DDO 88: A galaxy-sized hole in the interstellar medium

We present an H I and optical study of the gas-rich dwarf irregular galaxy DDO 88. Although the global optical and H I parameters of DDO 88 are normal for its morphological type, it hosts a large (3 kpc diameter) and unusually complete ring of enhanced H I emission. The normal appearance of this galaxy in the optical and the outer regions of the H I give no hint of the presence of the striking H I ring in the inner regions. The gas ring is located at approximately one-third of the total H I radius and one-half the optically defined Holmberg radius, and contains 30% of the total H I of the galaxy. The ring surrounds a central depression in the H I distribution. If the H I ring and central depression in the gas were formed by the energy input from winds and supernova explosions of massive stars formed in a starburst, as is thought often to be the case, the star-forming event would have formed 0.1% - 1% of the total stellar mass of the galaxy. However, the UBV colors in the H I 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, although there is an old (similar to 1 - 3 Gyr), red cluster near the center of the hole that is massive enough to have produced the hole in the H I. An age estimate for the ring is uncertain, however, 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 indicates 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. Therefore, uncertainties prevent us from concluding that the cluster and the H I hole are definitely related. If this ring and the depression in the gas that it surrounds were not formed by stellar winds and supernovae, this would indicate that some other, currently unidentified, mechanism is operating.