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& Rephaeli (2007, unpublished), who used a modified version of the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.