Bruce G. Elmegreen

Optical and Theoretical Studies of Giant Clouds in Spiral Galaxies

An optical study of four spiral galaxies, combined with radiative transfer models for transmitted and scattered light, has led to a determination of the opacities and masses of numerous dark patches and dust lanes that outline spiral structure. The observed compression factors for the spiral-like dust lanes are in accord with our expectations from the theory of gas flow in spiral density waves. Several low density (10 2 cm -3 ) clouds containing 10 6 to 10 7 M⊙ were also studied. We discuss these results in terms of recent theoretical models of cloud and star formation in spiral galaxies. The long-term evolution of giant molecular clouds is shown to have important consequences for the positions and ages of star formation sites in spiral arms.