Abstract
The rate of star formation in a galaxy is often determined by the observation of emission in the Hα line, which is related to the presence of short-lived massive stars. Disk galaxies show a strong cut-off in Hα 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 Hα 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 Hα luminosities2. In a galaxy-wide formulation, the clustered nature of star formation has recently led to the insight that the total galactic Hα 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 Hα surface luminosity than in the star-formation-rate surface density, in quantitative agreement with the observations, and that the observed Hα cut-off arises naturally.
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We thank K. S. de Boer for discussions.
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Pflamm-Altenburg, J., Kroupa, P. Clustered star formation as a natural explanation for the Hα cut-off in disk galaxies. Nature 455, 641–643 (2008). https://doi.org/10.1038/nature07266
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DOI: https://doi.org/10.1038/nature07266
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