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A Halo Substructure in Gaia Data Release 1

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Myeong, GC 
Evans, NW 
Koposov, S 
Sanders, JL 


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 (JR > 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 (vR, vϕ, vz) = (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.



galaxies: kinematics and dynamics, galaxies: structure

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Monthly Notices of the Royal Astronomical Society: Letters

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Oxford University Press
Science and Technology Facilities Council (ST/N000927/1)
Science and Technology Facilities Council (ST/N004493/1)
GCM thanks the Boustany Foundation, Cambridge Commonwealth, European & International Trust, and Issac Newton Studentship for their support on his work. SEK thanks the United Kingdom Science and Technology Council (STFC) for the award of an Ernest Rutherford fellowship (grant number ST/N004493/1).