A 5deg x 5deg deep HI survey of the M81 group
  A 25 $\rm deg^2$ region, including the M81 complex (M81, M82, NGC 3077), NGC
2976 and IC2574, was mapped during ~3000 hours with the DRAO synthesis
telescope. With a physical resolution of ~1 kpc, these observations allow us to
probe a large region down to column density levels of ~$\rm 1\times10^{18}\,
cm^{-2}$ over 16 $\rm km\, s^{-1}$, mapping the extent of the HI arm connecting
the system and NGC 2976, and resolving the HI clouds adjacent to the arm. The
observations also reveal a few clouds located between the system and IC 2574,
probably tidally stripped from a past interaction between the two systems.
Given the regular velocity distribution in the HI envelope of the system, we
attempt and derive an idealised large-scale rotation curve of the system. We
observe a flat trend for the rotation velocity of the overall system from 20
kpc out to 80 kpc, well beyond the outskirts of the M81 disk, although with
asymmetries like a wiggle at the vicinity of M82. This supports the assumption
that intergalactic gas and galaxies in the system participate to a large-scale
ordered rotation motion which is dominated by M81. Also, our HI analysis of the
group further supports the hypothesis that the galaxies forming the system
moved closer from afar, in agreement with numerical simulations.
