Young Metal-Rich Stars in the Halo of M82: a tidal dwarf galaxy?

ABSTRACT We present Hubble Space Telescope Advanced Camera for Surveys photometry of an arc structure of stars in the southern halo of M82. The structure is located at a projected height of ≈5 kpc from the disc of M82 but its stellar content exhibits a younger mean age than that of typical halo stars. We measured the distance, mean age, and peak metallicity of the stars in the arc and used them to determine the structure’s origin. The arc colour–magnitude diagram yields a tip of the red giant branch (RGB) distance of 3.63 ± 0.08 Mpc, similar to that of M82, confirming its association with the galaxy. The colour–magnitude diagram also reveals a population of predominantly young stars with a mean age of ≈100–160 Myr, which is similar to that of the young stars formed in the tidal debris between M81, M82, and NGC 3077. We derived the mean metallicity of the halo stars surrounding the arc to be Fe/H ≈ −1.0, similar to that of the RGB population of the halo of M81. The RGB stars in the arc, while they are slightly more metal-rich than the halo RGB, show no spatial correlation with the young stars, suggesting that the arc is consists of mainly young stellar population. Furthermore, the observed age gradient along the structure of the arc indicates that the arc began forming from the closer side to M82, then moving outward. Taken together, the results suggest that the formation of the young stars that give rise to the arc structure could have been triggered by the interactions between M82’s southern outflow and the H i gas clouds surrounding it, representing a snapshot of a tidal dwarf galaxy in the process of forming.