Session 81 - Dwarf Galaxies.
Display session, Friday, January 09
Exhibit Hall,

## [81.09] What Causes the HI Holes in Gas-Rich Dwarf Galaxies?

K. L. Rhode (Yale U.), J. J. Salzer (Wesleyan U.), D. J. Westpfahl (New Mexico Inst M and T)

Several nearby dwarf galaxies have been extensively mapped in HI using the VLA, revealing intricate structure in their interstellar gas component (Puche et al. 1992, Westpfahl amp; Puche 1994). An analysis of these detailed structures shows the neutral gas to contain a number of expanding HI holes. It has been suggested that these holes were created by multiple supernova events, with on the order of 40--100 supernovae required to produce many of the holes. From the sizes and expansion velocities of the holes, ages of typically 10^7 to 10^8 years are assigned to each hole. If the supernova scenario for the formation of the HI holes is correct, it implies the existence of star clusters with a substantial population of A and F main sequence stars at the locations of the hole centers. These clusters should be readily visible in deep images of the galaxies.

We analyze deep optical CCD images of several dwarf galaxies in order to test the supernova hypothesis for the formation of the HI holes. Deep broadband (BVR) and H\alpha narrowband images for a dozen nearby gas-rich irregular galaxies have been obtained. We compare the optical and HI data and carry out sensitive searches for evidence of the expected star clusters within the HI holes. We also use the HI data to constrain models of the expected remnant stellar population. Under the assumption that the HI holes are created by multiple SNe, that the number of SNe required can be determined from the observed energetics of the holes, and that the SNe represent the high mass population of a stellar cluster with a normal IMF, we show that in at least several of the holes the observed upper limits for the remnant cluster brightness are strongly inconsistent with the SNe hypothesis. Many of the HI holes are located in regions of very low optical surface brightness (i.e., lie beyond the Holmberg radius) which show no indication of recent star formation. We explore possible alternative explanations for the existence of the HI holes.