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

Abstract

As the nearest dramatic example of a poststarburst galaxy driving a galactic wind, NGC 1569 is an ideal test environment to understand the impact of ``feedback'' from massive star lives and deaths on the surrounding interstellar medium. We present Hubble Space Telescope Wide Field Planetary Camera 2 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]/Hβ and [S II]/Hα ratio maps to find that nonphotoionizing mechanisms (e.g., shocks) are responsible for 10%+/-3% of the Hα emission, 2.5-3 times larger than results from similar galaxies. Note that our method of determining this result is different from these past results, a point that we discuss further in this paper. The area of the nonphotoionized region is 10%-23% of the total. Our results for NGC 1569 indicate that these nonphotoionized areas do not lie in low surface brightness regions exclusively. A comparison with multiwavelength point-source catalogs of NGC 1569 indicates that the dominant nonphotoionizing mechanisms are shocks from supernovae or winds from massive stars. To explain this large percentage of nonphotoionized emission, we suggest that NGC 1569 is, indeed, in a poststarburst phase, as previous authors have claimed. We also derive slopes for the H II region luminosity function (-1.00+/-0.08) and size distribution (-3.02+/-0.27). The luminosity slope, although 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. Within 4 pc of the 10-20 Myr old super star clusters A1, A2, and B, no bright H II regions exist to a luminosity limit of 2.95×1036 ergs s-1, suggesting that the winds and shocks have effectively terminated star formation in this small cavity. In the three annular regions around the super star clusters, both the H II region luminosity function and H II region size distribution are consistent with respect to one another and the galaxy as a whole. The H II region surface densities within the annuli remain the same as the annuli are moved away from the super star clusters. These results indicate that feedback effects in NGC 1569 are confined to the immediate vicinity of the most recent massive star formation event on scales of ~1 pc.