  We present a comprehensive analysis of effective temperatures based on
fluctuation-dissipation relations in a model of an active fluid composed of
self-propelled hard disks. We first investigate the relevance of effective
temperatures in the dilute and moderately dense fluids. We find that a unique
effective temperature does not in general characterize the non-equilibrium
dynamics of the active fluid over this broad range of densities, because
fluctuation-dissipation relations yield a lengthscale-dependent effective
temperature. By contrast, we find that the approach to a non-equilibrium glass
transition at very large densities is accompanied by the emergence of a unique
effective temperature shared by fluctuations at all lengthscales. This suggests
that an effective thermal dynamics generically emerges at long times in very
dense suspensions of active particles due to the collective freezing occurring
at non-equilibrium glass transitions.
