The Starburst-Interstellar Medium Interaction of NGC 1569 I. Location and Nature of He II Sources Using Hubble Space Telescope WFPC2 Imagery

We present the detection of He II sources in the Im galaxy NGC 1569 from HST WFPC2 imagery. Out of the fifteen detections, seven were Wolf-Rayet stars, five were stellar clusters with associated He II emission, and three were sources of unknown origin. The detected Wolf-Rayet stars' colors and magnitudes are similar to Large Magellanic Cloud late-type WN stars. The physical origin of the other He II sources are discussed. We conclude that the equivalent of 51+/-19 WNL stars have been detected in NGC 1569, and we have estimated the total Wolf-Rayet population in NGC 1569 as 78+/-51. These numbers are compared with the Wolf-Rayet stellar populations in the SMC, LMC, Milky Way, and other starburst galaxies relative to their luminosities, dynamical masses, and ionized gas masses. Previous to this study, Wolf-Rayet stars in NGC 1569 were not detected using ground-based imagery, but were only indicated through longslit spectroscopy. This is the first time the exact locations of the Wolf-Rayet stars in this nearby, well-studied, "post-starburst" galaxy have been determined.