We have presented V and I CCD photometry of stars in the Local Group dIrr galaxy SagDIG. The colour-magnitude diagram shows a well defined red giant branch, and a sequence of blue stars. Based on the I magnitude of the TRGB we have derived the distance modulus of SagDIG to be for an adopted extinction of . From the mean colour of the RGB , a mean metallicity [Fe/H] is obtained. With this low Fe abundance, SagDIG could be the lowest metallicity star forming dIrr known.
The absolute total magnitude, the total colour, and the central surface brightness of SagDIG are derived to be , and mag arcsec-2. The radial star density distribution is fitted by an exponential law of scale length , corresponding to 140 pc.
The star formation history of SagDIG has been estimated from the number of blue and red stars in the colour-magnitude diagram and using a synthetic CMD as reference. The star formation rate has been evaluated for the intervals of time 0.2-15 Gyr, 0.05-0.2 Gyr and 0-0.05 Gyr, for which has been obtained yr-1, yr-1, and yr-1, respectively (see a summary in Table 1). The current
strong enhancement of the observed star formation rate is a common feature of galaxies classified as dIrrs.
© European Southern Observatory (ESO) 1999
Online publication: December 2, 1999