The surface mass density and structure of the outer disk of NGC 628

We study the kinematics of GALEX-selected Hα 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 Hα detections (6%), roughly the number expected given the different mean ages of the two populations. The measured vertical velocity dispersion of the Hα knots between 1and1.8 R ₂₅ (13.5-23.2 kpc) is <11 km s^-1. We assume that the Hα knots trace an "intermediate" vertical mass density distribution (between the isothermal sech²(z) and exponential distributions) with a constant scale height across the outer disk (h _z = 700 pc) and estimate a total surface mass density of 7.5 M 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 Hα 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.