  In nucleus--nucleus collisions, high-pT partons interact with a dense medium,
which possesses strong collective flow components. Here, we demonstrate that
the resulting medium-induced gluon radiation does not depend solely on the
energy density of the medium, but also on the collective flow. Both components
cannot be disentangled on the basis of leading hadron spectra, but the
measurement of particle production associated to high-pT trigger particles,
jet-like correlations and jets, allows for their independent characterization.
In particular, we show that flow effects lead to a characteristic breaking of
the rotational symmetry of the average jet energy and jet multiplicity
distribution in the $\eta \times \phi$-plane. We argue that data on the
medium-induced broadening of jet-like particle correlations in Au+Au collisions
at RHIC provide a first evidence for a significant distortion of parton
fragmentation due to the longitudinal collective flow.
