SIMBAD references

The star formation history of the dwarf elliptical galaxy NGC 185, together with its spatial variations, has been investigated for old, intermediate-age, and young stars, using new ground-based Hα and BVI photometry and synthetic color-magnitude diagrams (CMDs). We find that the bulk of the stars were formed in NGC 185 at an early epoch of its evolution. After that, star formation 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, ψ(t), for old and intermediate-age stars show a gradient, in the sense of exhibiting lower values for higher galactocentric radii. Moreover, recent star formation is detected in the central 150x90 pc² only, where the youngest, 100 Myr old, population is found. No traces of stars born more recently than 1 Gyr ago are found outside this central region. Since the larger concentration of stars of any age lies in the central part of a galaxy, it could be the case that the youngest stars originate from material ejected from dying 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 appearance and that a conspicuous main sequence is lacking in the CMD at our limiting 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 populations, 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 Hα images. The fact that a conspicuous main sequence is lacking in our CMD implies that the supernova (SN) had originated from a white dwarf progenitor. A consistent picture 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. Further support is found in the similar kinematic properties of gas and stars.