Scaling Characteristics of Azimuthal Anisotropy at RHIC

Recent differential measurements of elliptic flow are used to probe several hydrodynamic scaling predictions. Eccentricity scaling is observed for Cu+Cu and Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV, suggesting essentially complete thermalization of the high energy density matter produced in these collisions. An estimate of the speed of sound is also obtained from the eccentricity scaled elliptic flow. The predicted mass scaling is observed for particles with transverse kinetic energy KE${}_T$ up to $\sim 1$ GeV. For KE${}_T$ values above $\sim 1$ GeV, valence quark number scaling compatible with partonic degrees of freedom is observed.