G.A.S. II: Dust extinction in galaxies; Luminosity functions and
  InfraRed eXcess
  Dust is a crucial component of the interstellar medium of galaxies. The
presence of dust strongly affects the light produced by stars within a galaxy.
As these photons are our main information vector to explore the stellar mass
assembly and therefore understand a galaxy's evolution, modeling the luminous
properties of galaxies and taking into account the impact of the dust is a
fundamental challenge for semi-analytical models.We present the complete
prescription of dust attenuation implemented in the new semi-analytical model:
G.A.S. This model is based on a two-phase medium originating from a physically
motivated turbulent model of gas structuring (G.A.S. I paper). Dust impact is
treated by taking into account three dust components: Polycyclic Aromatic
Hydrocarbons, Very Small Grains, and Big Grains. All three components evolve in
both a diffuse and a fragmented/dense gas phase. Each phase has its own stars,
dust content and geometry. Dust content evolves according to the metallicity of
it associated phase.The G.A.S. model is used to predict both the UV and the IR
luminosity functions from $z=9.0$ to $z=0.1$. Our two-phase ISM prescription
catches very well the evolution of UV and IR luminosity functions. We note a
small overproduction of the IR luminosity at low redshift ($z<0.5$). We also
focus on the Infrared-Excess (IRX) and explore its dependency with the stellar
mass, UV slope, stellar age, metallicity and slope of the attenuation curves.
Our model predicts large scatters for relations based on IRX, especially for
the IRX-$\beta$ relation. Our analysis reveals that the slope of the
attenuation curve is more driven by absolute attenuation in the FUV band than
by disk inclination. We confirm that the age of the stellar population and the
slope of the attenuation curve can both shift galaxies below the fiducial
star-birth relation in the IRX-$\beta$ diagram.
