We present HST/WFPC2 observations across the disk of the nearby isolated dwarf S0 galaxy NGC 404, which hosts an extended gas disk. The locations of our fields contain a roughly equal mixture of bulge and disk stars. All of our resolved stellar photometry reaches m F814W = 26 (M F814W = -1.4), which covers 2.5 mag of the red giant branch and main-sequence stars with ages <300Myr. Our deepest field reaches m F814W = 27.2 (M F814W = -0.2), sufficient to resolve the red clump and main-sequence stars with ages <500Myr. Although we detect trace amounts of star formation at times more recent than 10Gyr ago 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 (>10Gyr) population. Detailed modeling of the color-magnitude diagram suggests that ∼70% of the stellar mass in the NGC 404 disk formed by z ∼2 (10Gyr ago) and at least ∼90% formed prior to z∼ 1 (8Gyr 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 Galaxy Evolution Explorer images show that the brightest FUV regions contain the youngest stars, but that some young stars (<160Myr) 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, ∼0.5-1Gyr 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 ∼ 0.5Gyr ago. Such an event could explain why this S0 galaxy hosts an extended gas disk.
- Galaxies: evolution
- Galaxies: individual (NGC 404)
- Galaxies: stellar content