2014ApJ...783...49H

We present a spectroscopic study of the four extended star clusters (ESCs) in NGC 6822 based on the data obtained with the Gemini Multi-Object Spectrograph on the Gemini-South 8.1 m telescope. The radial velocities derived from the spectra range from -61.2±20.4 km/s (for C1) to -115.34±57.9 km/s (for C4) and, unlike the intermediate-age carbon stars, they do not display any sign of systematic rotation around NGC 6822. The ages and metallicities derived using the Lick indices show that the ESCs are old ( ≥ 8 Gyr) and metal poor ([Fe/H] ≲ -1.5). NGC 6822 is found to have both metal poor ([Fe/H] ~-2.0) and metal rich ([Fe/H] ~-0.9) star clusters within 15' (2 kpc) from the center, whereas only metal poor clusters are observed in the outer halo with r ≥ 20'(2.6 kpc). The kinematics, old ages, and low metallicities of ESCs suggest that ESCs may have accreted into the halo of NGC 6822. Based on the velocity distribution of ESCs, we have determined the total mass and the mass-to-light ratio of NGC 6822: M_N6822 = 7.5_–0.1^+4.5 x 10⁹M_☉ and (M/L)_N6822 = 75_–1c+45^(M/L)_☉. It shows that NGC 6822 is one of the most dark matter dominated dwarf galaxies in the Local Group.