The Surface Mass Density and Structure of the Outer Disk of NGC 628
  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 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 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 isothermal
sech(z)^2 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
solar masses/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.
