar winds and supernovae, this would indicate that some other, currently unidentified, mechanism is operating.