Rate of star formation in normal disk galaxies

Photometry of the integrated Hα emission in a large sample of field spiral and irregular galaxies has been used to obtain quantitative estimates of the total star formation rate (SFR) in the galaxies. The photoionization properties of a stellar population have been modeled for a variety of choices for the initial mass function (IMF). The observed UBV colors and Hα emission equivalent widths place tight constraints on the slope of the IMF between 1 M /sub sun/ and 50 M /sub sun/ in the galaxies; excellent agreement with the observed galaxy colors and Hα emission is obtained with models using an IMF slope close to Salpeter's original value. The properties of late-type galaxies are not well reproduced by the Miller-Scalo solar neighborhood IMF. The extinction-corrected star formation rates are large, as high as 20 M /sub sun/ yr-1 in giant Sc galaxies (H0 = 50 km s-1 Mpc-1). The current rates in late-type galaxies are comparable to the past rates averaged over the age of the disk; late-type disk galaxies have evolved at a nearly constant rate, confirming earlier models by Searle, Sargent, and Bagnuolo. Little evidence is found for a strong correlation between the SFR and average gas density; if the SFR proportional rho/sup n/, then the exponent n must be much less than 1, corroborating earlier studies of star formation in the solar neighborhood by Miller, Scalo, and Twarog. Comparison of the present SFRs with the remaining supply of interstellar gas yields consumption time scales of only a few times 109 years in most cases, in agreement with the model estimates of Larson, Tinsley, and Caldwell