An Observational Estimate for the Mean Secular Evolution Rate in Spiral
  Galaxies
  We have observationally quantified the effect of gravitational torques on
stars in disk galaxies due to the stellar distribution itself and explored
whether these torques are efficient at transporting angular momentum within a
Hubble Time. We derive instantaneous torque maps for a sample of 24 spiral
galaxies, based on stellar mass maps that were derived using the pixel-by-pixel
mass-to-light estimator by Zibetti, Rix and Charlot. In conjunction with an
estimate of the rotation velocity, the mass maps allow us to determine the
torque-induced instantaneous angular momentum flow across different radii,
resulting from the overall stellar distributions for each galaxy in the sample.
By stacking the sample, which effectively replaces a time average by an
ensemble average, we find that the torques due to the stellar disk act to
transport angular momentum outward over much of the disk (within 3 disk scale
lengths). The strength of the ensemble-averaged gravitational torques within
one disk scale length have a timescale of ~ 4 Gyr for angular momentum
redistribution. This study is the first to observationally determine the
strength of torque-driven angular momentum flow of stars for a sample of spiral
galaxies, providing an important empirical constraint on secular evolution.
(abridged)
