http://arxiv.org/abs/1906.08271
$Context$. The assembly history experienced by the Milky Way is currently being unveiled thanks to the data provided the $Gaia$ mission. It is likely that the globular cluster system of our Galaxy has followed a similarly intricate formation path. $Aims$. To unravel this formation path, we explore the link between the globular clusters and the hitherto known merging events that the Milky Way has experienced. $Methods$. To this end, we have combined the kinematic information provided by $Gaia$ for almost all Galactic clusters, with the largest sample of cluster ages available after carefully correcting for systematics. To identify clusters with a common origin we analysed their dynamical properties, particularly in the space of integrals of motion. $Results$. We have found that about 40% of the clusters likely formed in-situ. A similarly large fraction, 35%, can be associated to the merger events, in particular to $Gaia$-Enceladus (19%), the Sagittarius dwarf galaxy (5%), the progenitor of the Helmi streams (6%) and to the Sequoia galaxy (5%), although some uncertainty remains due to some degree of overlap in their dynamical characteristics. Of the remaining clusters, 16% are tentatively associated to a group with high-binding energy, while the rest are all on loosely bound orbits and likely have a more heterogeneous origin. The resulting age-metallicity relations are remarkably tight and differ in their detailed properties depending on the progenitor, providing further confidence on the associations made. $Conclusions$. We provide a table listing the associations found which highlights the progress made in sorting out the assembly history of the Galactic globular clusters. Improved kinematic data by future $Gaia$ data releases and especially a larger, systematic-free sample of cluster ages would help to further pin down this history.
D. Massari, H. Koppelman and A. Helmi
Fri, 21 Jun 19
51/56
Comments: 7 pages, 5 figures, submitted to A&A Letters. The list of all globular clusters and their associated progenitors is provided in Table 1 in the Appendix