  In this study we investigate the dynamics of strongly interacting
parton-hadron matter by calculating the centrality dependence of direct photons
produced in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV within the
Parton-Hadron-String Dynamics (PHSD) transport approach. As sources for
'direct' photons, we incorporate the interactions of quarks and gluons as well
as hadronic interactions ($\pi+\pi\to\rho+\gamma$, $\rho+\pi\to\pi+\gamma$,
meson-meson bremsstrahlung $m+m\to m+m+\gamma$, meson-baryon bremsstrahlung
$m+B\to m+B+\gamma$), the decays of $\phi$ and $a_1$ mesons and the photons
produced in the initial hard collisions ('pQCD'). Our calculations suggest that
the channel decomposition of the observed spectrum changes with centrality with
an increasing (dominant) contribution of hadronic sources for more peripheral
reactions. Furthermore, the 'thermal' photon yield is found to scale roughly
with the number of participant nucleons as $N_{part}^\alpha$ with $\alpha
\approx$ 1.5, whereas the partonic contribution scales with an exponent
$\alpha_p \approx1.75$. Additionally, we provide predictions for the centrality
dependence of the direct photon elliptic flow $v_2(p_T)$. The direct photon
$v_2$ is seen to be larger in peripheral collisions compared to the most
central ones since the photons from the hot deconfined matter in the early
stages of the collision carry a much smaller elliptic flow than those from the
final hadronic interactions.
