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 dwarf elliptical galaxy NGC 185, together with its spatial variations, has been investigated for old, intermediate-a ge, and young stars, using new ground-based Ha and BVI photometry and synth etic color-magnitude diagrams (CMDs). We find that the bulk of the stars we re formed in NGC 185 at an early epoch of its evolution. After that, star f ormation proceeded at a low rate until the recent past, the age of the most recent traces of star formation activity detected in the galaxy being some 100 Myr. As for the spatial variations, the star formation rates, psi(t), for old and intermediate-age stars show a gradient, in the sense of exhibit ing lower values for higher galactocentric radii. Moreover, recent star for mation is detected in the central 150 x 90 pc(2) only, where the youngest, 100 Myr old, population is found. No traces of stars born more recently tha n 1 Gyr ago are found outside this central region. Since the larger concent ration of stars of any age lies in the central part of a galaxy, it could b e the case that the youngest stars originate from material ejected from dyi ng stars and that this process would only be efficient enough in the center of the galaxy. The luminous "blue stars" discovered by W. Baade in 1951 in the center of NGC 185 are discussed using new CCD images in B and Baade's original photographic plates. Considering their fuzzy, unresolved appearanc e and that a conspicuous main sequence is lacking in the CMD at our limitin g magnitude, we reach the conclusion that most of Baade's blue objects are in fact star clusters. These clusters, as well as the other stellar populat ions, are young (a few times 100 Myr) but not as young as they would be if they were individual stars (a few times 10 Myr). A supernova remnant (SNR) close to the center of NGC 185 has been analyzed from Ha images. The fact t hat a conspicuous main sequence is lacking in our CMD implies that the supe rnova (SN) had originated from a white dwarf progenitor. A consistent pictu re arises in which the gas observed in the central region of NGC 185 would have an internal origin. The rate at which evolved stars return gas to the interstellar medium is enough to seed the recent star formation observed in the center of the galaxy, and the SN rate is probably low enough to allow the galaxy to retain the gas not used in the new stellar generations. Furth er support is found in the similar kinematic properties of gas and stars.