  We present two-dimensional (2D) two-particle angular correlations on relative
pseudorapidity $\eta$ and azimuth $\phi$ for charged particles from Au-Au
collisions at $\sqrt{s_{\rm NN}} = 62$ and 200 GeV with transverse momentum
$p_t \geq 0.15$ GeV/$c$, $|\eta| \leq 1$ and $2\pi$ azimuth. Observed
correlations include a {same-side} (relative azimuth $< \pi/2$) 2D peak, a
closely-related away-side azimuth dipole, and an azimuth quadrupole
conventionally associated with elliptic flow. The same-side 2D peak and
away-side dipole are explained by semihard parton scattering and fragmentation
(minijets) in proton-proton and peripheral nucleus-nucleus collisions. Those
structures follow N-N binary-collision scaling in Au-Au collisions until
mid-centrality where a transition to a qualitatively different centrality trend
occurs within a small centrality interval. Above the transition point the
number of same-side and away-side correlated pairs increases rapidly {relative
to} binary-collision scaling, the $\eta$ width of the same-side 2D peak also
increases rapidly ($\eta$ elongation) and the $\phi$ width actually decreases
significantly. Those centrality trends are more remarkable when contrasted with
expectations of jet quenching in a dense medium. Observed centrality trends are
compared to {\sc hijing} predictions and to the expected trends for semihard
parton scattering and fragmentation in a thermalized opaque medium. We are
unable to reconcile a semihard parton scattering and fragmentation origin for
the observed correlation structure and centrality trends with heavy ion
collision scenarios which invoke rapid parton thermalization. On the other
hand, if the collision system is effectively opaque to few-GeV partons the
observations reported here would be inconsistent with a minijet picture.

Nuclear Experiment