We use Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) u, g, r, i, z photometry to study Milky Way halo substructure in the area around the North Galactic Cap. A simple color cut (g − r < 0.4) reveals the tidal stream of the Sagittarius dwarf spheroidal, as well as a number of other stellar structures in the field. Two branches (A and B) of the Sagittarius stream are clearly visible in an RGB-composite image created from 3 magnitude slices, and there is also evidence for a still more distant wrap behind the A branch. A comparison of these data with numerical models suggests that the shape of the Galactic dark halo is close to spherical.

The Sloan Extension for Galactic Understanding and Exploration (SEGUE) Survey obtained ≈240,000 moderateresolution (R ∼ 1800) spectra from 3900 Å to 9000 Å of fainter Milky Way stars (14.0 < g < 20.3) of a wide variety of spectral types, both main-sequence and evolved objects, with the goal of studying the kinematics and populations of our Galaxy and its halo. The spectra are clustered in 212 regions spaced over three quarters of the sky. Radial velocity accuracies for stars are σ (RV) ∼ 4 km s −1 at g < 18, degrading to σ (RV) ∼ 15 km s −1 at g ∼ 20. For stars with signal-to-noise ratio >10 per resolution element, stellar atmospheric parameters are 4377 4378 YANNY ET AL.Vol. 137 estimated, including metallicity, surface gravity, and effective temperature. SEGUE obtained 3500 deg 2 of additional ugriz imaging (primarily at low Galactic latitudes) providing precise multicolor photometry (σ (g, r, i) ∼ 2%), (σ (u, z) ∼ 3%) and astrometry (≈0 .1) for spectroscopic target selection. The stellar spectra, imaging data, and derived parameter catalogs for this survey are publicly available as part of Sloan Digital Sky Survey Data Release 7.

There has long been evidence that low-mass galaxies are systematically larger in radius, of lower central stellar mass density, and of lower central phase-space density, than are star clusters of the same luminosity. The larger radius, at a comparable value of central velocity dispersion, implies a larger mass at similar luminosity, and hence significant dark matter, in dwarf galaxies, compared to no dark matter in star clusters. We present a synthesis of recent photometric and kinematic data for several of the most dark-matter dominated galaxies. There is a bimodal distribution in half-light radii, with stable star clusters always being smaller than ∼ 30pc, while stable galaxies are always larger than ∼ 120pc. We extend the previously known observational relationships and interpret them in terms of a more fundamental pair of intrinsic properties of dark matter itself: dark matter forms cored mass distributions, with a core scale length of greater than about 100pc, and always has a maximum central mass density with a narrow range. The dark matter in dSph galaxies appears to be clustered such that there is a mean volume mass density within the stellar distribution which has the very low value of about 0.1 M ⊙ pc −3 (about 5GeV/c 2 cm −3 ). All dSphs have velocity

We present five new satellites of the Milky Way discovered in Sloan Digital Sky Survey (SDSS) imaging data, four of which were followed up with either the Subaru or the Isaac Newton Telescopes. They include four probable new dwarf galaxies-one each in the constellations of Coma Berenices, Canes Venatici, Leo, and Hercules-together with one unusually extended globular cluster, Segue 1. We provide distances, absolute magnitudes, half-light radii, and colormagnitude diagrams for all five satellites. The morphological features of the color-magnitude diagrams are generally well described by the ridge line of the old, metal-poor globular cluster M92. In the past two years, a total of 10 new Milky Way satellites with effective surface brightness v k 28 mag arcsec À2 have been discovered in SDSS data. They are less luminous, more irregular, and apparently more metal-poor than the previously known nine Milky Way dwarf spheroidals. The relationship between these objects and other populations is discussed. We note that there is a paucity of objects with half-light radii between $40 and $100 pc. We conjecture that this may represent the division between star clusters and dwarf galaxies.

We confirm and extend the recent finding that the central surface density r_0*rho_0 galaxy dark matter halos, where r_0 and rho_0 are the halo core radius and central density, is nearly constant and independent of galaxy luminosity. Based on the co-added rotation curves of about 1000 spiral galaxies, mass models of individual dwarf irregular and spiral galaxies of late and early types with high-quality rotation curves and, galaxy-galaxy weak lensing signals from a sample of spiral and elliptical galaxies, we find that log(r_0*rho_0) = 2.15 +- 0.2, in units of log(Msol/pc^2). We also show that the observed kinematics of Local Group dwarf spheroidal galaxies are consistent with this value. Our results are obtained for galactic systems spanning over 14 magnitudes, belonging to different Hubble Types, and whose mass profiles have been determined by several independent methods. In the same objects, the approximate constancy of rho_0*r_0 is in sharp contrast to the systematical variations, by several orders of magnitude, of galaxy properties, including rho_0 and central stellar surface density.Comment: Accepted for publication in MNRAS. 9 pages, 4 figure