The Stellar Content and Star Formation History of the Late-Type Spiral Galaxy NGC 300 fromHubble Space TelescopeObservations

Abstract

We present the first Hubble Space Telescope (HST) WFPC2 V and I photometry for the Sculptor Group galaxy NGC 300 in four fields ranging from the center to the outer edge. We have made the first measurement of the star formation histories in two disk fields: the oldest stars were born at similar epochs and formation activity increased but at different mean rates. The main disk stellar population is predominantly old, consisting of red giant branch (RGB) and asymptotic giant branch (AGB) stars, based on a synthetic color-magnitude diagram analysis. The metallicity Z is found to have been less than 0.006 (or 0.33 Z⊙), with no evidence for significant change in mean Z over time in both disk fields. In the central region, we find a dearth of bright stars with respect to the two disk fields that cannot be explained by observational effects. Taken at face value, this finding would agree with the Davidge report of suppressed star formation there during the past 109 yr with respect to his disk fields at larger radii; but the possibility remains that significant central extinction affects our finding. We have also determined the first distance modulus estimate based on the tip of the red giant branch method. On the Cepheid distance scale of Ferrarese et al., we find (m-M)0 = 26.56 ± 0.07(±0.13) mag and a similar value from the Cepheid-independent empirical method of Lee, Freedman, & Madore, both in good agreement with the Cepheid distance determined by Freedman et al. A discrepancy between this value and the theoretical calibration of the red giant branch tip magnitude method remains. Finally, we report a newly detected young (up to about 10 Myr) stellar association of about average size (∼140 pc) in one of the disk fields.