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

We present the first WFPC2 V, I photometry for the Sculptor Group galaxy NGC300 in four fields ranging from the centre to the outer edge. We have made thefirst measurement of the star formation histories in two disk fields: theoldest stars were born at similar epochs and formation activity increased butat different mean rates. The main disk stellar population is predominantly old,consisting of RGB and AGB stars, based on a synthetic colour magnitude diagramanalysis. 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 bothdisk fields. In the central region, we find a dearth of bright stars withrespect to the two disk fields that cannot be explained by observationaleffects. Taken at face value, this finding would agree with the Davidge (1998)report of suppressed star formation there during the past 1Gyr with respect tohis disk fields at larger radii; but the possibility of significant centralextinction affecting our finding remains. We have also determined the firstdistance modulus estimate based on the tip of the red giant branch method: onthe 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-independentempirical method by Lee et al. (1993), both in good agreement with the Cepheiddistance determined by Freedman et al. (2001). A discrepancy between this andthe theoretical calibration of the red giant branch tip magnitude methodremains. Finally, we report a newly detected young (up to about 10Myr) stellarassociation of about average size (~140pc) in one of the disk fields.Comment: 47 pages, 12 figures (the quality of Figures 1 and 6 has been degraded), Corrected minor textual errors and two reference omissions to match the version to be published in The Astronomical Journa