Lyman-alpha emission properties of simulated galaxies: interstellar
  medium structure and inclination effects
  [abridged] Aims. The aim of this paper is to assess the impact of the
interstellar medium (ISM) physics on Lyman-alpha (Lya) radiation transfer and
to quantify how galaxy orientation with respect to the line of sight alters
observational signatures. Methods. We compare the results of Lya radiation
transfer calculations through the ISM of a couple of idealized galaxy
simulations with different ISM models. Results. First, the small-scale
structuration of the ISM plays a determinant role in shaping a galaxys Lya
properties.The artificially warm, and hence smooth, ISM of G1 yields an escape
fraction of 50 percent at the Lya line center, and produces symmetrical
double-peak profiles. On the contrary, in G2, most young stars are embedded in
thick star-forming clouds, and the result is a 10 times lower escape fraction.
G2 also displays a stronger outflowing velocity field, which favors the escape
of red-shifted photons, resulting in an asymmetric Lya line. Second, the Lya
properties of G2 strongly depend on the inclination at which it is observed:
From edge-on to face-on, the line goes from a double-peak profile with an
equivalent width of -5 Angstrom to a 15 times more luminous red-shifted
asymmetric line with EW 90 Angstrom. Conclusions. Lya radiation transfer
calculations can only lead to realistic properties in simulations where
galaxies are resolved into giant molecular clouds, putting these calculations
out of reach of current large scale cosmological simulations. Finally, we find
inclination effects to be much stronger for Lya photons than for continuum
radiation. This could potentially introduce severe biases in the selection
function of narrow-band Lya emitter surveys, which could indeed miss a
significant fraction of the high-z galaxy population.
