Torques and angular momentum: Counter-rotation in galaxies and ring
  galaxies
  We present an alternate origin scenario to explain the observed phenomena of
(1) counter-rotation between different galaxy components and (2) the formation
of ring galaxies. We suggest that these are direct consequences of the galaxy
being acted upon by a torque which causes a change in its primordial spin
angular momentum first observed as changes in the gas kinematics or
distribution. We suggest that this torque is exerted by the gravitational force
between nearby galaxies. This origin requires the presence of at least one
companion galaxy in the vicinity - we find a companion galaxy within 750 kpc
for 51/57 counter-rotating galaxies and literature indicates that all ring
galaxies have a companion galaxy thus giving observable credence to this
origin. Moreover in these 51 galaxies, we find a kinematic offset between
stellar and gas heliocentric velocities $>50$ kms$^{-1}$ for several galaxies
if the separation between the galaxies $<$ 100 kpc. This, we suggest, indicates
a change in the orbital angular momentum of the torqued galaxies. A major
difference between the torque origin suggested here and the existing model of
gas accretion/galaxy collision, generally used to explain the above two
phenomena, is that the torque acts on the matter of the same galaxy whereas in
the latter case gas, with different properties, is brought in from outside. An
important implication of our study is that mutual gravity torques acting on the
galaxy can explain the formation of warps, polar ring galaxies and lenticular
galaxies. We conclude that mutual gravity torques play an important role in the
dynamical evolution of galaxies and that they naturally explain several galaxy
observables.
