Neutral hydrogen in IC 342. I. The large-scale structure.

An aperture synthesis survey of H I in the Scd galaxy IC 342 is described, with a radial velocity resolution 16 km/sec and a maximum angular resolution of 1.9 x 2.0 arcmin. There is large-scale asymmetry, with a low-brightness extension to the northwest reaching 52 kpc from the nucleus. The velocity field shows deviations from normal rotation in the outer parts of the galaxy which are interpreted as warping of the galactic plane. The data are consistent with a flat rotation curve out to at least 42 kpc. If the rotation curve is flat at greater radii, the 'northwest extension' is also warped, but not so strongly as expected from an extrapolation of the perturbations observed at smaller radii. A tidal interaction is an attractive explanation for the disturbances.