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

TL;DR

This paper reports the discovery of a nascent tidal dwarf galaxy within M82's northern HI streamer, characterized by active star formation, distinct kinematics, and a significant gas overdensity, indicating early-stage galaxy formation.

Contribution

First identification of a forming tidal dwarf galaxy within M82's tidal stream using multi-wavelength observations and spectroscopy.

Findings

Active star formation in a 600 pc region within the HI streamer.

Kinematic decoupling from M82's disk confirmed via spectroscopy.

Presence of substantial gas overdensity supporting dwarf galaxy formation.

Abstract

We identify a $\sim$600 pc-wide region of active star formation located within a tidal streamer of M82 via $H\alpha$ emission ($F_{H\alpha}\sim 6.5\times10^{-14}$ 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 $\sim5\times10^7$ 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.