The systemic velocity and internal kinematics of the dwarf galaxy LGS 3: An optical foray beyond the milky way

We have obtained radial velocities of three K giants and one faint carbon star in LGS 3, a dwarf companion of M31, based on 12 individual spectra obtained with the HIRES spectrograph on the Keck I telescope. The mean precision of these measurements is 3.8 km s^-1. The mean systemic velocity of LGS 3 is -282.2 ± 3.5 km s^-1. Monte Carlo simulations that take into account the individual velocity uncertainties and the maximum observed velocity difference reveal that the central velocity dispersion of LGS 3 is in the range 2.6-30.5 km s^-1, with 95% confidence; the most likely value for the central dispersion is 7.9^+5.3_-2.9 km s^-1. These results agree with the kinematics of H I gas in LGS 3. This contrasts with the tendency for the gas and stars in other low-luminosity Local Group dwarfs to exhibit distinct spatial and kinematic properties. Taking into account the relative youth of LGS 3, we conclude that the " asymptotic " M/L ratio - the value the galaxy would exhibit if it were composed only of ancient stars - is M/L_v,LGS3 ≥ 11 (at a 97.5% confidence level), with a most probable value of 95⁺¹⁷⁵_-56. These values are consistent with the M/L_v ratios observed in other well-studied early-type dwarfs of the Local Group. We have also estimated the mass of LGS 3 using modified Newtonian dynamics. These data represent the first moderately high precision optical spectra of giants in a dwarf system beyond the Galactic halo. We suggest future studies that are now feasible to study the dynamics of dwarf galaxies throughout the Local Group and beyond.