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We study the kinematics of GALEX-selected H_alpha knots in the outer disk(beyond R25) of NGC 628 (M74), a galaxy representative of large, undisturbed,extended UV (Type 1 XUV) disks. Our spectroscopic target sample of 235 of thebluest UV knots surrounding NGC 628 yielded 15 H_alpha detections (6%), roughlythe number expected given the different mean ages of the two populations. Themeasured vertical velocity dispersion of the H_alpha knots between 1 - 1.8 R25(13.5 - 23.2 kpc) is < 11 km/s. We assume that the H_alpha knots trace an'intermediate' vertical mass density distribution (between the isothermalsech(z)^2 and exponential distributions) with a constant scaleheight across theouter disk (h_z = 700 pc) and estimate a total surface mass density of 7.5solar masses/pc^2. This surface mass density can be accounted for by theobserved gas and stars in the outer disk (little or no dark matter in the diskis required). The vertical velocity dispersion of the outer disk H_alpha knotsnearly matches that measured from older planetary nebulae near the outskirts ofthe optical disk by Herrmann et al., suggesting a low level of scattering inthe outer disk. A dynamically cold stellar component extending nearly twice asfar as the traditional optical disk poses interesting constraints on theaccretion history of the galaxy.

The Surface Mass Density and Structure of the Outer Disk of NGC 628