Vortex formation in the Vicsek model with internal chirality of self-propelling objects

Effect of internal chirality on collective motion of a large number of active objects is studied by simulations of appropriately modified Vicsek model. We add a fixed angle to the noise and consider small ratios, $p$, between this angle and the maximal deviation from the average local direction of motion. When the above ratio is $p=1/120$, the traveling bands observed with the symmetrical noise are destroyed, and small bands moving in different directions appear. Circular rotating flocks of objects with the same orientation are formed for $p=1/7.5$. Stable vortexes in the stationary state were found from $p=1/60$ to $p=1/20$. Velocity autocorrelation function shows equilibrium between the inflow and the outflow to and from the vortex. Long-time evolution is considerably influenced by a temporary trapping of the objects in the vortex. The ballistic behavior for the symmetrical noise changes to the diffusive behavior for the chirality leading to the onset of vortexes.