A Halo Substructure in Gaia Data Release 1

Abstract

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 $(v_R,v_{\varphi },v_z)=(167.33,0.86,-94.85)$ 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.

5 pages, 4 figures, MNRAS (Letters), accepted

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• StarGO: A New Method to Identify the Galactic Origins of Halo Stars
• The S2 Stream: the shreds of a primitive dwarf galaxy
• Dynamically Tagged Groups of Metal-Poor Stars from the Best \& Brightest Survey
• Chemo-Dynamically Tagged Groups of CEMP Stars in the Halo of the Milky Way. I. Untangling the Origins of CEMP-$s$ and CEMP-no Stars
• Kinematic structures of the Solar neighbourhood revealed by Gaia DR1/TGAS and RAVE
• The R-Process Alliance: Chemo-Dynamically Tagged Groups II. An Extended Sample of Halo $r$-Process-Enhanced Stars
• Dynamically Tagged Groups of Metal-Poor Stars II. The Radial Velocity Experiment Data Release 6
• Tidal ribbons