Decreased Frequency of Strong Bars in S0 Galaxies: Evidence for Secular
  Evolution?
  Using data from the Near-Infrared S0 Survey (NIRS0S) of nearby, early-type
galaxies, we examine the distribution of bar strengths in S0 galaxies as
compared to S0/a and Sa galaxies, and as compared to previously published bar
strength data for Ohio State University Bright Spiral Galaxy Survey (OSUBSGS)
spiral galaxies. Bar strengths based on the gravitational torque method are
derived from 2.2 micron Ks-band images for a statistical sample of 138 (98 S0,
40 S0/a,Sa) galaxies having a mean total blue magnitude <B_T> <= 12.5 and
generally inclined less than 65 degrees. We find that S0 galaxies have weaker
bars on average than spiral galaxies in general, even compared to their closest
spiral counterparts, S0/a and Sa galaxies. The differences are significant and
cannot be due entirely to uncertainties in the assumed vertical scale-heights
or in the assumption of constant mass-to-light ratios. Part of the difference
is likely due simply to the dilution of the bar torques by the higher mass
bulges seen in S0s. If spiral galaxies accrete external gas, as advocated by
Bournaud & Combes, then the fewer strong bars found among S0s imply a lack of
gas accretion according to this theory. If S0s are stripped former spirals, or
else are evolved from former spirals due to internal secular dynamical
processes which deplete the gas as well as grow the bulges, then the weaker
bars and the prevalence of lenses in S0 galaxies could further indicate that
bar evolution continues to proceed during and even after gas depletion
