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
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
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.
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.
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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