  We numerically study photon escape rates from three-dimensional atomic gases
and investigate the respective roles of cooperative effects and disorder in
photon localization, while taking into account the vectorial nature of light. A
scaling behavior is observed for the escape rates, and photons undergo a
crossover from delocalization toward localization as the optical thickness of
the cloud is increased. This result indicates that light localization is
dominated by cooperative effects rather than disorder. We compare our results
with those obtained in the case of a scalar radiation field and find no
significant differences. We conclude that the scalar model constitutes an
excellent approximation when considering photon escape rates from atomic gases.
