We confirm the reality of the recently discovered Milky Way stellar cluster Gaia 1 using spectra acquired with the HERMES and AAOmega spectrographs of the Anglo-Australian Telescope. This cluster had been previously undiscovered due to its close angular proximity to Sirius, the brightest star in the sky at visual wavelengths. Our observations identified 41 cluster members, and yielded an overall metallicity of [Fe/H] = -0.13 ± 0.13 and barycentric radial velocity of vr = 58.30 ± 0.22 km s-1. These kinematics provide a dynamical mass estimate of 12.9+4.6-3.9 x 103 M⊙. Isochrone fits to Gaia, 2MASS, and Pan-STARRS1 photometry indicate that Gaia 1 is an intermediate age (∼3 Gyr) stellar cluster. Combining the spatial and kinematic data we calculate Gaia 1 has a circular orbit with a radius of about 12 kpc, but with a large out of plane motion: zmax = 1.1+0.4-0.3kpc. Clusters with such orbits are unlikely to survive long due to the number of plane passages they would experience.
Bibliographical noteThis article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, Volume 471, Issue 4, 11 November 2017, Pages 4087–4098, https://doi.org/10.1093/mnras/stx1892. © 2017 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
- Galaxy: general
- Galaxy: structure