Gas at large radii
Abstract
Three reasons are cited for investigating this outlying gas. The first is that, even though it represents only a very small fraction of the total mass, it can be used as tracer of the kinematics to determine the total mass and the mass distribution of the system. The second is that it contributes to the galactic cross sections needed to explain QSO absorption lines, and the third is that its physical and chemical properties may provide information on the formation and evolution of galactic disks and their evolution. The main properties of the density distribution are discussed, with attention also given to the rotation curves in the outermost regions of galaxies. The outlying gas of spiral galaxies is seen as undergoing a more or less abrupt change of its properties at some radius, a radius that is approximately at the Van der Kruit and Searle (1981) edge of the optical disk and generally close to the Holmberg radius. The three main effects observed at approximately this same radius are: (1) the gas layer is warped and probably thicker; (2) the density profile exhibits a drop-off by a factor of five and a 'shoulder" appears in the outer parts; and (3) a decrese of radial velocity takes place which may imply a drop-off in the rotation curve.
- Publication:
-
Internal Kinematics and Dynamics of Galaxies
- Pub Date:
- 1983
- Bibcode:
- 1983IAUS..100...55S
- Keywords:
-
- Density Distribution;
- Hydrogen;
- Interstellar Gas;
- Neutral Gases;
- Spiral Galaxies;
- Absorption Spectra;
- Galactic Evolution;
- Galactic Rotation;
- Gas Density;
- Gas Dynamics;
- Line Spectra;
- Mass Distribution;
- Quasars;
- Radii;
- Astrophysics