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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 astraightforward test of gravity at low accelerations. Specifically, therotation curve at large radius sets the degree of non-standard acceleration andthen the predicted scaleheight of the knots at that radius provides the test ofthe scenario. For our demonstration, we presume that the H alpha knots, whichare young (age < 10 Myr), are distributed like the gas from which they haverecently formed and find a marginal (> 97% confidence) discrepancy with amodified gravity scenario given the current data. More interestingly, wedemonstrate that there is no inherent limitation that prevents such a test fromreaching possible discrimination at the > 4 sigma level with a reasonableinvestment of observational resources.

Outer Galactic Disks and a Quantitative Test of Gravity at Low Accelerations