Abundance gradients in extragalactic H II regions and internal absorption by dust

TL;DR: In this article, the radio, optical, and infrared observations of giant spiral arm H II regions in the Sc galaxies M33, M51, and M101 are discussed, in order to determine the effect of the observed abundance gradients on the structure of the regions.

Abstract: The radio, optical, and infrared observations of giant spiral arm H II regions in the Sc galaxies M33, M51, and M101 are discussed, in order to determine the effect of the observed abundance gradients on the structure of the regions. The radio continuum observations require the presence of a considerable amount of internal extinction, which appears to increase with the heavy-element abundance. We suggest that these three effects are best explained by assuming that the dust-to-gas ratio within the nebulae increases with increasing Z. A model analysis of the emission of these regions shows that the observations cannot be explained by an abundance gradient alone; an excitation gradient, or softening of the ionizing radiation field in the inner regions, is also required. Inhomogeneous models are presented in which this excitation gradient is provided by ultraviolet-absorbing dust. (AIP)