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

## TL;DR

This study confirms Gaia 1 as an intermediate-age Milky Way stellar cluster near Sirius, using spectroscopic data to determine its properties, orbit, and mass, revealing its dynamic nature and potential longevity challenges.

## Contribution

First spectroscopic confirmation of Gaia 1, providing detailed kinematic, chemical, and orbital characterization of this previously unconfirmed cluster.

## Key findings

- Gaia 1 has a metallicity of [Fe/H] = -0.13.
- The cluster's radial velocity is 58.30 km/s.
- Gaia 1 has an estimated mass of about 13,000 solar masses.

## Abstract

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. These kinematics provide a dynamical mass estimate of $12.9^{+4.6}_{-3.9}\times10^3$ M$_{\odot}$. Isochrone fits to $\textit{Gaia}$, 2MASS, and Pan-STARRS1 photometry indicate that $\textit{Gaia}$ 1 is an intermediate age ($\sim3$ Gyr) stellar cluster. Combining the spatial and kinematic data we calculate $\textit{Gaia}$ 1 has a circular orbit with a radius of about 12~kpc, but with a large out of plane motion: $z_\textrm{max}=1.1^{+0.4}_{-0.3}$ kpc. Clusters with such orbits are unlikely to survive long due to the number of plane passages they would experience.