  In this paper, we explore experimentally the phase behavior of a dense active
suspension of self- propelled colloids. In addition to a solid-like and a
gas-like phase observed for high and low densities, a novel cluster phase is
reported at intermediate densities. This takes the form of a stationary
assembly of dense aggregates, with an average size which grows with activity as
a linear function of the self-propelling velocity. While different possible
scenarii can be considered to account for these observations - such as a
generic velocity weakening instability recently put forward -, we show that the
experimental results are reproduced by a chemotactic aggregation mechanism,
originally introduced to account for bacterial aggregation, and accounting here
for diffusiophoretic chemical interaction between colloidal swimmers.
