Velocity measurements in Segue 1

We have obtained velocity measurements for 98.2% of the stars within 67pc (10', or 2.3 half-light radii) of the center of Segue 1 that have colors and magnitudes consistent with membership, down to a magnitude limit of r=21.7. Based on photometric, kinematic, and metallicity information, we identify 71 stars as probable Segue 1 members, including some as far out as 87pc.

After correcting for the influence of binary stars using repeated velocity measurements, we determine a velocity dispersion of 3.7^+1.4^_-1.1_km/s. The mass within the half-light radius is 5.8^+8.2^_-3.1_x10^5^M_{sun}_. The stellar kinematics of Segue 1 require very high mass-to-light ratios unless the system is far from dynamical equilibrium, even if the period distribution of unresolved binary stars is skewed toward implausibly short periods.