The Sagittarius Dwarf Irregular galaxy (SagDIG): distance and star formation history

The distance, star formation history and global properties of the Local Group dIrr galaxy SagDIG are derived based on an [I-(V-I)] colour--magnitude diagram of ~1550 stars. A distance of 1.06+/-0.10 Mpc is obtained from the I magnitude of the TRGB. This corresponds to 1.17 Mpc to the barycenter of the Local Group and 1.34 to M 31, being DDO 210, at 0.35 Mpc, the nearest galaxy to SagDIG. The metallicity is estimated from the colour of the RGB to be [Fe/H]=-2.45+/-0.25. SagDIG is hence a probable member of the Local Group and a candidate for the lowest-metallicity star forming galaxy known. The radial density profile of the galaxy has been obtained together with other integrated properties (magnitude, colour and central surface density). The galaxy density profile is fitted by an exponential law of scale length 27.1", corresponding to 140 pc. The star formation history of SagDIG has been analysed, based on synthetic colour-magnitude diagrams. The galaxy is currently in a high star formation activity epoch, forming stars at a rate about 10 times greater than the average for its entire life. This is a common feature of galaxies classified as dIrrs.