1. Argelander-Institut für Astronomie, Universität Bonn, 53121 Bonn, Germany

Correspondence to: Jan Pflamm-Altenburg¹ Correspondence and requests for materials should be addressed to J.P.-A. (Email: jpflamm@astro.uni-bonn.de).

Abstract

The rate of star formation in a galaxy is often determined by the observation of emission in the H line, which is related to the presence of short-lived massive stars. Disk galaxies show a strong cut-off in H radiation at a certain galactocentric distance, which has led to the conclusion that star formation is suppressed in the outer regions of disk galaxies. This is seemingly in contradiction to recent observations¹ in the ultraviolet which imply that disk galaxies have star formation beyond the H cut-off, and that the star-formation-rate surface density is linearly related to the underlying gas surface density, which is a shallower relationship than that derived from H luminosities². In a galaxy-wide formulation, the clustered nature of star formation has recently led to the insight that the total galactic H luminosity is nonlinearly related to the galaxy-wide star formation rate³. Here we show that a local formulation of the concept of clustered star formation naturally leads to a steeper radial decrease in the H surface luminosity than in the star-formation-rate surface density, in quantitative agreement with the observations, and that the observed H cut-off arises naturally.

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