Context :

Aims : This paper is the first of a series investigating Lyman-alpha ( hereafter Ly \alpha ) radiation transfer through hydrodynamical simulations of galaxy formation .
Its aim is to assess the impact of the interstellar medium ( ISM ) physics on Ly \alpha radiation transfer and to quantify how galaxy orientation with respect to the line of sight alters observational signatures .

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 \sim 10 ^ { 10 } M _ { \odot } .
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 , and hence smooth , ISM of G1 yields an escape fraction of \sim 50 % at the Ly \alpha 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 \sim 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 of \sim - 5 Å to a 15 times more luminous red-shifted asymmetric line with EW \sim 90 Å .

Conclusions : The remarkable discrepancy in the Ly \alpha properties we derived for two ISM models raises a fundamental issue .
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 out of 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 these surveys could indeed miss a significant fraction of the high- z galaxy population .