Outer Galactic Disks and a Quantitative Test of Gravity at Low
  Accelerations
  We use the recent measurement of the velocity dispersion of star-forming,
outer-disk knots by Herbert-Fort et al. in the nearly face-on galaxy NGC 628,
in combination with other data from the literature, to execute a
straightforward test of gravity at low accelerations. Specifically, the
rotation curve at large radius sets the degree of non-standard acceleration and
then the predicted scaleheight of the knots at that radius provides the test of
the scenario. For our demonstration, we presume that the H alpha knots, which
are young (age < 10 Myr), are distributed like the gas from which they have
recently formed and find a marginal (> 97% confidence) discrepancy with a
modified gravity scenario given the current data. More interestingly, we
demonstrate that there is no inherent limitation that prevents such a test from
reaching possible discrimination at the > 4 sigma level with a reasonable
investment of observational resources.
