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We identify a ∼600 pc wide region of active star formation located within a tidal streamer of M82 via Hαemission (F_Hα∼ 6.5 × 10⁻¹⁴erg s⁻¹cm⁻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 Hiand molecular hydrogen within the “northern Histreamer” induced by the passage of M81 several hundred Myr ago. From Hidata, we estimate that ∼5 × 10⁷M_⊙of gas is present in the specific overdensity coincident with the Hαsource. 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.

 

Abstract We present the discovery of a giant cloud of ionized gas in the field of the starbursting galaxy M82. Emission from the cloud is seen in H α and [N ii ] λ 6583 in data obtained though a small pathfinder instrument used to test the key ideas that will be implemented in the Dragonfly Spectral Line Mapper, an upcoming ultranarrow-bandpass imaging version of the Dragonfly Telephoto Array. The discovered cloud has a shell-like morphology with a linear extent of 0.°8 and is positioned 0.°6 northwest of M82. At the heliocentric distance of the M81 group, the cloud’s longest angular extent corresponds to 55 kpc and its projected distance from the nucleus of M82 is 40 kpc. The cloud has an average H α surface brightness of 2 × 10 −18 erg cm − 2 s − 1 arcsec − 2 . The [N ii ] λ 6583/H α line ratio varies from [N ii ]/H α ∼ 0.2 to [N ii ]/H α ∼ 1.0 across the cloud, with higher values found in its eastern end. Follow-up spectra obtained with Keck LRIS confirm the existence of the cloud and yield line ratios of [N ii ] λ 6583/H α = 0.340 ± 0.003 and [S ii ] λλ 6716, 6731/H α = 0.64 ± 0.03 in the cloud. This giant cloud of material could be lifted from M82 by tidal interactions or by its powerful starburst. Alternatively, it may be gas infalling from the cosmic web, potentially precipitated by the superwinds of M82. Deeper data are needed to test these ideas further. The upcoming Dragonfly Spectral Line Mapper will have 120 lenses, 40× more than in the pathfinder instrument used to obtain the data presented here.