The physics of Lyman a escape from high-redshift galaxies

Lyman a (Lya) photons from ionizing sources and cooling radiation undergo a complex resonant scattering process that generates unique spectral signatures in high-redshift galaxies. We present a detailed Lya radiative transfer study of a cosmological zoom-in simulation from the Feedback In Realistic Environments (FIRE) project. We focus on the time, spatial, and angular properties of the Lya emission over a redshift range of z = 5-7, after escaping the galaxy and being transmitted through the intergalactic medium (IGM). Over this epoch, our target galaxy has an average stellar mass of M∗ 5 × 108 M∗. We find that many of the interesting features of the Lya line can be understood in terms of the galaxy's star formation history. The time variability, spatial morphology, and anisotropy of Lya properties are consistent with current observations. For example, the rest-frame equivalent width has an EWLya,0 > 20 Å duty cycle of 62 per cent with a non-negligible number of sightlines with > 100 Å, associated with outflowing regions of a starburst with greater coincident UV continuum absorption, as these conditions generate redder, narrower (or single-peaked) line profiles. The lowest equivalent widths correspond to cosmological filaments, which have little impact on UV continuum photons but efficiently trap Lya and produce bluer, broader lines with less transmission through the IGM. We also show that in dense self-shielding, low-metallicity filaments and satellites, Lya radiation pressure can be dynamically important. Finally, despite a significant reduction in surface brightness with increasing redshift, Lya detections and spectroscopy of high-z galaxies with the upcoming James Webb Space Telescope is feasible.