Paper detail

On the Origin of the Globular Cluster FSR 1758

Globular clusters in the Milky Way are thought to have either an {\it in situ} origin, or to have been deposited in the Galaxy by past accretion events, like the spectacular Sagittarius dwarf galaxy merger. We aim to probe the origin of the recently discovered globular cluster FSR 1758, often associated with some past merger event, and which happens to be projected toward the Galactic bulge, by a detailed study of its Galactic orbit, and to assign it to the most suitable Galactic component. We employ three different analytical time-independent potential models to calculate the orbit of the cluster by using the Gauss Radau spacings integration method. In addition, a time-dependent bar potential model is added to account for the influence of the Galactic bar. We run a large suite of simulations to account for the uncertainties in the initial conditions, in a Montecarlo fashion. We confirm previous indications that the globular cluster FSR 1758 possesses a retrograde orbits with high eccentricity. The comparative analysis of the orbital parameters of star clusters in the Milky Way, in tandem with recent metallicity estimates, allows us to conclude that FSR1758 is indeed a Galactic bulge intruder. The cluster can therefore be considered an old metal poor halo globular cluster formed {\it in situ} and which is passing right now in the bulge region. Its properties, however, can be roughly accounted for also assuming that the cluster is part of some stream of extra-Galactic origin. We conclude that assessing the origin, either Galactic or extra-galactic, of globular clusters is surely a tantalising task. In any case, by using an {\it Occam's razor} argument, we tend to prefer an {\it in situ} origin for FSR 1758.