The Starburst-Interstellar Medium Interaction in NGC 1569 II.   Small-Scale Examination of Nebular Emission, H II Region Size Distribution,   and H II Region Luminosity Function
  (abridged) NGC 1569 is an ideal test environment to understand the impact of "feedback" from massive stars on the surrounding ISM. We present HST WFPC2 narrowband imagery of NGC 1569 in an attempt to understand the underlying ionizing emission mechanisms on a 3 pc scale and to generate a H II region size distribution and luminosity function. We use [O III]/Hbeta and [S II]/Halpha ratio maps to find that non-photoionizing mechanisms (e.g. shocks) are responsible for 10%+/-3% of the Halpha emission, 2.3-3.3 times larger than results from similar galaxies. Our results for NGC 1569 indicate that these non-photoionized areas do not lie in low surface brightness regions exclusively. To explain this large percentage of non-photoionized emission, we suggest that NGC 1569 is, indeed, in a post-starburst phase as previous authors have claimed. We also derived slopes for the H II region luminosity function (-1.00+/-0.08) and size distribution (-3.02+/-0.27). The luminosity slope, though shallow, is similar to previous work on this galaxy and other irregular galaxies. The size distribution slope is shallower than previous slopes found for irregular galaxies, but our slope value fits into their confidence intervals and vice versa. 
Brent A. Buckalew (1) and Henry A. Kobulnicky (2) ((1) Caltech and (2)   University of Wyoming)