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 .
Here , 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 , a value that we confirm from the calcium triplet region .
This GC shows mild evidence for rotation with an amplitude of A _ { rot } \sim 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 } / \sigma _ { 0 } ratios suggest that M 75 is a slow rotator driven by internal dynamics rather than being effected by the weak Galactic tides at its large distances .
Here , M 75 ( A _ { rot } / \sigma = 0.31 ) is fully consistent with the properties of other , younger halo clusters .
At A _ { rot } / \sigma _ { 0 } = 0.8 \pm 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 can not be excluded , based on the eccentric orbit , which we derived from the recent data release 2 of Gaia , and considering its younger age .