Application of the Density-Wave Theory of Spiral Structure: Shock Formation Along the Perseus Arm

An examination of (1) recent optical data on 0 associations, young open clusters, H ii regions, and interstellar absorption lines and (2) recent high-resolution 21-cm hydrogen maps of the Perseus spiral arm indicates that the systematic motion of order up to 20 km s ' associated with the interstellar medium may be a significantly more extended phenomenon than previously recognized, stretching from a longitude of t = 100 all the way around to t = 200 and perhaps beyond. In light of the observational data, a model based on the density-wave and galactic-shock-wave concepts is proposed for the Perseus arm. The model considered is the two-armed spiral shock (TASS) model that delineates the large-scale spiral structure and describes the large-scale motion of the interstellar gas, and the young optical objects born out of the gas, over the galactic disk. In this model, the Perseus arm is visualized to consist of a galactic shock wave embedded in a background density wave. The systematic motion observed along the entire length of the Perseus arm can he accounted for as the systematic motion predicted in the TASS model. Splitting of an H r feature into multiple subcomponents of the t ?e found in observed profiles of neutral hydrogen arises rather naturally in the presence of a galactic shock. When the observational data are interpreted through the TASS model, the apparent discrepancy between the position of the optical spiral arm and that of the neutral-hydrogen spiral arm is found to disappear, and the overall Perseus feature becomes one composite arm. Secondary arms are found to be possible in the outer parts of the TASS model, and these bear some resemblence to the secondary arms, "spurs," and "feathers" which are often observed in the outer parts of external spiral galaxies and our own MiWy Way system.