Gas at Large Radii

Abstract

A considerable fraction of the neutral hydrogen in spiral galaxies is generally found at large radii and beyond the optical image. This outlying gas either forms an extension of the stellar disk and of the inner HI layer, or, is concentrated in a ring. It shows well ordered motion around the galaxy, although the plane of the orbits is generally inclined with respect to the inner disk. The study of this gas is important for at least three reasons: i) although it represents only a very small fraction of the total mass it can be used as a tracer of the kinematics to determine the total mass and the mass distribution of the system, ii) it contributes to the galactic cross-sections needed to explain QSO absorption lines, iii) its physical and chemical properties may throw some light on the formation of galactic disks and their evolution.

The general kinematical properties of the HI have already been reviewed by Bosma (this symposium). I will therefore discuss the main properties of the density distribution and add only a few comments on the rotation curves in the outermost regions of galaxies.