Old open clusters in the Sagittarius dSph tidal stream -- kith or kin?

TL;DR

This study suggests that two old open star clusters in the outer Galactic disc may have originated from the Sagittarius dwarf spheroidal galaxy, indicating a possible extragalactic origin linked to galactic accretion processes.

Contribution

It provides evidence supporting the hypothesis that certain outer open clusters originated in the Sgr dSph and were deposited in the Milky Way through tidal interactions.

Findings

Berkeley 29 and Saurer 1 are located in the Sgr dSph trailing tail.

Their properties are consistent with an extragalactic origin.

Supports the idea of accretion contributing to the Galactic disc buildup.

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 a such 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 indicate clearly that the trailing tail passes through the outer Galactic disc, at the same galactocentric distance as some anti-centre 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, lends further support to this possible association. Hence, we find it likely that the old open star clusters Berkeley~29 and Saurer~1 have extra-galactic origins.