We explain, following Gromov, how to produce uniform isometric actions of
groups starting from isometric actions without fixed point, using common
ultralimits techniques. This gives in particular a simple proof of a result by
Shalom: Kazhdan's property (T) defines an open subset in the space of marked
finitely generated groups.
We prove that the limits of Baumslag-Solitar groups which we previously
studied are non-linear hopfian C*-simple groups with infinitely many twisted
conjugacy classes. We exhibit infinite presentations for these groups, classify
them up to group isomorphism, describe their automorphisms and discuss the word
and conjugacy problems. Finally, we prove that the set of these groups has
non-zero Hausforff dimension in the space of marked groups on two generators.