I present results from an Hα imaging survey of 51 nearby early-type (Sa-Sab) spiral galaxies. Hα emission line flux is one of the best tracers of high mass star formation. Prior to this imaging campaign early-type spirals were considered as a homogeneous group of galaxies with predominantly low rates massive star formation rates. One of the most surprising results from my dissertation has been the discovery of a significant fraction of early-type spirals that host giant HII regions and exhibit massive star formation rates comparable to the high star forming Sc galaxies. A determination of the Hα morphology and a measure of the Hα luminosity suggest that early- type spirals can be divided into two categories based on the Hα luminosity of the largest HII region in the disk. The first category includes galaxies for which the individual HII regions have Hα luminosity LHα < 1039erg s-1. On the other hand, there is at least one HII in the disk of category 2 galaxies with LHα > 1039erg s-1. Category 1 galaxies exhibit diverse nuclear properties but appear undisturbed in the continuum image. Dust lanes and/or tidal tails are more frequent in Category 2 galaxies and have high Hα and far-infrared luminosities. Approximately 37% of nearby early-type spirals belong to Category 2. Our observations reveal early-type spirals to be a heterogeneous class of galaxies that are evolving in the current epoch. HII region luminosity functions for a sub- sample of galaxies have also been obtained to understand the properties of individual star forming regions in early-type spirals. The results reveal a wide diversity in luminosity function shapes. In general, HII regions in Category 2 galaxies are more luminous than Category 1 early-type spirals. Thus, the difference in global Hα luminosities between the two categories of early-type spirals is not attributable to just one anomalous HII region, but rather suggests intrinsic differences in the ensemble of HII regions found in the two categories of galaxies. The surprising presence of giant HII regions and high massive star formation rates in Sa-Sab galaxies may be related to interactions. HI maps of 9 high star forming early-type spirals have been analyzed and our HI maps reveal startling results. Eight out of nine high star forming Sa-Sab galaxies show either tidal tails or bridges in HI, suggesting that, perhaps, interactions play an important role in the evolution of early-type spirals.
- Pub Date:
- December 2001
- Physics: Astronomy and Astrophysics