  A theoretical derivation of photon bremsstrahlung, induced by the
interactions of an energetic quark in a hot and dense quark-gluon plasma, is
given in the framework of the reaction operator approach. For the physically
relevant case of hard jet production, followed by few in-medium interactions,
we find that the Landau-Pomeranchuk-Migdal suppression of the bremsstrahlung
photon intensity is much stronger than in the previously discussed limit of
on-shell quarks and a large number of soft scatterings. We present the first
systematic study of direct photon production in minimum bias d+Cu and d+Au and
central Cu+Cu and Au+Au heavy ion collisions at the Relativistic Heavy Ion
Collider at center of mass energies of 62.4 GeV and 200 GeV. We find that the
contribution of the final-state photon production at pT < 5 GeV is limited to
35%, and at high transverse momenta, the modification of the direct photon
cross section is dominated by initial-state cold nuclear matter effects.
