Accurate radial velocity and metallicity of the Large Magellanic Cloud old globular clusters NGC 1928 and NGC 1939

A.E. PIATTI, N. HWANG, A.A. COLE, M.S. ANGELO AND B. EMPTAGE

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2018 vol. 481 p. 49 - 58

0035-8711

We present results obtained from spectroscopic observations of red giants located inthe fields of the Large Magellanic Cloud (LMC) globular clusters (GCs) NGC 1928and NGC 1939. We used the GMOS and AAOmega+2dF spectrographs to obtainspectra centred on the Ca II triplet, from which we derived individual radial velocities(RVs) and metallicities. From cluster members we derived mean RVs of RV NGC 1928 =249.58±4.65 km/s and RV NGC 1939 = 258.85±2.08 km/s, and mean metallicities of[Fe/H] NGC 1928 = −1.30±0.15 dex and [Fe/H] NGC 1939 = −2.00±0.15 dex. We foundthat both GCs have RVs and positions consistent with being part of the LMC disc, sothat we rule out any possible origin but that in the same galaxy. By computing thebest solution of a disc that fully contains each GC, we obtained circular velocities forthe 15 known LMC GCs. We found that 11/15 of the GCs share the LMC rotationderived from HST and Gaia DR2 proper motions. This outcome reveals that theLMC disc existed since the very early epoch of the galaxy formation and experiencedthe steep relatively fast chemical enrichment shown by its GC metallicities. The fourremaining GCs turned out to have circular velocities not compatible with an in situcluster formation, but rather with being stripped from the SMC.