The Systemic Velocity and Internal Kinematics of the Dwarf Galaxy LGS 3: An Optical Foray beyond the Milky Way

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Abstract
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 Hi 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/LV,LGS3≥11 (at a 97.5% confidence level), with a most probable value of 95+175-56. These values are consistent with the M/LV 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.