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Session 66 - Galaxies.
Display session, Thursday, June 11
Atlas Ballroom,
The Diverse Nature of Early-type Spirals
Preliminary results of an on-going H\alpha imaging survey of nearby, bright, early-type (Sa-Sab) spirals has revealed them to be a heterogeneous class of galaxies.The H\alpha images indicate that early-type spirals can be divided into two broad categories based on the H\alpha luminosity of the largest H-II region in the disk. The first category includes galaxies for which the individual H-II regions have H\alpha luminosity, L_H\alpha < 10^39ergs^-1 . Previously it was thought that all H-II regions in early-type spirals have L_H\alpha < 10^39 ergs^-1, but our new observations have revealed that some early-type spirals host giant H-II regions that rival the most luminous giant H-II regions in late-type spirals.
Most of the category 1 galaxies appear morphologically undisturbed. Despite the similarities in the continuum images, category 1 galaxies exhibit diverse nuclear properties. The early-type spirals with the lowest H\alpha luminosities host nuclear emission spirals whereas the galaxies that have the highest H\alpha luminosities host nuclear starbursts. Category 2 galaxies include those which have at least one H-II region in the disk with L_H\alpha \ge 10^39ergs^-1. Dust lanes and tidal tails are present in the continuum images of all category 2 galaxies suggesting a recent interaction, which may also explain the presence of giant H-II regions in these galaxies.
