We study the kinematics of GALEX-selected H \alpha knots in the outer disk ( beyond R _ { 25 } ) of NGC 628 ( M74 ) , a galaxy representative of large , undisturbed , extended UV ( Type 1 XUV ) disks .
Our spectroscopic target sample of 235 of the bluest UV knots surrounding NGC 628 yielded 15 H \alpha detections ( 6 \% ) , roughly the number expected given the different mean ages of the two populations .
The measured vertical velocity dispersion of the H \alpha knots between 1 - 1.8 R _ { 25 } ( 13.5 - 23.2 kpc ) is < 11 km s ^ { -1 } .
We assume that the H \alpha knots trace an ‘ intermediate ’ vertical mass density distribution ( between the isothermal sech ^ { 2 } ( z ) and exponential distributions ) with a constant scaleheight across the outer disk ( h _ { z } = 700 pc ) and estimate a total surface mass density of 7.5 M _ { \sun } pc ^ { -2 } .
This surface mass density can be accounted for by the observed gas and stars in the outer disk ( little or no dark matter in the disk is required ) .
The vertical velocity dispersion of the outer disk H \alpha knots nearly matches that measured from older planetary nebulae near the outskirts of the optical disk by Herrmann et al. , suggesting a low level of scattering in the outer disk .
A dynamically cold stellar component extending nearly twice as far as the traditional optical disk poses interesting constraints on the accretion history of the galaxy .