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We investigate the kinematic and photometric properties of the Segue 3 MilkyWay companion using Keck/DEIMOS spectroscopy and Magellan/IMACS g and r-bandimaging. Using maximum likelihood methods to analyze the photometry, we studythe structure and stellar population of Segue 3. We find the half-light radiusof Segue 3 is 26" +/- 5" (2.1 +/- 0.4 pc, for a distance of 17 kpc) and theabsolute magnitude is a mere M_V = 0.0 +/- 0.8 mag, making Segue 3 the leastluminous old stellar system known. We find Segue 3 to be consistent with asingle stellar population, with an age of 12.0 +1.5/-0.4 Gyr and an [Fe/H] of-1.7 +0.07/-0.27. Line-of-sight velocities from the spectra are combined withthe photometry to determine a sample of 32 stars which are likely associatedwith Segue 3. The member stars within three half-light radii have a velocitydispersion of 1.2 +/- 2.6 km/s. Photometry of the members indicates the stellarpopulation has a spread in [Fe/H] of <0.3 dex. These facts, together with thesmall physical size of Segue 3, imply the object is likely an old, faintstellar cluster which contains no significant dark matter. We find tentativeevidence for stellar mass loss in Segue 3 through the eleven candidate memberstars outside of three half-light radii, as expected from dynamical arguments.Interpretation of the data outside of three half-light radii, is complicated bythe object's spatial coincidence with a previously known halo substructure,which may enhance contamination of our member sample.

Segue 3: An Old, Extremely Low luminosity Star Cluster in the Milky Way's Halo