11.3.1. Density Distribution
a) Extent of Distribution
The integral properties of galaxies have revealed several interesting correlations of total mass and hydrogen abundance with morphological type, but have not answered the question as to why the different morphological types exist. We can hope to gain insight into this, and the question of evolution and star formation in galaxies, only if we know more details of the structure and distribution of the different components of a galaxy. Particularly lacking in this respect is a detailed knowledge of the distribution of the neutral hydrogen, the most basic component of a galaxy. The mapping of neutral hydrogen in external galaxies allows us to find such things as:
The overall form of the HI distribution, how this compares with that of the total mass distribution, and where the highest hydrogen concentrations are.
The extent of the hydrogen distribution. Is it coextensive with the luminous population of a galaxy? How sharp are its boundaries ?
Neutral hydrogen wings, bridges between galaxies and companions or satellites with the same sort of relationship as the Magellanic Clouds are believed to have to the Milky Way.
b) Spiral Arms
The above questions can be answered with fairly limited resolution. With higher resolution we may comment further on whether the neutral hydrogen has the form of spiral arms and if these are coincident with the spiral arms defined by the bright HII regions or displaced from them.
A detailed correlation can be made of the surface densities of the young O and B stars, the HII regions they excite, and the neutral hydrogen from which they have been formed. This is a very important field, for in our galaxy the situation is confused by the complex velocity-distance relationship. In an external galaxy one has an overall view of the distribution of the different components.