Proper motions in Terzan 5: membership of the multi-iron sub-populations and first constrain to the orbit

TL;DR

This study uses high-resolution HST images over a decade to measure proper motions in Terzan 5, confirming sub-population memberships and suggesting the cluster is a remnant of the Galactic bulge's formation.

Contribution

It provides the first orbit estimate of Terzan 5, linking its kinematics to the Galactic bulge and confirming sub-population memberships with high-precision proper motions.

Findings

Confirmed membership of multiple sub-populations with different iron abundances

Derived the orbit of Terzan 5, excluding external accretion origin

Supported the idea that Terzan 5 is a fossil remnant of bulge formation

Abstract

By exploiting two sets of high-resolution images obtained with HST ACS/WFC over a baseline of ~10 years we have measured relative proper motions of ~70,000 stars in the stellar system Terzan 5. The results confirm the membership of the three sub-populations with different iron abudances discovered in the system. The orbit of the system has been derived from a first estimate of its absolute proper motion, obtained by using bulge stars as reference. The results of the integration of this orbit within an axisymmetric Galactic model exclude any external accretion origin for this cluster. Terzan 5 is known to have chemistry similar to the Galactic bulge; our findings support a kinematic link between the cluster and the bulge, further strengthening the possibility that Terzan 5 is the fossil remnant of one of the pristine clumps that originated the bulge.