Summary

Using HST WFPC2 imagery, we find the following on the morphology and location of He II sources in the post-starburst galaxy NGC 1569.

1. Seven He II sources are associated with a star, indicating that they are WR stars. The four WR stars with both magnitudes and colors are consistent with LMC WN stars. Spectroscopy of the individual WR stars will be performed to confirm their spectral class.

2. Five clusters have associated He II. All clusters (excluding SSC A) have emission which would be due to 2-4 WNL equivalent WR stars. SSC A, which was discovered by González-Delgado et al. (1997) to have 25-40 WNL equivalent WR stars, has about 50 WNL equivalents according to our study.

3. De Marchi et al. (1997) showed that SSC A was a superposition of two separate clusters. We have shown the redder of their two clusters (SSC A1) is the location of a higher concentration of red supergiants, and the WR stars are more concentrated in SSC A2. The coexistence of these two populations is consistent with massive stellar evolution.

4. Three sources close to SSC A were unidentified. The interpretation for these sources is that the emission is nebular in origin and possibly related to SSC A. However, all three have He II emission comparable to the WR stars detected in this survey.

5. The total detected number of WR stars in NGC 1569 is 51$\pm$19.The estimated total number of WR stars in NGC 1569 is 78$\pm$51.

If some of these sources are nebular in origin, they could be attributed to either hot stellar ionizing continua (which requires the presence of WR stars in most models) or photoionization due to X-rays. Presently, preliminary analysis of the Chandra observations of this object do not show any of our sources near X-ray sources. However, the model of [Schaerer & Vacca(1998)] and data from KS97 derive similar starburst ages to those from previous and recent stellar models. Therefore, a majority of the nebular He II is associated with the ionizing continua of the massive stars.

This paper is the first in a series of papers on the morphological interaction between the starburst and interstellar medium of NGC 1569. Using other WFPC2 interference-filter images, we will examine next the detailed morphology of the ionized gas components. The excitation mechanisms of the ionized gas will be modeled both empirically and theoretically, and compared with the neutral and molecular gas distribution.

Acknowledgments:

We would like to thank the referee for his/her insightful comments and suggestions. We would also like to thank Henry Kobulnicky for allowing us to look at his thesis spectra of NGC 1569. B.B.'s research is supported by a NASA-GSRP fellowship (NGT 2-52252). Obtaining the HST observations and funding for R.J.D., P.L.S., and D.K.W. via grants related to the Cycle 8 General Observer Program GO-8133 were supported by AURA/STScI.