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

Abstract

We identify a $\sim$600 pc-wide region of active star formation located within a tidal streamer of M82 via $H\alpha$ emission ($F_{H\alpha }\sim 6.5\times 10^{-14}$ 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 HI and molecular hydrogen within the "northern HI streamer" induced by the passage of M81 several hundred Myr ago. From HI data, we estimate that $\sim 5\times 10^7$ M${}_{\odot }$ of gas is present in the specific overdensity coincident with the $H\alpha$ source. The object's derived metallicity (12+$\log (O/H)\simeq 8.6$), 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.

Version accepted to ApJL. 9 pages, 4 figures

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• The pathfinder Dragonfly Spectral Line Mapper: Pushing the limits for ultra-low surface brightness spectroscopy
• Young, blue, and isolated stellar systems in the Virgo Cluster. I. 2-D Optical spectroscopy