Star Formation in Early-Type(Sa-Sab) Spiral Galaxies
- Hameed, Devereux, Holtzman

Early-Type (Sa-Sab) spirals are widely perceived to be the most inert of spiral galaxies. The perception is based, in large part, on the optical morphology which is dominated by an inert stellar bulge. There are other observational results, however, which suggest that the early-type spirals are not as quiescent as once believed. The Far-infrared Luminosity Function of nearby (D < 40Mpc) spiral galaxies has revealed a previously unsuspected population of early-type spirals with massive star formation rates that rival the most prodigiously star forming Sc galaxies.

We are conducting an H-alpha imaging survey of a complete sample of 57 bright, nearby early-type spiral galaxies. We have so far obtained images of 27 galaxies using the 3.5m telescope at the Apache Point Observatory in New Mexico, and the 1.5m telescope at CTIO in Chile. Our analysis of this initial data suggests that the early-type spirals can be classified into two broad categories based on the H-alpha luminosity of the H-II regions. Classical early-type spirals have no H-II regions with L(H-alpha)>10^39 ergs/s whereas galaxies in the second category contain at least one H-II region with luminosity greater than that.

Category 1:

Classical Early-type Spirals with small H-II Regions in the Disk.

NGC 1371, Continuum(left) and H-alpha(right)

NGC 1371 is an example of low luminous early-type spiral with small H-II regions in the disk. It also seem to have a nuclear emission spiral, similar to the one observed at the center of M81.

Category 2:

Early-type Spirals with Giant H-II Regions in the Disk.

NGC 5915, Continuum(left) and H-alpha(right)

The continuum emission of NGC 5915 is dominated by an inert stellar bulge. When the continuum light from the stellar bulge is subtracted, however, the underlying HII regions are clearly revealed in the H-alpha image.

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