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We present high-resolution CO(1-0) and CO(2-1) maps of the LINER 2 galaxyNGC1961. This galaxy is unusual among late-type (Sc) disk galaxies in having avery large radial extent and inferred dynamical mass. We propose a head-oncollision scenario to explain the perturbed morphology of this galaxy- both theoff-centered rings and the inflated radius. This scenario is supported by thedetection of a steep velocity gradient in the CO(1-0) map at the position of asouthwest peak in radio continuum and near-infrared emission. This peak wouldrepresent the remnant of the disrupting companion. We use numerical models todemonstrate the plausibility of the scenario. While ram pressure strippingcould in principle be important for shocking the atomic gas and produce thestriking head-tail morphology, the non detection of this small galaxy group inX-ray emission suggests that any hot intragroup medium has too low a density.The collision with the companion of mass ratio 1:4 produces a lopsided wave,which accounts for the sharp boundary observed in the atomic gas on thesouthern side. We argue that NGC1961 has a strongly warped disk, which givesthe false impression of a nearly face-on system; the main disk is actually moreedge-on, and this error in the true inclination has led to the surprisinglyhigh dynamical mass for a morphologically late-type galaxy. In addition, theoutward propagating ring artificially enlarges the disk. The collisionde-stabilizes the inner disk and can provide gas inflow to the active nucleus.

Molecular Gas in NUclei of GAlaxies (NUGA): XII. The head-on collision in NGC1961