Strong suppression of star formation and spiral arm formation in disk
  galaxies with counter-rotating gas disks
  Galaxy-wide star formation can be quenched by a number of physical processes
such as environmental effects (e.g., ram pressure stripping) and supernova
feedback. Using numerical simulations, we here demonstrate that star formation
can be severely suppressed in disk galaxies with their gas disks
counter-rotating with respect to their stellar disks. This new mechanism of
star formation suppression (or quenching) does not depend so strongly on model
parameters of disk galaxies, such as bulge-to-disk- ratios and gas mass
fractions. Such severe suppression of star formation is due largely to the
suppression of the gas density enhancing mechanism i.e spiral arm formation in
disk galaxies with counter-rotating gas. Our simulations also show that
molecular hydrogen and dust can be rather slowly consumed by star formation in
disk galaxies with counter-rotating gas disks (i.e., long gas depletion
timescale). Based on these results, we suggest that spiral and S0 galaxies with
counter-rotation can have rather low star formation rate for their gas
densities. Also we suggest that a minor fraction of S0 galaxies have no
prominent spiral arms, because they have a higher fraction of counter-rotating
gas. We predict that poststarburst E+A disk galaxies with cold gas could have
counter-rotating gas.
