A Giant Shell of Ionized Gas Discovered near M82 with the Dragonfly Spectral Line Mapper Pathfinder

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 Hα and [N ii] λ6583 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 Hα surface brightness of 2 × 10−18 erg cm − 2 s − 1 arcsec − 2 . The [N ii] λ6583/Hα line ratio varies from [N ii]/Hα ∼ 0.2 to [N ii]/Hα ∼ 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] λ6583/Hα = 0.340 ± 0.003 and [S ii] λλ6716, 6731/Hα = 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.

The Pathfinder Dragonfly Spectral Line Mapper: pushing the limits for ultra-low surface brightness spectroscopy

“…In this imaging not only is the Hα cap clearly visible but also a variety of new features including a tidal dwarf galaxy candidate at the edge of the M82 disk (Ref. 20; within the dashed gray box), Hα emission from the tidal streamer appearing to connect the edge of the disk to the Hα cap, and the most striking discovery of a giant cloud of gas seeming to hover above the M82 galaxy, the Hα shell 16.…”

“…These observations served as proof-of-concept imaging to determine the surface brightness limits of the instrument, as well as scientific imaging, the results of which are presented in Ref. 16. The observations followed the Dragonfly automated observing model, where the telescope is set up every night for observing at the beginning of the night and the telescope carries out observations autonomously, adapting to changing weather conditions and pausing observations when necessary.…”

“…20; within the dashed gray box), Hα emission from the tidal streamer appearing to connect the edge of the disk to the Hα cap, and the most striking discovery of a giant cloud of gas seeming to hover above the M82 galaxy, the Hα shell. 16 [Nii] science images, respectively (see Ref. 15 for further details).…”

“…3), which is analyzed further in Ref. 16. The main goal with this instrument was to reach the surface brightnesses required to directly image the ultra-faint line emission from gas in the surroundings of galaxies, and determine what improvements upon the instrument would be necessary before undertaking the construction of the full-scale Dragonfly Spectral Line Mapper.…”

“…The resulting data revealed many new gaseous features in the group, including a giant cloud of gas in the outskirts of the M82 galaxy. 16 Here we describe the pathfinder Dragonfly Spectral Line Mapper and the observing methods used to carry out the imaging survey. This includes several upgrades and additions to the observing methods and software used by the Dragonfly Telephoto Array, such as calibration of the Dragonfly Filter-Tilter and additional hardware for flatfield imaging.…”