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.GCM thanks Boustany Foundation, Cambridge Commonwealth, European & International Trust and Isaac Newton Studentship for their support on his work. ... The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. 308024

Dark Matter in the Milky Way's Dwarf Spheroidal Satellites

The Milky Way's dwarf spheroidal satellites include the nearest, smallest and least luminous galaxies known. They also exhibit the largest discrepancies between dynamical and luminous masses. This article reviews the development of empirical constraints on the structure and kinematics of dSph stellar populations and discusses how this phenomenology translates into constraints on the amount and distribution of dark matter within dSphs. Some implications for cosmology and the particle nature of dark matter are discussed, and some topics/questions for future study are identified.Comment: A version with full-resolution figures is available at http://www.cfa.harvard.edu/~mwalker/mwdsph_review.pdf; 70 pages, 22 figures; invited review article to be published in Vol. 5 of the book "Planets, Stars, and Stellar Systems", published by Springe

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.GCM thanks Boustany Foundation, Cambridge Commonwealth, European & International Trust and Isaac Newton Studentship for their support on his work. ... The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. 308024

The globular cluster systems of 54 Coma ultra-diffuse galaxies: statistical constraints from HST data

We use data from the HST Coma Cluster Treasury program to assess the richness of the Globular Cluster Systems (GCSs) of 54 Coma ultra-diffuse galaxies (UDGs), 18 of which have a half-light radius exceeding 1.5 kpc. We use a hierarchical Bayesian method tested on a large number of mock datasets to account consistently for the high and spatially varying background counts in Coma. These include both background galaxies and intra-cluster GCs (ICGCs), which are disentangled from the population of member GCs in a probabilistic fashion. We find no candidate for a GCS as rich as that of the Milky Way, our sample has GCSs typical of dwarf galaxies. For the standard relation between GCS richness and halo mass, 33 galaxies have a virial mass $M_{vir}\leq10^{11}M_\odot$ at 90% probability. Only three have $M_{vir}>10^{11}M_\odot$ with the same confidence. The mean colour and spread in colour of the UDG GCs are indistinguishable from those of the abundant population of ICGCs. The majority of UDGs in our sample are consistent with the relation between stellar mass and GC richness of 'normal' dwarf galaxies. Nine systems, however, display GCSs that are richer by a factor of 3 or more (at 90% probability). Six of these have sizes $\lesssim1.4$ kpc. Our results imply that the physical mechanisms responsible for the extended size of the UDGs and for the enhanced GC richness of some cluster dwarfs are at most weakly correlated.Comment: Matches accepted versio

The remnant of a merger between two dwarf galaxies in Andromeda II

Driven by gravity, massive structures like galaxies and clusters of galaxies are believed to grow continuously through hierarchical merging and accretion of smaller systems. Observational evidence of accretion events is provided by the coherent stellar streams crossing the outer haloes of massive galaxies, such as the Milky Way or Andromeda. At similar mass-scales, around $10^{11}$ solar masses in stars, further observational evidence of merging activity is also ample. Mergers of lower-mass galaxies are expected within the hierarchical process of galaxy formation, but have hitherto not been seen for galaxies with less than about $10^9$ solar masses in stars. Here, we report the kinematic detection of a stellar stream in one of the satellite galaxies of Andromeda, the dwarf spheroidal galaxy Andromeda II, which has a mass of only $10^7$ solar masses in stars. The properties of the stream show that we are observing the remnant of a merger between two dwarf galaxies. This had a dramatic influence on the dynamics of the remnant, which is now rotating around its projected major axis. The stellar stream in Andromeda II illustrates the scale-free character of the formation of galaxies, down to the lowest galactic mass scales.Comment: Published in Nature on February 23rd, 2014 (online); includes Methods, 13 pages, 5 figures; see http://dx.doi.org/10.1038/nature1299