THE HISTORY OF STAR FORMATION IN GALAXY DISKS IN THE LOCAL VOLUME AS MEASURED BY THE ADVANCED CAMERA FOR SURVEYS NEARBY GALAXY SURVEY TREASURY

We present a measurement of the age distribution of stars residing in spiral disks and dwarf galaxies. We derive a complete star formation history of the similar to 140 Mpc(3) covered by the volume-limited sample of galaxies in the Advanced Camera for Surveys (ACS) Nearby Galaxy Survey Treasury (ANGST). The total star formation rate density history (rho(SFR)(t)) is dominated by the large spirals in the volume, although the sample consists mainly of dwarf galaxies. Our rho(SFR)(t) shows a factor of similar to 3 drop at z similar to 2, in approximate agreement with results from other measurement techniques. While our results show that the overall rho(SFR)(t) has decreased since z similar to 1, the measured rates during this epoch are higher than those obtained from other measurement techniques. This enhanced recent star formation rate appears to be largely due to an increase in the fraction of star formation contained in low-mass disks at recent times. Finally, our results indicate that despite the differences at recent times, the epoch of formation of similar to 50% of the stellar mass in dwarf galaxies was similar to that of similar to 50% of the stellar mass in large spiral galaxies (z greater than or similar to 2), despite the observed galaxy-to-galaxy diversity among the dwarfs.