H-alpha Imaging of Early-type (Sa-Sab) Spiral Galaxies II. Global Properties


New results, based on one of the most comprehensive Hα imaging surveys of nearby Sa–Sab spirals completed to date, reveals early-type spirals to be a diverse group of galaxies that span a wide range in massive star formation rates. While the majority of Sa–Sab galaxies in our sample are forming stars at a modest rate, a significant fraction (~29%) exhibit star formation rates greater than 1 M yr-1, rivaling the most prolifically star-forming late-type spirals. A similar diversity is apparent in the star formation history of Sa–Sab spirals as measured by their Hα equivalent widths. Consistent with our preliminary results presented in the first paper in this series, we find giant H II regions [L(Hα) ≥ 1039 ergs s-1] in the disks of ~37% of early-type spirals. We suspect that recent minor mergers or past interactions are responsible for the elevated levels of Hα emission and, perhaps, for the presence of giant H II regions in these galaxies. Our results, however, are not in total agreement with the Hα study of Kennicutt & Kent, who did not find any early-type spirals with Hα equivalent widths >14 Å. A close examination of the morphological classification of galaxies, however, suggests that systematic differences between the Revised Shapley-Ames Catalog and the Second Reference Catalogue may be responsible for the contrasting results.

Hameed, S. and Devereux, N. 2005, Astronomical Journal, volume 129, 2597

Keywords: galaxies,galaxies evolution, ISM, photometry, spiral, HII regions
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