“…An alternative hydrodynamic limit can be obtained [37,38,39] by taking an isothermal ansatz f (x, v, t) = (2π) −d/2 ρ(x, t) exp(−|v − u(x, t)| 2 /2), in which case ∇ x • R becomes a pressure term ∇ x ρ. The resulting isothermal Euler-alignment system was investigated in [8,13]. A more general class of isentropic Euler-alignment system with a pressure term ∇ x (ρ γ ), γ ≥ 1 was considered in [14,12].…”