Molecular Gas in NUclei of GAlaxies (NUGA): XII. The head-on collision
  in NGC1961
  We present high-resolution CO(1-0) and CO(2-1) maps of the LINER 2 galaxy
NGC1961. This galaxy is unusual among late-type (Sc) disk galaxies in having a
very large radial extent and inferred dynamical mass. We propose a head-on
collision scenario to explain the perturbed morphology of this galaxy- both the
off-centered rings and the inflated radius. This scenario is supported by the
detection of a steep velocity gradient in the CO(1-0) map at the position of a
southwest peak in radio continuum and near-infrared emission. This peak would
represent the remnant of the disrupting companion. We use numerical models to
demonstrate the plausibility of the scenario. While ram pressure stripping
could in principle be important for shocking the atomic gas and produce the
striking head-tail morphology, the non detection of this small galaxy group in
X-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 the
southern side. We argue that NGC1961 has a strongly warped disk, which gives
the false impression of a nearly face-on system; the main disk is actually more
edge-on, and this error in the true inclination has led to the surprisingly
high dynamical mass for a morphologically late-type galaxy. In addition, the
outward propagating ring artificially enlarges the disk. The collision
de-stabilizes the inner disk and can provide gas inflow to the active nucleus.
