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. 
N. G. Kantharia (National Centre for Radio Astrophysics, Tata   Institute of Fundamental Research, Pune)
astro-ph.GA