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[8.13] The Spatial Distribution of HII Regions in Irregular Galaxies

E.W. Roye (Yale University), D.A. Hunter (Lowell Observatory)

CCD H\alpha and V-band images were used to examine the distributions of star-forming regions in 34 irregular galaxies, 29 of which are normal Ims and 6 of which are Blue Compact Dwarf (BCD)/starburst irregulars. The V-band images were used to determine the center, position angle, and inclination of the galaxies. The H\alpha images were used to trace the star-formation through the HII regions. HII region distributions in the planes of the galaxies were compared to turnovers in the rotation curves, the sizes of the galaxies, and locations relative to stellar bars. The overall symmetry and concentration of the HII distributions were also determined.

In general, the HII regions are concentrated towards the centers of the galaxies, with the giant HII regions and complexes being even more centrally concentrated. Furthermore, most of the HII regions and complexes are located within R₂₅, the radius at a B surface brightness of 25 magnitudes per arcsec², as well as within the radius at which the rotation curve turns over. The locations of HII regions, giant HII regions, and complexes are not otherwise correlated with these particular radii. There are no obvious differences in the distribution of HII regions in BCDs and starburst galaxies relative to that of typical irregulars. However, in the two BCD/starburst galaxies for which rotation curves are available, both had HII complexes located well beyond the turnover in the rotation curve. There appears to be no preferential location of giant HII regions or complexes relative to stellar bars. Finally, the overall distribution of HII regions tends to be symmetric.

I would like to thank the National Science Foundation for providing funding for the Research Experiences for Undergraduates program at Northern Arizona University, and Kathy Eastwood for directing the program.