Abstract
Since extreme Population I dominates three rocketborne telescope UV images of the Sc I galaxy NGC 5457, the delineation of the spiral structure is sharper than that of ground photographs. The UV pictures, in which the H II regions appear as reflection nebulae and illumine the nearby dust, reveal faint arms which, although correlated with the 21 cm H I radiograph, are only marginally detectable on deep visual photographs. The surface brightness of the arms is explainable in terms of UV light scattering off dust from hot stars and bright H II regions, in which case the dust would have extinction properties and a dust-to-gas ratio similar to that in the solar neighborhood and would in addition suggest that the NGC 5457 outer arm metal content is similar to that of the galaxy.
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