initial elliptic spatial anisotropy, $\epsilon$, as a function of the charge particle density per unit transverse area. The bulk hydrodynamic limit is only attained at RHIC.[]{data-label="line1fig"}](NA49STARv2_hydro.eps "fig:"){height="0.35\textheight" width="50.00000%"}

Figure (\[line1fig\]) shows the striking bulk collectivity elliptic flow signature of QGP formation at RHIC. Unlike at SPS and lower energies, the observed large elliptic deformation ($(1+2 v_2)/(1-2 v_2)\sim 1.5$) of the final transverse momentum distribution agrees for the first time with non-viscous hydrodynamic predictions [@Kolb:2000fh]-[@Hirano:2003pw] at least up to about $p_T\sim 1$ GeV/c. However, the right panel shows that when the local rapidity density per unit area [@Voloshin:1999gs; @Alt:2003ab] drops below the values achieved at RHIC $\sim 30/{\rm fm}^2$, then the elliptic flow (scaled by the initial spatial ellipticity, $\epsilon=\langle (y^2-x^2)/(y^2+x^2)\rangle$) falls below the perfect fluid hydrodynamic predictions. We will discuss in more detail the origin of the large discrepancy at SPS energies in the next section.

The most impressive feature in Fig.(\[line1fig\]) is the agreement of the observed hadron mass dependence of the elliptic flow pattern for all hadron species, $\pi, K, p,\Lambda$, with the hydrodynamic predictions below 1 GeV/c. This is the QGP fingerprint that shows that there is a common bulk collective azimuthally asymmetric flow velocity field, $u^\mu(\tau,r,\phi)$.

