  Two dimensional low-$p_T$ dihadron correlations in azimuthal angle $\phi$ and
pseudo-rapidity $\eta$ in high-energy heavy-ion collisions are investigated
within both the HIJING Monte Carlo model and an event-by-event (3+1)D ideal
hydrodynamic model. Without final-state interaction and collective expansion,
dihadron correlations from HIJING simulations have a typical structure from
minijets that contains a near-side two-dimensional peak and an away-side ridge
along the $\eta$-direction. In contrast, event-by-event (3+1)D ideal
hydrodynamic simulations with fluctuating initial conditions from the
HIJING+AMPT model produce a strong dihadron correlation that has an away-side
as well as a near-side ridge. Relics of intrinsic dihadron correlation from
minijets in the initial conditions still remain as superimposed on the two
ridges. By varying initial conditions from HIJING+AMPT, we study effects of
minijets, non-vanishing initial flow and longitudinal fluctuation on the final
state dihadron correlations. With a large rapidity gap, one can exclude
near-side correlations from minijet relics and dihadron correlations can be
described by the superposition of harmonic flows up to the 6th order. When
long-range correlations with a large rapidity gap are subtracted from
short-range correlations with a small rapidity gap, the remaining near-side
dihadron correlations result solely from relics of minijets. Low transverse
momentum hadron yields per trigger ($p_T^{\rm trig} <4$ GeV/$c$, $p_T^{\rm
asso}<2$ GeV/$c$) in central heavy-ion collisions are significantly enhanced
over that in p+p collisions while widths in azimuthal angle remain the same, in
qualitative agreement with experimental data.
