Hα emission from the disks of spiral galaxies

Abstract

Observations of the amount of Ha emission from the disks of 53 galaxies have been made using large entrance apertures so that the maximum possible fraction of the total area of the galaxy is included. These observations reveal a relationship between B - V color and Ha emission; for a given morphological type the bluer galaxies have more emission at Ha. The effect of the strong emission lines on the colors is not sufficient to produce this trend. Furthermore, the Ha emission comes from discrete H II regions ionized by starlight which are limited in the crudest approximation by the number of hot stars rather than the availability of gas. From the range in Ha emission, we obtain the ratio of hot stars to cooler stars corresponding to the observed range in B - V color. We then compare this with a theoretical model by Searle, Sargent, and Bagnuolo which predicts B - V color as a function of the stellar luminosity function in the galaxy. The two separate methods of determining the change in ratio of hot-to-cool stars agree well. We thus give further observational support to the models of Searle, Sargent, and Bagnuolo.