Long-ranged velocity correlations in dense systems of self-propelled particles

Experiments demonstrated that epithelial cell monolayers exhibit equal-time velocity correlations that extend over several cell sizes. Equal-time velocity correlations have also been observed in models of active matter systems. These correlations originate from the activity and are absent in equivalent equilibrium systems. Previous work that examined amorphous solid-like active systems rationalized the origin of the velocity correlations in terms of the elasticity of the solid, and found that the velocity correlations grow with the square-root of the persistence time. Here we use computer simulations to show that velocity correlations also exist in active fluids and also grow with the square root of the persistence time. In active fluids these correlations develop due to the combination of the activity and the virial bulk modulus that originates from the repulsive interactions between fluid's constituents.