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

Abstract

The photometric and morphological properties, as well as the star formation history, of the Draco dwarf spheroidal galaxy are analyzed on the basis of wide field, CCD photometry of the resolved stars covering about 1deg^2. No metallicity gradient is detected. The star surface density distribution can be fitted by a single exponential law of scale-length a=5'.0+/-0'.1. The central surface 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 found for 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) of Draco. There are hence no evidences of a tidal-tail associated to Draco. The tidal radius of the galaxy is found to be r_t~42' (~1 kpc). The possibility that the large mass-to-light relation in Draco could be accounted for by a convenient 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-age population is present in Draco, most of the star formation (up to 90%) took place before ~10 Gyr ago. No significant star formation activity is detected in the last ~2 Gyr. Two methods ("partial model" and "subgiant") have been used to investigate the star formation history of Draco.