Diffuse continuum gamma rays from the galaxy

A new study of the diffuse Galactic gamma-ray continuum radiation is presen ted, using a cosmic-ray propagation model which includes nucleons, antiprot ons, electrons, positrons, and synchrotron radiation. Our treatment of the inverse Compton scattering includes the effect of anisotropic scattering in the Galactic interstellar radiation held (ISRF) and a new evaluation of th e ISRF itself. Models based on locally measured electron and nucleon spectr a and synchrotron constraints are consistent with gamma-ray measurements in the 30-500 MeV range, but outside this range excesses are apparent. A hard er nucleon spectrum is considered but fitting to gamma-rays causes it to vi olate limits from positrons and antiprotons. A harder interstellar electron spectrum allows the gamma-ray spectrum to be fitted above 1 GeV as well, a nd this can be further improved when combined with a modified nucleon spect rum which still respects the limits imposed by antiprotons and positrons. A large electron/inverse Compton halo is proposed which reproduces well the high-latitude variation of gamma-ray emission; this is taken as support for the halo size for nucleons deduced from studies of cosmic-ray composition. Halo sizes in the range 4-10 kpc are favored by both analyses. The halo co ntribution of Galactic emission to the high-latitude gamma-ray intensity is large, with implications for the study of the diffuse extragalactic compon ent and signatures of dark matter. The constraints provided by the radio sy nchrotron spectral index do not allow all of the gamma-ray emission at less than 30 MeV to be explained in terms of a steep electron spectrum unless t his takes the form of a sharp upturn below 200 MeV. This leads us to prefer a source population as the origin of the excess low-energy gamma-rays, whi ch can then be seen as a continuation of the hard X-ray continuum measured by OSSE, Ginga, and RXTE.