Lyman-α emission properties of simulated galaxies: interstellar medium structure and inclination effects

Aims. This paper is the first in a series investigating Lyman-alpha (hereafter Ly alpha) radiation transfer through hydrodynamical simulations of galaxy formation. Its aim is to assess the impact of interstellar medium (ISM) physics on Ly alpha radiation transfer and to quantify how galaxy orientation alters observational signatures with respect to the line of sight. Methods. We compare the results of Ly alpha radiation transfer calculations through the ISM of a couple of idealized galaxy simulations in a dark matter halo of similar to 10(10) M-circle dot. In the first one, G1, this ISM is modeled using physics typical of large-scale cosmological hydrodynamics simulations of galaxy formation, where gas is prevented from radiatively cooling below 10(4) K. In the second one, G2, gas is allowed to radiate away more of its internal energy via metal lines and consequently fragments into dense star-forming clouds. Results. First, as expected, the small-scale structuration of the ISM plays a determinant role in shaping a galaxy's Ly alpha properties. The artificially warm, hence smooth, ISM of G1 yields an escape fraction of similar to 50% at the Ly alpha line center, and produces symmetrical double-peak profiles. In contrast, in G2, most young stars are embedded in thick star-forming clouds, and the result is a similar to 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 Ly alpha line. Second, the Ly alpha 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 (EW) of similar to-5 angstrom to a 15 times more luminous, red-shifted asymmetric line with EW similar to 90 angstrom. Conclusions. The remarkable discrepancy in the Ly alpha properties we derived for two ISM models raises a fundamental question. In effect, it demonstrates that Ly alpha radiation transfer calculations can only lead to realistic properties in simulations where galaxies are resolved into giant molecular clouds. Such a stringent requirement translates into severe constraints both in terms of ISM physics modeling and numerical resolution, putting these calculations beyond the reach of current large-scale cosmological simulations. Finally, we find inclination effects to be much stronger for Ly alpha photons than for continuum radiation. This could potentially introduce severe biases in the selection function of narrow-band Ly alpha emitter surveys and in their interpretation, and we predict that these surveys could indeed miss a significant fraction of the high-z galaxy population.