Diameters of H II Regions in M31 and Comparison with the Largest Regions in M33

Abstract

In order to obtain diameters of H ii regions which are comparable to those that are measured in more distant Sb spirals, small-scale plates in Ha wavelengths have been obtained for M31. It is found that H ii regions with low and medium surface brightness mostly range from less than 7 to about 40 arc sec in diameter at the distance of M31. High-surface-brightness H ii regions show a wider distribution of sizes, ranging from less than 7 to about 70 arc sec in apparent diameter. Two features in M3 1 appear to be an agglomeration of smaller H ij regions into single large emission regions of 150 and 180 arc sec, respectively. Observations carried out with exactly the same plate-filter and exposure combinations on M33 show differences in the stiucture of the average H ii region in M31 and M33. The M33 emission regions tend to be single large loops and rings. The M31 emission regions tend to be more stellar and occur in associated groups. If the larger H ii complexes in M31 are classed as single regions, then there is a contradiction to the generally quoted result that H ii regions are larger in Sc than in Sb galaxies. Because of the gradual way in which the intrinsic diameter-number plot falls off toward large diameters, the apparent size of H ii regions is not a very precise indicator of extragalactic distances. The results reported here on M31 and M33 support, however, the idea that the intrinsic sizes of H ii regions are reliable as gross indicators of distance, accurate within approximately a factor of 2. Subject headings: galaxies nebulae