Distribution and kinematics of gas in galaxy discs

Summary

Abstract Three topics are briefly discussed concerning the gas distribution and kinematics in spiral galaxies. The first concerns the relative location of neutral hydrogen, HII regions, dust, molecules and non-thermal radio continuum emission in spiral arms. The second is the asymmetrical structure and the presence of large non-circular motions in spiral galaxies, as shown by the observations of M 101. Finally, attention is drawn to the presence of spiral arm structure and to some puzzling HI features in the outermost parts of gaseous discs. Observational evidence seems to indicate that infall of gas has important effects on the kinematics of discs and on their evolution.

Structure of spiral arms

Detailed, multifrequency observations of recent years of two nearby spiral galaxies, M 51 and M 83, have led to a new picture of the relative distributions of the various ingredients of the interstellar medium. In the classical schematic picture of spiral arm structure the HI is concentrated on the inner side of spiral arms on the dust lanes, which mark the location of spiral shocks. The observations of HII and HI regions by Allen et al. (1986) and Tilanus & Allen (1989) show that both HI and HII are displaced from the dust lanes toward the outer parts of the arm, although there is no small scale agreement between the distribution of HI and HII. The radio continuum ridge in M 51 coincides with the dust lanes (Tilanus et al. 1988). Its profile across the arm is much broader than expected.