High star formation (SF) and supernova (SN) rates in starburst (SB) galaxies (SBGs) boost the density of energetic nonthermal particles, whose main constituents are protons and electrons. Coulomb, synchrotron and Compton energy losses by the electrons, and the decay of pions following their production in energetic proton interactions with protons in the ambient gas, result in emission over the full electromagnetic spectrum, from radio to very high-energy (VHE, 100 GeV) -rays. The relatively high intensity emission in SBGs, as compared with emission form `normal' star-forming galaxies (SFGs), makes nearby members of this class the most likely non-AGN targests for -ray telescopes, such as Fermi and the (Cherenkov arrays) H.E.S.S., MAGIC, and VERITAS.
Interest in -ray emission from SFGs clearly stems from the prospects for improved understanding of the origin and propagation mode of energetic electrons and protons and their coupling to interstellar media. This interest has been enhanced by recent detections of the two nearby SBGs M 82 & NG C253 by Fermi (Abdo et al. 2010a) and, respectively, by H.E.S.S (Acciari et al. 2009) and VERITAS (Acero et al. 2009). M 31, the closest normal spiral galaxy, was also detected by Fermi (Abdo et al. 2010b).
A realistic estimate of the expected -ray emission requires a detailed account of all relevant energy loss processes of energetic electrons and protons as they move out from the central SB source region into the outer galactic disk. Calculations of the predicted X--ray spectra of nearby galaxies were made long ago with varying degree of detail (e.g., Goldshmidt & Rephaeli 1995, Paglione et al. 1996, Romero & Torres 2003, Domingo-Santamaría & Torres 2005). A more quantitative numerical approach was initiated by Arieli & Rephaeli (2007, unpublished), who used a modified version of the GALPROP code (Moskalenko & Strong 1998, Moskalenko et al. 2003) to solve the Fokker-Planck diffusion-convection equation (e.g., Lerche & Schlickeiser 1982) in 3D with given source distribution and boundary conditions for electrons and protons. This numerical treatment was implemented to predict the high-energy spectra of the two nearby galaxies M 82 (Persic, Rephaeli, & Arieli 2008, hereafter PRA) and NGC 253 (Rephaeli, Arieli, & Persic 2010, hereafter RAP). The predictions made in these papers agree well with observations made with Fermi and TeV arrays, as will be discussed in the next section.
Particle acceleration and propagation in galactic environments are largely similar in all SFGs. What mainly distinguishes a SBG from a normal SFG is the dominance of a relatively small central region of intense star formation activity. The overall validity of the numerical treatments of the two nearby SBGs provides a solid basis for generalizing the model to SFGs in general.
We briefly review the calculation of steady-state particle spectra and their predicted radiative spectra for the above two nearby SBGs, and discuss similar calculations for conditions in a SFG. The particle energy density can be determined in several different ways. In order to assess and gauge the SN-energetic particle connection we compare estimates of the energetic proton (which dominate the) energy density in SFGs by three different methods, finding overall agreement, which provides further evidence for the validity of the basic approach.