Hα Imaging of Early-Type (Sa-Sab) Spiral Galaxies. I.

Hα and continuum images are presented for 27 nearby early-type (Sa–Sab) spiral galaxies. Contrary to popular perception, the images reveal copious massive star formation in some of these galaxies. A determination of the Hα morphology and a measure of the Hα luminosity suggest that early-type spirals can be classified into two broad categories based on the luminosity of the largest H II region in the disk. The first category includes galaxies for which the individual H II regions have LHα < 1039 ergs s-1. Most of the category 1 galaxies appear to be morphologically undisturbed but show a wide diversity in nuclear Hα properties. The second category includes galaxies that have at least one H II region in the disk with LHα ≥ 1039 ergs s-1. All category 2 galaxies show either prominent dust lanes or other morphological peculiarities such as tidal tails, which suggests that the anomalously luminous H II regions in category 2 galaxies may have formed as a result of a recent interaction. The observations, which are part of an ongoing Hα survey, reveal early-type spirals to be a heterogeneous class of galaxies that are evolving in the current epoch. We have also identified some systematic differences between the classifications of spiral galaxies in the Second General Catalog and the Revised Shapley-Ames Catalog that may be traced to subtle variations in the application of the criteria used for classifying spiral galaxies. An examination of earlier studies suggests that perceptions concerning the Hubble-type dependence of star formation rates among spiral galaxies depends on the choice of catalog.