  The surface brightness profile of H-alpha emission in galaxies is generally
thought to be confined by a sharp truncation, sometimes speculated to coincide
with a star formation threshold. Over the past years, observational evidence
for both old and young stellar populations, as well as individual H II regions,
has demonstrated that the outer disk is an actively evolving part of a galaxy.
To provide constraints on the origin of the aforementioned H-alpha truncation
and the relation of H-alpha emission in the outer disk to the underlying
stellar population, we measure the shape of the outer H-alpha surface
brightness profile of 15 isolated, edge-on late-type disk galaxies using deep,
long-slit spectroscopy. Tracing H-alpha emission up to 50% beyond the optical
radius, R_25, we find a composite H-alpha surface brightness profile, well
described by a broken-exponential law, that drops more steeply in the outer
disk, but which is not truncated. The stellar continuum and H-alpha surface
brightness both exhibit a break at ~0.7 R_25, but the H-alpha emission drops
more steeply than the stellar continuum beyond that break. Although profiles
with truncations or single exponential laws correctly describe the H-alpha
surface brightness profiles of some individual galaxies, flexible
broken-exponentials are required in most cases and are therefore the more
appropriate generic description. The common existence of a significant second
surface brightness component beyond the H-alpha break radius disfavors the
hypothesis that this break is a purely stochastic effect.
