Composition Gradients in Spiral Galaxies: a Consistency Check on the - Wave Theory

Radial composition gradients in spiral galaxies are compared with expectations based on the spiral density-wave picture. It is presumed that star formation along the spiral arms is triggered when disk gas encounters the density-wave pattern and is compressed in a shock region near the wave crest. The frequency of star-forming events at a given radius in the galaxy is thus proportional to the difference between the angular speed of the gas and that of the pattern. The efficiency of star formation may be related to the compression suffered by the disk gas as it passes through the wave pattern. Radial changes in disk gas chemical composition can be related to variations in the star-formation frequency and in the star-formation efficiency across the face of a galaxy. The observations of emission-line strengths in the disks of 12 late-type galaxies, reported here, support this picture but do not exclude other, more ad hoc, star-formation and metal-enrichment scenarios. Subject headings: galaxies: general - galaxies: structure - abundances - image processing