Ocular Shock Front in the Colliding Galaxy IC 2163
  ALMA observations in the CO 1 - 0 line of the interacting galaxies IC 2163
and NGC 2207 at 2" x 1.5" resolution reveal how the encounter drives gas to
pile up in narrow, ~ 1 kpc wide, "eyelids" in IC 2163. IC 2163 and NGC 2207 are
involved in a grazing encounter, which has led to development in IC 2163 of an
eye-shaped (ocular) structure at mid-radius and two tidal arms. The CO data
show that there are large velocity gradients across the width of each eyelid,
with a mixture of radial and azimuthal streaming of gas at the outer edge of
the eyelid relative to its inner edge. The sense of the radial streaming in the
eyelids is consistent with the idea that gas from the outer part of IC 2163
flows inward until its radial streaming slows down abruptly and the gas piles
up in the eyelids. The radial compression at the eyelids causes an increase in
the gas column density by direct radial impact and also leads to a high rate of
shear. We find a strong correlation between the molecular column densities and
the magnitude of dv/dR across the width of the eyelid at fixed values of
azimuth. Substantial portions of the eyelids have high velocity dispersion in
CO, indicative of elevated turbulence there.
