3.6 Extended dark halos
The evidence for very extended dark halos around spiral galaxies does not come from a single tracer such as HI. Other tracers are brought to bear, such as satellites, binary galaxy statistics, etc. The use of these tracers is much less straightforward, and needs the development of mass estimators, which take into account the statistical effects of the orbits of the tracer galaxy around the parent galaxy.
An example of this is the work by Zaritsky & White (1994) and Zaritsky et al. (1997) on a sample of spiral galaxies with dwarf satellites. These authors have been slowly collecting data on dwarfs around large, inclined spirals, in order to put together a well defined sample in which the satellite distribution around the primaries can be treated statistically. They conclude that large massive halos, with radii as large as 200 kpc, do indeed exist around large spirals. However, the statistics of the satellite distribution show some interesting details, which are not fully understood in the framework of current ideas on galaxy formation and evolution.
A completely different way to probe the extent of dark halos around individual galaxies comes from the analysis of weak but measurable changes to the shapes of distant galaxies due to the gravitational lensing by foreground galaxies. Brainerd et al. (1996) report on one such analysis, and find that large halos, of ~ 100 h-1 kpc do indeed exist, in agreement with the conclusions from Zaritsky & White (1994).