Old open clusters in the Sagittarius dwarf spheroidal galaxy tidal stream - kith or kin?

Abstract

Abstract A widely supported formation scenario for the Galactic disc is that it formed inside-out from material accumulated via accretion events. The Sagittarius dwarf spheroidal galaxy (Sgr dSph) is the best example of such an accretion, and its ongoing disruption has resulted in that its stars are being deposited in the Milky Way halo and outer disc. It is therefore appealing to search for possible signatures of the Sgr dSph contribution to the build-up of the Galactic disc. Interestingly, models of the Sgr dSph stream clearly indicate that the trailing tail passes through the outer Galactic disc, at the same Galactocentric distance as some anticentre old open star clusters. We investigate in this Letter the possibility that the two outermost old open clusters, Berkeley 29 and Saurer 1, could have formed inside the Sgr dSph and then left behind in the outer Galactic disc as a result of tidal interaction with the Milky Way. The actual location of these two star clusters, inside the Sgr dSph trailing tail, is compatible with this scenario, and their chemical and kinematical properties, together with our present understanding of the age–metallicity relationship in the Sgr dSph, lend further support to this possible association. Hence, we find it likely that the old open star clusters, Berkeley 29 and Saurer 1, have extragalactic origins.