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With the Multi Unit Spectroscopic Explorer (MUSE), it is now possible todetect spatially extended Lyman alpha emission from individual faint (M_UV ~-18) galaxies at redshifts, 3 < z < 6, tracing gas out to circum-galacticscales comparable to the dark matter halo virial radius. To explore theimplications of such observations, we present a cosmological radiationhydrodynamics simulation of a single galaxy, chosen to be typical of the Lymanalpha-emitting galaxies detected by MUSE in deep fields. We use this simulationto study the origin and dynamics of the high-redshift circum-galactic medium(CGM). We find that the majority of the mass in the diffuse CGM is comprised ofmaterial infalling for the first time towards the halo center, but with theinner CGM also containing a comparable amount of mass that has moved pastfirst-pericentric passage, and is in the process of settling into arotationally supported configuration. Making the connection to Lyman alphaemission, we find that the observed extended surface brightness profile is dueto a combination of three components: scattering of galactic Lyman alphaemission in the CGM, in-situ emission of CGM gas (mostly infalling), and Lymanalpha emission from small satellite galaxies. The weight of these contributionsvary with distance from the galaxy such that (1) scattering dominates the innerregions (r < 7 kpc), at surface brightness larger than a few 10^-19 cgs, (2)all components contribute equally around r ~ 10 kpc (or SB ~10^-19), and (3)the contribution of small satellite galaxies takes over at large distances (orSB ~10^-20). Our simulation fails to reproduce the characteristic observedLyman alpha spectral morphology that is red-shifted with respect to thesystemic velocity, with the implication that the simulation is missing animportant component of neutral outflowing gas.

Tracing the simulated high-redshift circum-galactic medium with Lyman alpha emission