The Star Formation History and Morphological Evolution of the Draco Dwarf Spheroidal Galaxy

The photometric and morphological properties, as well as the star formationhistory, of the Draco dwarf spheroidal galaxy are analyzed on the basis of widefield, CCD photometry of the resolved stars covering about 1deg^2. Nometallicity gradient is detected. The star surface density distribution can befitted by a single exponential law of scale-length a=5'.0+/-0'.1. The centralsurface magnitude is mu_V=24.4+/-0.5 and the core radius r_c=7'.5+/- 0.'3(equivalent to r_c=175+/-7 pc). Within errors, the same scale-lengths are foundfor the density profiles along the semi-major axis and the semi-minor axis(re-scaled to semi-major axis units, using the ellipticity of the galaxy) ofDraco. There are hence no evidences of a tidal-tail associated to Draco. Thetidal radius of the galaxy is found to be r_t~42' (~1 kpc). The possibilitythat the large mass-to-light relation in Draco could be accounted for by aconvenient spatial orientation is rejected, which implies that other mechanisms(e.g., dark matter) are required. The stellar population of Draco is mainly old. Although some intermediate-agepopulation is present in Draco, most of the star formation (up to 90%) tookplace before ~10 Gyr ago. No significant star formation activity is detected inthe last ~2 Gyr. Two methods ("partial model" and "subgiant") have been used toinvestigate the star formation history of Draco.Comment: Accepted in the A