11.1.1. Hydrogen Distribution and Kinematics
Hydrogen is the building material for stars and galaxies, and a study of its distribution and kinematics in external galaxies is essential to understanding the dynamics and evolution of these objects. Much can be learned by comparing the overall or integral properties of different galaxies, but an understanding of the detailed distribution of neutral hydrogen in a galaxy can be derived only from a high-resolution map.
The Magellanic Clouds were the first extra-galactic objects observed in the 21-cm line (Kerr, Hindman, and Robinson, 1954). The large angular extent of these objects enabled a detailed map to be made of the neutral hydrogen density and velocity distribution across the Clouds. The aim of observations today is to make similarly detailed maps for ordinary spiral and irregular galaxies.
The story of mapping neutral hydrogen in external galaxies is one of a continuing quest for angular resolution, but we shall first briefly review the classification and properties of the galaxies we will be talking about.