2021ApJ...923L..21P

We identify a ∼600 pc wide region of active star formation located within a tidal streamer of M82 via Hα emission (F_Hα ∼ 6.5 x 10^–14 erg s^–1 cm^–2), using a pathfinder instrument based on the Dragonfly Telephoto Array. The object is kinematically decoupled from the disk of M82 as confirmed via Keck/LRIS spectroscopy and is spatially and kinematically coincident with an overdensity of H I and molecular hydrogen within the "northern H I streamer" induced by the passage of M81 several hundred Myr ago. From H I data, we estimate that ∼5 x 10⁷ M_☉ of gas is present in the specific overdensity coincident with the Hα source. The object's derived metallicity (12+log(O/H)≃8.6), position within a gas-rich tidal feature, and morphology (600 pc diameter with multiple star-forming clumps), indicate that it is likely a tidal dwarf galaxy in the earliest stages of formation.