A Variation in the Present Star Formation Activity of Spiral Galaxies

The star formation rate in spiral galaxies is considered to be continuously decreasing with time on a time scale of 109 yr. The present star formation activity, on the other hand, occurs in molecular clouds with a time scale of 107 yr, and shows various degrees among galaxies. We have made a new data set of 1681 nearby spiral galaxies from available databases, and have studied the statistics of the present star formation activity. We analyzed the far-infrared and optical B-band surface brightnesses of the HII regions and the non-HII regions in M31, and have shown that the far-infrared-to-optical B-band flux ratio (ƒFIR/ ƒB) is a useful indicator of the present star formation activity of spiral galaxies. For the sample galaxies, we made a distribution diagram of log(ƒFIR/ ƒB) versus log LB for each morphological type. The distribution of ƒFIR/ ƒB has a dispersion of one to two orders of magnitude, even within the same morphological type of galaxies, implying that the star formation activities of spiral galaxies change discontinuously within a short time scale. After analyzing the log(ƒFIR/ ƒB) versus log LB correlation, we suggest that the most active star formation in galaxies does not continue longer than 108 yr. We also constructed a universal histogram of log(ƒFIR/ ƒB) for each morphological type. The earlier-type spirals tend to show a larger variation in the present star formation activity. We discuss the correlation between the present star formation activity and the structures and environments of galaxies. We suggest that a short-term variation occurs primarily due to internal processes which may change with the morphological type of galaxies.