Article

Science with Rotating Slits: Star Formation in the Extreme Outskirts of Galaxies

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Abstract

The extreme outer regions of disk galaxies - beyond the stellar disk - are being recognized as a diverse region that plays a major role in the formation and evolution of galaxies. They are host to stellar populations as well as to low-level star formation. But what is the relation of those two components to each other? Are the stars on the outskirts of galaxies generated in situ by this star formation activity, and if so, what does this tell us about the growth of galaxies? Or was the outer disk populated in other ways (for example, by tidal debris)? To clarify the relation between star formation and stars in the outer disk, we suggest a novel observational strategy that would simultaneously get a complete census of the H-alpha emitting gas far beyond the stellar disk, measure the surface brightness profiles of both components, obtain constraints on the metallicity, and determine the vertical velocity dispersion of the disk gas, which is a measure of the disk surface mass density. By comparing the H-alpha and stellar surface brightness profiles, we will be able to evaluate whether the outer disk population could have been generated in situ by the present distribution of star formation. Correlations with morphology will clarify whether the tidal debris hypothesis is applicable. Metallicity and velocity dispersion will illuminate the conditions under which outer-disk star formation is progressing and, via the surface mass density of the outer disk, measure the total gas reservoir that is available for star formation in the outer disk.

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