Kinematics of outer halo globular clusters: M 75 and NGC 6426^⋆

¹ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12, 69120 Heidelberg, Germany
e-mail: andreas.koch@uni-heidelberg.de
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 27 March 2018
Accepted: 13 May 2018

Abstract

Globular clusters (GCs) and their dynamic interactions with the Galactic components provide an important insight into the structure and formation of the early Milky Way. We present a kinematic study of two outer halo GCs based on a combination of VLT/FORS2, VLT/FLAMES, and Magellan/MIKE low- and high-resolution spectroscopy of 32 and 27 member stars, respectively. Although both clusters are located at Galactocentric distances of 15 kpc, they have otherwise very different properties. M 75 is a luminous and metal-rich system at [Fe/H] = −1.2 dex, which is a value that we confirm from the calcium triplet region. This GC shows mild evidence for rotation with an amplitude of A_rot ~ 5 km s⁻¹. One of the most metal-poor GCs in the Milky Way (at [Fe II/H] = −2.3 dex), NGC 6426 exhibits marginal evidence of internal rotation at the 2 km s⁻¹ level. Both objects have velocity dispersions that are consistent with their luminosity. Although limited by small-number statistics, the resulting limits on their A_rot/σ₀ ratios suggest that M 75 is a slow rotator driven by internal dynamics rather than being affected by the weak Galactic tides at its large distances. In this work, M 75 (A_rot/σ₀ = 0.31) is fully consistent with the properties of other, younger halo clusters. At A_rot/σ₀ = 0.8 ± 0.4, NGC 6426 appears to have a remarkably ordered internal motion for its low metallicity, but the large uncertainty does not allow for an unambiguous categorization as a fast rotator. An accretion origin of M 75 cannot be excluded, based on the eccentric orbit, which we derived from the recent data release 2 of Gaia, and considering its younger age.