Stars Born in the Wind. II. Widespread Extra-planar Star Formation in M82’s Halo

Abstract Galaxies evolve in tandem with their environments—mergers and gas inflows drive galaxy growth while galactic outflows launched by supernovae may seed the galactic environment with gas, metals, and energy, fueling star formation far from the main bodies of galaxies. The formation histories of young stars in the stellar halos of nearby galaxies can help understand this interplay. We thus present the most detailed map to date of young stars in the stellar halo of M82, a starburst galaxy in the M81 Group that hosts a prototypical outflow, using Hubble Space Telescope and Subaru Hyper-Suprime Cam observations. We find widespread extraplanar populations of stars with ages ≲630 Myr, with clear detections of stars up to ∼5 kpc to the south in unique arc-like stellar features (Southern Arcs) and in a new stellar trail up to ∼20 kpc to the east (M82’s tail), originating from the Southern Arcs. We estimate a total halo star formation of ∼4 × 10 6 M ⊙ in the last 630 Myr. Overall, the star formation history of the M82 tail is correlated with periods of heightened star cluster formation in the M82 disk, which suggests the influence of the starburst outflow. Further, the fraction of young stars decreases as we move away from M82 to the east. We forward a picture where the M82 tail formed from ram-pressure-stripped gas arising from M82’s westward motion, triggered by shocks from the outflow.