D. M. Elmegreen

Spiral arm amplitude variations and pattern speeds in the grand design galaxies M51, M81, and M100

In the modal theory of galactic spiral structure, the amplitude of a prominent two-arm spiral pattern should oscillate slightly with galactocentric distance because of an interference between the outward and inward propagating waves. In the stellar dynamical theory, the spiral arm amplitudes should oscillate because of differential crowding near and between wave-orbit resonances. Two and three cycles of such oscillations have been found in computer-enhanced images at B and I passbands of the grand design galaxies M81 and M100, respectively, and what is probably one cycle of such an amplitude variation in M51. These three galaxies are the most symmetric and global of the two-arm spirals in the near-IR survey of Elmegreen (1981), so the occurrence of such spiral amplitude oscillations could be common among galaxies of this type. The positions of the features discussed are used to suggest possible arm pattern speeds. 23 refs.

On the relative frequency of flocculent and grand design spiral structures in barred galaxies

The bars in late-type spiral galaxies (SBbc to SBm and SABbc to SABm) show little correlation with grand design spiral structure, unlike the bars in early-type galaxies. This is further evidence for two different types of bars in the SB morphological class, and two different types of ovals in the SAB class. The weak, short, and exponential-shaped bars that tend to occur in late-type galaxies may end far inside the corotation resonance and be unable to produce large-scale wave patterns in the outer stellar disks. The strong, long, and flat-shaped bars that tend to occur in early-type galaxies may extend closer to the corotation resonance and be able to produce strong and symmetric arms. This difference in bar-spiral correlation is consistent with the theoretical prediction that short-wavelength trailing spirals propagate outward beyond corotation and inward inside corotation. 30 refs.

Spiral Structure of the Milky Way and External Galaxies

The spiral structure of the Milky Way galaxy has always been somewhat elusive because of our internal vantage point. This review will present methods and data for determining the overall pattern, and will summarize various models that have been proposed. Observations of spirals in external galaxies will also be discussed, because they can provide insight into the spiral structure of the Milky Way.

Computer Processed Color Images of Spiral Galaxies

The painstaking and extensive work on spiral galaxies carried out by Antoinette and Gerard de Vaucouleurs is at the same time impressive and intimidating. It is clear from their work, and the work of other fine investigators in the field, that the job of collecting the extensive data necessary to understand the structure and evolution of spiral galaxies is not easy. The reduction of the astronomical plates to extract the appropriate data is difficult and time consuming. For that reason, we began a program to investigate how the addition of color and digital processing could provide more informative images. Computerized color imagery using multiple passbands may be an important new tool for analyzing galaxies. Computers can superimpose the various images of a galaxy, and then realign the result to a face-on orientation for a better view of the spirals, or subtract the underlying disk light to emphasize only the azimuthal variations. The computer processed images show the shapes and color distributions of spiral structure better then conventional photographs. Spiral arms can often be traced for more than a complete turn around the nucleus, inner ovals are sometimes revealed, and blue patterns of star formation can be identified.