  We simulate clustering, phase separation and hexatic ordering in a
monolayered suspension of active squirming disks subject to an attractive
Lennard-Jones-like pairwise interaction potential, taking hydrodynamic
interactions between the particles fully into account. By comparing the
hydrodynamic case with counterpart simulations for passive and active Brownian
particles, we elucidate the relative roles of self-propulsion, interparticle
attraction, and hydrodynamic interactions in determining clustering and phase
behaviour. Even in the presence of an attractive potential, we find that
hydrodynamic interactions strongly suppress the motility induced phase
separation that might a priori have been expected in a highly active
suspension. Instead, we find only a weak tendency for the particles to form
stringlike clusters in this regime. At lower activities we demonstrate phase
behaviour that is broadly equivalent to that of the counterpart passive system
at low temperatures, characterized by regimes of gas-liquid, gas-solid and
liquid-solid phase coexistence. In this way, we suggest that a dimensionless
quantity representing the level of activity relative to the strength of
attraction plays the role of something like an effective non-equilibrium
temperature, counterpart to the (dimensionless) true thermodynamic temperature
in the passive system. However there are also some important differences from
the equilibrium case, most notably with regards the degree of hexatic ordering,
which we discuss carefully.
