A Pair of Boötes: A New Milky Way Satellite

As part of preparations for a southern sky search for faint Milky Way dwarf galaxy satellites, we report the discovery of a stellar overdensity in the Sloan Digital Sky Survey Data Release 5, lying at an angular distance of only 1.5 degrees from the recently discovered Boötes dwarf. The overdensity was detected well above statistical noise by employing a sophisticated data mining algorithm and does not correspond to any catalogued object. Overlaid isochrones using stellar population synthesis models show that the color-magnitude diagram of that region has the signature of an old (12Gyr), metal-poor (Fe/H ≈ −2.0) stellar population at a tentative distance of 60 kpc, evidently the same heliocentric distance as the Boötes dwarf. We estimate the new object to have a total magnitude of MV ∼ −3.1 ± 1.1mag and a half-light radius of rh = 4 .1 ± 1.6 (72 ± 28 pc) placing it in an apparent 40 < rh < 100 pc void between globular clusters and dwarf galaxies, occupied only by another recently discovered Milky Way Satellite, Coma Berenices. Subject headings: galaxies: dwarf — Local Group