Atomic hydrogen in the disturbed edge-on galaxy NGC 4631

We present WSRT HI observations of the nearby, disturbed, edge-on galaxy NGC 4631. A low-resolution (45 in. x 87 in.) map shows previously unknown tidal debris at large distances from the plane, and two dwarf companions. A high resolution (12 in. x 22 in.) map reveals a very disturbed gas layer in NGC 4631, with a wealth of small-scale structure. The most striking discovery is a supershell in the eastern half of the disk with a diameter of about 3 kpc, a mass of approximately 10 exp 8 solar mass and a tentative expansion velocity of 45 km/s. If the expansion is real, the energy which must have been injected by supernovae to explain the shell's current parameters is roughly 4 x 10(exp 55) ergs. Such a high energy requirement suggests an alternative formation mechanism, such as a collision with a small companion.