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We calculate the centrality dependence of direct photons produced in Au+Aucollisions at sqrt{s}=200GeV and their transverse momentum spectra within theParton-Hadron-String Dynamics (PHSD) transport approach. As sources for'direct' photons, we incorporate the interactions of quarks and gluons as wellas hadronic interactions (pi+pi->rho+gamma, rho+pi->pi+gamma, meson-mesonbremsstrahlung m+m->m+m+gamma, meson-baryon bremsstrahlung m+B->m+B+gamma), thedecays of phi and a_1 mesons and the photons produced in the initial hardcollisions ('pQCD'). We find that the pT spectra of the 'thermal' photons (i.e.the direct photons after the pQCD contribution is subtracted) deviate fromexponential distributions and, consequently, observe a strong dependence of theinverse slope parameter T_eff on the fitting range in pT. On the other hand,all the obtained 'effective temperatures' are well above the criticaltemperature for the deconfinement phase transition even for peripheralcollisions, reflecting primarily a 'blue shift' due to radial collective motionof hadrons. Our calculations suggest that the channel decomposition of theobserved 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 ofparticipant nucleons as N_part^alpha with alpha=1.5, whereas the partoniccontribution scales with an exponent alpha_p=1.75. Additionally, we providepredictions for the centrality dependence of the direct photon elliptic flowv_2(pT). The direct photon $v_2$ is seen to be larger in peripheral collisionscompared to the most central ones since the photons from the hot deconfinedmatter in the early stages of the collision carry a much smaller elliptic flowthan the final hadrons.

Centrality dependence of the direct photon yield and elliptic flow in heavy-ion collisions at sqrt(s)=200 GeV