We have measured luminescence of atomically clean GaAs(100) samples induced by a scanning tunneling microscope (STM) under ultrahigh vacuum (UHV) conditions. GaAs(100) samples were grown by molecular beam epitaxy and capped with a thick arsenic overlayer to protect the surface during transfer through ambient atmosphere to the UHV STM chamber equipped with a light collection system. The luminescence arises from the radiative recombination of the excited carriers across the band gap induced by the tunneling current. Maps of luminescence as a function of lateral position of the STM tip (photon maps) have been recorded. Arsenic related features of the size of similar to 20 nm appear as dark areas in the photon maps indicating increased local nonradiative recombination. Photon maps acquired at positive and negative sample bias show significant differences in contrast. Mechanisms responsible for the STM excited luminescence and contrast observed in the photon maps are discussed.