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° 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 cataloged object. Overlaid isochrones using stellar population synthesis models show that the color-magnitude diagram of that region has the signature of an old (12 Gyr), 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 ± 1.1 mag and a half-light radius of rh = 4.1′ ± 1.6′ (72 ± 28 pc), placing it in an apparent 40 pc < rh < 100 pc void between globular clusters and dwarf galaxies, occupied only by another recently discovered Milky Way satellite, Coma Berenices.