  Using deep 21-cm HI data from the Green Bank Telescope we have detected an
~18.3 kpc-long gaseous extension associated with the starbursting dwarf galaxy
IC 10. The newly-found feature stretches 1.3 deg to the northwest and has a
large radial velocity gradient reaching to ~65 km/s lower than the IC 10
systemic velocity. A region of higher column density at the end of the
extension that possesses a coherent velocity gradient (~10 km/s across ~26
arcmin) transverse to the extension suggests rotation and may be a satellite
galaxy of IC 10. The HI mass of IC 10 is 9.5x10^7 (d/805 kpc)^2 Msun and the
mass of the new extension is 7.1x10^5 (d/805 kpc)^2 Msun. An IC 10-M31 orbit
using known radial velocity and proper motion values for IC 10 show that the HI
extension is inconsistent with the trailing portion of the orbit so that an
M31-tidal or ram pressure origin seems unlikely. We argue that the most
plausible explanation for the new feature is that it is the result of a recent
interaction (and possible late merger) with another dwarf galaxy. This
interaction could not only have triggered the origin of the recent starburst in
IC 10, but could also explain the existence of previously-found
counter-rotating HI gas in the periphery of the IC 10 which was interpreted as
originating from primordial gas infall.
