Ca ɪɪ triplet spectroscopy of RGB stars in NGC 6822: kinematics and metallicities

We present a detailed analysis of the chemistry and kinematics of red giants in the dwarf irregular galaxy NGC 6822. Spectroscopy at ≈8500 Å was acquired for 72 red giant stars across two fields using FORS2 at the VLT. Line-of-sight extinction was individually estimated for each target star to accommodate the variable reddening across NGC 6822. The mean radial velocity was found to be 〈υ_rad〉 = −52.8 ± 2.2 km s⁻¹ with dispersion σ_υ = 24.1 km s⁻¹, in agreement with other studies. Ca ɪɪ triplet equivalent widths were converted into [Fe/H] metallicities using a V magnitude proxy for surface gravity. The average metallicity was 〈[Fe/H]〉 = −0.84 ± 0.04 with dispersion σ = 0.31 dex and interquartile range 0.48. Our assignment of individual reddening values makes our analysis more sensitive to spatial variations in metallicity than previous studies. We divide our sample into metal-rich and metal-poor stars; the former were found to cluster towards small radii with the metal-poor stars more evenly distributed across the galaxy. The velocity dispersion of the metal-poor stars was found to be higher than that of the metal-rich stars (σ_υMP = 27.4 km s⁻¹; σ_υMR = 21.1 km s⁻¹); combined with the age–metallicity relation this indicates that the older populations have either been dynamically heated during their lifetimes or were born in a less disc-like distribution than the younger stars. The low ratio υ_rot/σ_υ suggests that within the inner 10 arcmin, NGC 6822's stars are dynamically decoupled from the H ɪ gas, and possibly distributed in a thick disc or spheroid structure.