Halo intruders in the Galactic bulge revealed by HST and Gaia : the globular clusters Terzan 10 and Djorgovski 1

Abstract

Context. The low-latitude globular clusters Terzan 10 and Djorgovski 1 are projected in the Galactic bulge, in a Galactic region highly affected by extinction. A discrepancy of a factor of ∼2 exists in the literature in regards to the distance determination of these clusters. Aims. We revisit the colour-magnitude diagrams (CMDs) of these two globular clusters with the purpose of disentangling their distance determination ambiguity and, for the first time, of determining their orbits to identify whether or not they are part of the bulge/bar region. Methods. We use Hubble Space Telescope CMDs, with the filters F606W from ACS and F160W from WFC3 for Terzan 10, and F606W and F814W from ACS for Djorgosvski 1, and combine them with the proper motions from Gaia Data Release 2. For the orbit integrations, we employed a steady Galactic model with bar. Results. For the first time the blue horizontal branch of these clusters is clearly resolved. We obtain reliable distances of d⊙ = 10.3 ± 1.0 kpc and 9.3 ± 0.5 kpc for Terzan 10, and Djorgovski 1 respectively, indicating that they are both currently located in the bulge volume. From Gaia DR2 proper motions, together with our new distance determination and recent literature radial velocities, we are able to show that the two sample clusters have typical halo orbits that are passing by the bulge/bar region, but that they are not part of this component. For the first time, halo intruders are identified in the bulge.