We have mapped the emission from atomic hydrogen at \lambda =21 cm from the galaxy NGC 3783 with the Australia Telescope Compact Array .
Our main results are : a ) the HI morphology is irregular and perturbed , gathered in three blobs apparently unrelated to the optical morphology ; b ) the observed HI velocity distribution indicates a normal disk in differential rotation with a constant velocity out to a radius of 160 ^ { \prime \prime } ( 30 kpc ) , c ) the inclination of the disk is about 25 ^ { \circ } with the kinematic major axis at a position angle slightly different from that of the stellar bar , d ) the HI mass inside a radius of 18 ^ { \prime \prime } is only 2.1 \times 10 ^ { 7 } M _ { \odot } , the total HI mass within 180 ^ { \prime \prime } is 1.1 \times 10 ^ { 9 } M _ { \odot } and the dynamical mass is 2 \times 10 ^ { 11 } M _ { \odot } .
The bulk of the gas in NGC 3783 is outside the diameter of the stellar bar ; e ) Numerical simulations of the gas flow in the barred potential derived from the red image indicate that the pattern speed is \Omega _ { p } = 38 km/s/kpc : the ring of H \alpha emitting regions encircling the bar would then correspond to UHR , and the H \alpha accumulation in the center to a nuclear ring .
Various possibilities are discussed to account for the active nucleus fuelling .