Halo Substructure in the SDSS-Gaia Catalogue : Streams and Clumps
  We use the SDSS-Gaia Catalogue to identify six new pieces of halo
substructure. SDSS-Gaia is an astrometric catalogue that exploits SDSS data
release 9 to provide first epoch photometry for objects in the Gaia source
catalogue. We use a version of the catalogue containing $245\,316$ stars with
all phase space coordinates within a heliocentric distance of $\sim 10$ kpc. We
devise a method to assess the significance of halo substructures based on their
clustering in velocity space. The two most substantial structures are multiple
wraps of a stream which has undergone considerable phase mixing (S1, with 94
members) and a kinematically cold stream (S2, with 61 members). The member
stars of S1 have a median position of ($X,Y,Z$) = ($8.12, -0.22, 2.75$) kpc and
a median metallicity of [Fe/H] $= -1.78$. The stars of S2 have median
coordinates ($X,Y,Z$) = ($8.66, 0.30, 0.77$) kpc and a median metallicity of
[Fe/H] $= -1.91$. They lie in velocity space close to some of the stars in the
stream reported by Helmi et al. (1999). By modelling, we estimate that both
structures had progenitors with virial masses $\approx 10^{10} M_\odot$ and
infall times $\gtrsim 9$ Gyr ago. Using abundance matching, these correspond to
stellar masses between $10^6$ and $10^7 M_\odot$. These are somewhat larger
than the masses inferred through the mass-metallicity relation by factors of 5
to 15. Additionally, we identify two further substructures (S3 and S4 with 55
and 40 members) and two clusters or moving groups (C1 and C2 with 24 and 12)
members. In all 6 cases, clustering in kinematics is found to correspond to
clustering in both configuration space and metallicity, adding credence to the
reliability of our detections.
