A Nascent Tidal Dwarf Galaxy Forming within the Northern H i Streamer of M82

We identify a ∼600 pc wide region of active star formation located within a tidal streamer of M82 via Hα emission (FHα ∼ 6.5 × 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 × 107 M⊙ of gas is present in the specific overdensity coincident with the Hα source. The object's derived metallicity (12+$\mathrm{log}({\rm{O}}/{\rm{H}})\simeq 8.6$), 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.