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We present HST/WFPC2 observations across the disk of the nearby isolateddwarf S0 galaxy NGC 404, which hosts an extended gas disk. Our deepest fieldreaches the red clump and main-sequence stars with ages <500 Myr. Although wedetect trace amounts of star formation at times more recent than 10 Gyr for allfields, the proportion of red giant stars to asymptotic giants and mainsequence stars suggests that the disk is dominated by an ancient (>10 Gyr)population. Detailed modeling of the color-magnitude diagram suggests that ~70%of the stellar mass in the NGC 404 disk formed by z~2 (10 Gyr ago) and at least~90% formed prior to z~1 (8 Gyr ago). These results indicate that the stellarpopulations of the NGC 404 disk are on average significantly older than thoseof other nearby disk galaxies, suggesting that early and late type disks mayhave different long-term evolutionary histories, not simply differences intheir recent star formation rates. Comparisons of the spatial distribution ofthe young stellar mass and FUV emission in GALEX images show that the brightestFUV regions contain the youngest stars, but that some young stars (<160 Myr)lie outside of these regions. FUV luminosity appears to be strongly affected byboth age and stellar mass within individual regions. Finally, we use ourmeasurements to infer the relationship between the star formation rate and thegas density of the disk at previous epochs. We find that most of the history ofthe NGC 404 disk is consistent with star formation that has decreased with thegas density according to the Schmidt law. However, 0.5-1 Gyr ago, the starformation rate was unusually low for the inferred gas density, consistent withthe possibility that there was a gas accretion event that reignited starformation ~0.5 Gyr ago. Such an event could explain why this S0 galaxy hosts anextended gas disk.

The ACS Nearby Galaxy Survey Treasury VI. The Ancient Star Forming disk of NGC 404