1 Elliptic flow from partially thermalized heavy-ion collisions ∗

We study to what extent the measured elliptic flow at RHIC constrains viscous deviations from ideal hydrodynamics. We solve a toy model where only transverse momenta are thermalized while the system undergoes longitudinal free-streaming. We show that RHIC data exclude such a model and thus require fast 3-dimensional thermalization. 1.