The dynamical evolution of the large ( > 100 kpc ) , barred spiral galaxy NGC 6872 and its small companion IC 4970 in the southern group Pavo is investigated .
We present N -body simulations with stars and gas and 21 cm H i observations carried out with the Australia Telescope Compact Array of the large-scale distribution and kinematics of atomic gas .
H i is detected toward the companion , corresponding to a gas mass of \sim 1.3 \times 10 ^ { 9 } M _ { \odot } .
NGC 6872 contains \sim 1.4 \times 10 ^ { 10 } M _ { \odot } of H i gas , distributed in an extended rotating disk .
Massive concentrations of gas ( \sim 10 ^ { 9 } M _ { \odot } ) are found at the tip of both tidal tails and towards the break seen in the optical northern arm near the companion .
We detect no H i counterpart to the X-ray trail between NGC 6872 and NGC 6876 , the dominant elliptical galaxy in the Pavo group located \sim 8 ^ { \prime } to the southeast .
At the sensitivity and the resolution of the observations , there is no sign in the overall H i distribution that NGC 6876 has affected the evolution of NGC 6872 .
There is no evidence of ram pressure stripping either .
The X-ray trail could be due to gravitational focusing of the hot gas in the Pavo group behind NGC 6872 as the galaxy moves supersonically through the hot medium .
The simulations of a gravitational interaction with a small nearby companion on a low-inclination prograde passage are able to reproduce most of the observed features of NGC 6872 , including the general morphology of the galaxy , the inner bar , the extent of the tidal tails and the thinness of the southern tail .