(color online) (a) Cumulative rotation angle β of a rigid circle as a function of time t for different area fractions ρ of chiral active particles with l = 3, ω=0.1, n_B=4, and γ = π/2. The straight black lines are linear fitting curves. (b) Average angular velocity and (c) resultant moment M_Z of a rigid circle as a function of ρ.