Gamma-ray spectrum E ² dN/ dE of MW, M82, and NGC 253, where E is gamma-ray energy and dN/ dE the differential photon flux. The MW spectrum is the diffuse background flux in units shown on the right ordinate, and the others are the flux from a source in units shown on the left ordinate. For the purpose of presentation, the spectrum and the data points of M82 are multiplied by 100. The MW data (open squares) are the Galactic diffuse gamma-rays averaged over | b|>10° (Abdo et al. 2010c). The Fermi data of M82 (filled circles) and NGC 253 (open circles) are obtained by Abdo et al. (2010a). The data in the very high energy range of M82 (filled triangles; VERITAS Collaboration 2009) and NGC 253 (open triangles; Acero et al. 2009) are also plotted. The solid curves are the model spectra used to calculate EGRB (see the text for details).