# $\textit{Siriusly}$, a newly identified intermediate-age Milky Way stellar cluster: A spectroscopic study of $\textit{Gaia}$ 1 [GA]

We confirm the reality of the recently discovered Milky Way stellar cluster $\textit{Gaia}$ 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]$=-0.13\pm0.13$ and barycentric radial velocity of $v_r=58.30\pm0.22$ km/s. Isochrone fits to $\textit{Gaia}$, 2MASS, and Pan-STARRS1 photometry indicate that $\textit{Gaia}$ 1 is likely an intermediate age ($\sim3$ Gyr) stellar cluster. Combining the positional and kinematic information we estimated an orbit with maximum height out of the Galactic plane of $z_\textrm{max}=1.7^{+2.1}_{-0.9}$ kpc and an eccentricity of $e=0.3\pm0.2$. These orbital properties suggest that it may be of an extragalactic origin, or be the result of an older cluster interacting with high altitude gas, which triggered star formation.

J. Simpson, G. Silva, S. Martell, et. al.
Tue, 14 Mar 17
61/74

Comments: 9 pages (and 3 pages of appendices), 7 figures. Submitted to MNRAS. Comments welcome. Table B1 & B2 available as a machine-readable table by clicking on “Other formats” on the right