PAST AND FUTURE STAR-FORMATION IN DISK GALAXIES

We have combined H alpha and UBV measurements of 210 nearby Sa-Irr gal axies with new photometric synthesis models to reanalyze the past and future star formation timescales in disks. The integrated photoionizat ion rates and colors of disks are best fitted by a stellar initial mas s function (IMF) which is enriched in massive stars by a factor of 2-3 relative to the Scale solar neighborhood IMF. We have used published surface photometry of spiral galaxies to analyze the star formation hi stories of disks independent of their bulge properties. The ratio of t he current star formation rate (SFR) to the average past rate increase s from of order 0.01 in Sa galaxies to 1 in Sc-Irr disks. This confirm s that the pronounced change in the photometric properties of spiral g alaxies along the Hubble sequence is predominantly due to changes in t he star formation histories of disks, and only secondarily to changes in the bulge/disk ratio. A comparison of current SFRs and gas masses o f the sample yields median timescales for gas consumption of similar t o 3 Gyr, in the absence of stellar recycling. However, a proper time-d ependent treatment of the gas return from stars shows that recycling e xtends the gas lifetimes of disks by factors of 1.5-4 for typical disk parameters. Consequently the current SFRs in many (but not all) disks can be sustained for periods comparable to the Hubble time.