  We present a kinematic study of the outer regions (R_25<R<2 R_25) of 17
edge-on disk galaxies. Using deep long-slit spectroscopy (flux sensitivity a
few 10^-19 erg s^-1 cm^-2 arcsec^-2), we search for H-alpha emission, which
must be emitted at these flux levels by any accumulation of hydrogen due to the
presence of the extragalactic UV background and any other, local source of UV
flux. We present results from the individual galaxy spectra and a stacked
composite. We detect H-alpha in many cases well beyond R_25 and sometimes as
far as 2 R_25. The combination of sensitivity, spatial resolution, and
kinematic resolution of this technique thus provides a powerful complement to
21-cm observations. Kinematics in the outer disk are generally disk-like (flat
rotation curves, small velocity dispersions) at all radii, and there is no
evidence for a change in the velocity dispersion with radius. We place strong
limits, few percent, on the existence of counter-rotating gas out to 1.5 R_25.
These results suggest that thin disks extend well beyond R_25; however, we also
find a few puzzling anomalies. In ESO 323-G033 we find two emission regions
that have velocities close to the systemic velocity rather than the expected
rotation velocity. These low relative velocities are unlikely to be simply due
to projection effects and so suggest that these regions are not on disk-plane,
circular orbits. In MCG-01-31-002 we find emission from gas with a large
velocity dispersion that is co-rotating with the inner disk.
