A halo substructure in Gaia Data Release 1

We identify a halo substructure in the Tycho Gaia Astrometric Solution (TGAS) data set, cross-matched with the Radial Velocity Experiment (RAVE-on) data release. After quality cuts, the stars with large radial action (J_R > 800 km s^-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 centred on (X, Y, Z) ≈ (9.0, -1.0, -0.6) kpc and form a coherently moving structure in the halo with median (v_R, v_ϕ, v_z) = (167.33, 0.86, -94.85) km s^-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 8 out of the 14 stars where this is possible. We find that six of the stars are still comoving. These six stars also have a much tighter [Fe/H] distribution ∼-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 globular cluster accreted long ago.