  We present measurements of the excitation function of elliptic flow at
midrapidity in Au+Au collisions at beam energies from 0.09 to 1.49 GeV per
nucleon. For the integral flow, we discuss the interplay between collective
expansion and spectator shadowing for three centrality classes. A complete
excitation function of transverse momentum dependence of elliptic flow is
presented for the first time in this energy range, revealing a rapid change
with incident energy below 0.4 AGeV, followed by an almost perfect scaling at
the higher energies. The equation of state of compressed nuclear matter is
addressed through comparisons to microscopic transport model calculations.
