  Direct photons are a powerful probe to study the properties of the unique
matter created in ultrarelativistic heavy-ion collisions. They carry
information on the various stages of a heavy-ion collision. At different
transverse momenta (pT), different production processes dominate the direct
photon yield in heavy-ion collisions. Photons at high pT can be used to study
initial hard scattering processes while photons at low and intermediate pT
provide direct information on the hot and dense medium created in such
collisions since they origin predominantly from jet-medium interactions and
from thermal radiation from the medium itself. PHENIX has measured
direct-photon yields over a broad pT and energy range in different collision
systems such as Au+Au and Cu+Cu, allowing systematic studies of the behavior of
direct photons in heavy-ion collisions. Two different methods have been used to
measure the direct photons. An excess of direct photons in Au+Au collisions at
low pT beyond the expectation from p+p collisions was measured for the first
time via internal conversion. Furthermore, the azimuthal anisotropy parameter
v2 has been measured. The direct photon v2 sheds light on the different
processes contributing to the production of direct photons. The p+p data at the
same energy provide a baseline for understanding the heavy-ion data, but are
also interesting in their own right, e.g. for testing pQCD calculations or - as
RHIC collides polarized protons - for constraining models on the gluon
contribution to the proton spin.
