  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$\alpha$ and
[NII]$\lambda$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 $0.8^{\circ}$ and is positioned $0.6^{\circ}$ northwest of M82. At
the heliocentric distance of the M81 group, the cloud's extent corresponds to
55 kpc and its projected distance from the nucleus of M82 is 40 kpc. The cloud
has an average H$\alpha$ surface brightness of $2\times10^{-18}$ $\mathrm{erg}
~\mathrm{cm}^{-2} ~\mathrm{s}^{-1} ~\mathrm{arcsec}^{-2}$. The
[NII]$\lambda$6583/H$\alpha$ line ratio varies from [NII]/H$\alpha$ $\sim0.2$
to [NII]/H$\alpha$ $\sim1.0$ 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 [NII]$\lambda$6583/H$\alpha$ = 0.340 $\pm$
0.003 and [SII]$\lambda$6716,6731/H$\alpha$ = 0.64 $\pm$ 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, $40\times$ more than in the pathfinder instrument used to
obtain the data presented here.
