Ionised hydrogen at large galactocentric distances

Abstract

We summarize recent attempts to detect warm ionized gas at large galactocentric distances. This includes searching for gas at the edges of spirals, in between cluster galaxies, towards extragalactic HI clouds, and towards high velocity clouds and the Magellanic Stream in the Galaxy. With the exception of extragalactic HI clouds, all of these experiments have proved successful. Within each class, we have only observed a handful of objects. It is premature to assess what fraction of the missing baryonic mass fraction might be in the form of ionized gas. But, in most cases, the detections provide a useful constraint on the ambient ionizing flux, and in the case of spiral edges, can even trace dark matter halos out to radii beyond the reach of radio telescopes.