  Fermi/LAT observations of star-forming galaxies in the ~0.1-100GeV range have
made possible a first population study. Evidence was found for a correlation
between gamma-ray luminosity and tracers of the star formation activity.
Studying galactic cosmic rays (CRs) in various global conditions can yield
information about their origin and transport in the interstellar medium (ISM).
This work addresses the question of the scaling laws that can be expected for
the interstellar gamma-ray emission as a function of global galactic
properties, with the goal of establishing whether the current experimental data
in the GeV range can be constraining. I developed a 2D model for the
non-thermal emissions from steady-state CR populations interacting with the ISM
in star-forming galaxies. Most CR-related parameters were taken from Milky Way
studies, and a large number of galaxies were then simulated with sizes from 4
to 40kpc, several gas distributions, and star formation rates (SFR) covering
six orders of magnitude. The evolution of the gamma-ray luminosity over the
100keV-100TeV range is presented, with emphasis on the contribution of the
different emission processes and particle populations, and on the transition
between transport regimes. The model can reproduce the normalisation and trend
inferred from the Fermi/LAT population study over most of the SFR range. This
is obtained with a plain diffusion scheme, a single diffusion coefficient, and
the assumption that CRs experience large-scale volume-averaged interstellar
conditions. There is, however, no universal relation between high-energy
gamma-ray luminosity and star formation activity, as illustrated by the scatter
introduced by different galactic global properties and the downturn in
gamma-ray emission at the low end (abridged).
