Previous
abstract 
Next
abstract
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 \& 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 100 supernovae required to produce each hole. 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, then one would predict the existence of star clusters with a substantial population of A and F main sequence stars at the locations of the centers of the holes. These clusters should be readily visible in deep images of the galaxies as blue intensity enhancements. In order to test the supernova hypothesis, images were obtained at Kitt Peak National Observatory in February 1994 using the 0.9m telescope. Deep broadband (BVR) and H$\alpha$ narrowband images of seven dwarf irregular galaxies (Holmberg I, Holmberg II, Holmberg IX, Sextans A, M81dwA, IC 2574, and K73) were acquired. We present these images along with the VLA maps and photometry of hole and inter-hole regions carried out to detect the blue knots which would be the telltale evidence for the star clusters that spawned the multiple supernovae. Somewhat surprisingly, we do not see any evidence for these blue knots. Many of the HI holes are located in regions of very low optical surface brightness which show no indication of recent star formation. From the available evidence, we conclude that the supernovae hypothesis for the creation of the HI holes observed in these galaxies is incorrect.
