2018A&A...616A..74K -
Astronomy and Astrophysics, volume 616A, 74-74 (2018/8-1)
Kinematics of outer halo globular clusters: M 75 and NGC 6426.
KOCH A., HANKE M. and KACHAROV N.
Abstract (from CDS):
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 15kpc, they have otherwise very different properties. M 75 is a luminous and metal-rich system at [Fe/H]=-1.2dex, which is a value that we confirm from the calcium triplet region. This GC shows mild evidence for rotation with an amplitude of Arot/σ0∼5km/s. One of the most metal-poor GCs in the Milky Way (at [FeII/H]=-2.3dex), NGC 6426 exhibits marginal evidence of internal rotation at the 2km/s level. Both objects have velocity dispersions that are consistent with their luminosity. Although limited by small-number statistics, the resulting limits on their Arot/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 (Arot/σ0=0.31) is fully consistent with the properties of other, younger halo clusters. At Arot/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.
Abstract Copyright:
© ESO 2018
Journal keyword(s):
globular clusters: individual: M 75 - globular clusters: individual: NGC 6426 - Galaxy: kinematics and dynamics - Galaxy: structure - Galaxy: halo
Simbad objects:
61
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