A Halo Substructure in Gaia Data Release 1

We identify a halo substructure in the Tycho Gaia Astrometric Solution (TGAS) dataset, cross-matched with the RAVE-on data release. After quality cuts, the stars with large radial action ($J_R > 800$ kms$^{-1}$ kpc) are extracted. A subset of these stars is clustered in longitude and velocity and can be selected with further cuts. The 14 stars are centered on $(X,Y,Z) \approx (9.0,-1.0,-0.6)$ kpc and form a coherently moving structure in the halo with median $(v_R,v_\phi,v_z) = (167.33,0.86,-94.85)$ kms$^{-1}$. They are all metal-poor giants with median [Fe/H] $=-0.83$. To guard against the effects of distance errors, we compute spectrophotometric distances for the 8 out of 14 stars where this is possible. We find that 6 of the stars are still comoving. These 6 stars also have a much tighter [Fe/H] distribution $\sim -0.7$ with one exception ([Fe/H] = -2.12). We conclude that the existence of the comoving cluster is stable against changes in distance estimation and conjecture that this is the dissolving remnant of a long-ago accreted globular cluster.