Future Possibilities for Detecting HI at High Redshift

The 21-cm line of neutral hydrogen is in many respects the most valuable tracer of neutral gaseous mass in astrophysics. Even though neutral gas becomes predominantly molecular at high densities, the 21-cm line emission of the associated atomic component allows the total gaseous mass of (proto) galactic concentrations to be estimated to about a factor of two, even in the most extreme cases observed to date. This is in marked contrast to, for example, the luminosity of carbon monoxide emission lines originating in the molecular component. For the same total gaseous mass, these emission lines are observed to vary in luminosity over a factor of about 104 depending on the abundance of heavy elements and the intensity of illumination to which they are subjected.