The ACS Nearby Galaxy Survey Treasury VI. The Ancient Star Forming disk
  of NGC 404
  We present HST/WFPC2 observations across the disk of the nearby isolated
dwarf S0 galaxy NGC 404, which hosts an extended gas disk. Our deepest field
reaches the red clump and main-sequence stars with ages <500 Myr. Although we
detect trace amounts of star formation at times more recent than 10 Gyr for all
fields, the proportion of red giant stars to asymptotic giants and main
sequence 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 stellar
populations of the NGC 404 disk are on average significantly older than those
of other nearby disk galaxies, suggesting that early and late type disks may
have different long-term evolutionary histories, not simply differences in
their recent star formation rates. Comparisons of the spatial distribution of
the young stellar mass and FUV emission in GALEX images show that the brightest
FUV regions contain the youngest stars, but that some young stars (<160 Myr)
lie outside of these regions. FUV luminosity appears to be strongly affected by
both age and stellar mass within individual regions. Finally, we use our
measurements to infer the relationship between the star formation rate and the
gas density of the disk at previous epochs. We find that most of the history of
the NGC 404 disk is consistent with star formation that has decreased with the
gas density according to the Schmidt law. However, 0.5-1 Gyr ago, the star
formation rate was unusually low for the inferred gas density, consistent with
the possibility that there was a gas accretion event that reignited star
formation ~0.5 Gyr ago. Such an event could explain why this S0 galaxy hosts an
extended gas disk.
