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

Abstract

We identify a ∼600 pc wide region of active star formation located within a tidal streamer of M82 via H α emission ( F _{H α} ∼ 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 × 10 ⁷ M _⊙ of gas is present in the specific overdensity coincident with the H α source. The object’s derived metallicity (12+ $\log (\mathrm{O}/\mathrm{H})\simeq 8.6$ ), 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.