The Star Formation History of the Local Group Dwarf Elliptical Galaxy NGC 185. II. Gradients in the Stellar Population

The star formation history of the dE NGC 185, together with its spatialvariations, has been investigated using new ground-based $H_\alpha$ and $BVI$photometry, and synthetic color--magnitude diagrams (CMDs). We find that thebulk of the stars were formed in NGC 185 at an early epoch of its evolution.After that, the star formation proceeded at a low rate until the recent past,the age of the most recent traces of star formation activity detected in thegalaxy being some 100 Myr. The star formation rate, $\psi(t)$ for old and intermediate ages shows agradient in the sense of taking smaller values for higher galactocentric radii.Moreover, recent star formation is detected in the central $150 \times 90$ pc$^2$ only, where the youngest, 100 Myr old population isfound. The luminous blue {\it stars} discovered by Baade (1951) in the centerof NGC 185 are discussed using new CCD images in $B$ and Baade's originalphotographic plates, reaching the conclusion that most of them are in fact starclusters. A consistent picture arises in which the gas observed in the centralregion of NGC 185 would have an internal origin. The rate at which evolvedstars return gas to the ISM is enough to seed the recent star formationobserved in the center of the galaxy and the SN rate is probably low enough toallow the galaxy to retain the gas not used in the new stellar generations.Comment: 34 pages, 17 figures, 5 Tables, to be published in AJ October 9