Active glass: ergodicity breaking dramatically affects response to self-propulsion

We study experimentally the response of a dense sediment of Brownian particles to self-propulsion. We observe that the ergodic supercooled liquid relaxation is monotonically enhanced by activity. By contrast the nonergodic glass shows an order of magnitude slowdown at low activities with respect to passive case. This observation cannot be rationalized from the concept of effective temperature. Fluidization is recovered at higher activities due to collective motion. Analyzing particle trajectories we find that cooperative rearrangements become less efficient with self-propulsion due to rotation-diffusion coupling. Our results show that loss of ergodicity dramatically affects active systems.