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

The surface brightness profile of Hα 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 Hα truncation and the relation of Hα emission in the outer disc to the underlying stellar population, we measure the shape of the outer Hα surface brightness profile of 15 isolated, edge-on late-type disc galaxies using deep, long-slit spectroscopy. Tracing Hα emission up to 50 per cent beyond the optical radius, R₂₅, we find a composite Hα 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 Hα surface brightness both exhibit a break at ∼ 0.7 R₂₅, but the Hα emission drops more steeply than the stellar continuum beyond that break. Although profiles with truncations or single exponential laws correctly describe the Hα 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 Hα break radius disfavours the hypothesis that this break is a purely stochastic effect.