A spectroscopic study of the Hα surface brightness profiles in the outer discs of galaxies

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 disc 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 disc to the underlying stellar population, we measure the shape of the outer H alpha surface brightness profile of 15 isolated, edge-on late-type disc galaxies using deep, long-slit spectroscopy. Tracing H alpha emission up to 50 per cent 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 disc, but which is not truncated. The stellar continuum and H alpha surface brightness both exhibit a break at similar to 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 disfavours the hypothesis that this break is a purely stochastic effect.