The Stellar Content and Star Formation History of the late-type spiral galaxy NGC 300 from Hubble Space Telescope observations

Abstract

We present the first WFPC2 V, I photometry for the Sculptor Group galaxy NGC 300 in four fields ranging from the centre 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 RGB and AGB stars, based on a synthetic colour magnitude diagram analysis. Z is found to have been more metal poor than 0.006 (or 0.33Zsolar) with no evidence for significant change in the mean Z value 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 (1998) report of suppressed star formation there during the past 1Gyr with respect to his disk fields at larger radii; but the possibility of significant central extinction affecting our finding remains. 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. (2000) we find (m-M)o = 26.56+/-0.07 (+/-0.13) mag; and a similar value from the Cepheid-independent empirical method by Lee et al. (1993), both in good agreement with the Cepheid distance determined by Freedman et al. (2001). A discrepancy between this and the theoretical calibration of the red giant branch tip magnitude method remains. Finally, we report a newly detected young (up to about 10Myr) stellar association of about average size (~140pc) in one of the disk fields.