Segue 3: An Old, Extremely Low luminosity Star Cluster in the Milky
  Way's Halo
  We investigate the kinematic and photometric properties of the Segue 3 Milky
Way companion using Keck/DEIMOS spectroscopy and Magellan/IMACS g and r-band
imaging. Using maximum likelihood methods to analyze the photometry, we study
the structure and stellar population of Segue 3. We find the half-light radius
of Segue 3 is 26" +/- 5" (2.1 +/- 0.4 pc, for a distance of 17 kpc) and the
absolute magnitude is a mere M_V = 0.0 +/- 0.8 mag, making Segue 3 the least
luminous old stellar system known. We find Segue 3 to be consistent with a
single 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 with
the photometry to determine a sample of 32 stars which are likely associated
with Segue 3. The member stars within three half-light radii have a velocity
dispersion of 1.2 +/- 2.6 km/s. Photometry of the members indicates the stellar
population has a spread in [Fe/H] of <0.3 dex. These facts, together with the
small physical size of Segue 3, imply the object is likely an old, faint
stellar cluster which contains no significant dark matter. We find tentative
evidence for stellar mass loss in Segue 3 through the eleven candidate member
stars outside of three half-light radii, as expected from dynamical arguments.
Interpretation of the data outside of three half-light radii, is complicated by
the object's spatial coincidence with a previously known halo substructure,
which may enhance contamination of our member sample.
