  With the Multi Unit Spectroscopic Explorer (MUSE), it is now possible to
detect spatially extended Lyman alpha emission from individual faint (M_UV ~
-18) galaxies at redshifts, 3 < z < 6, tracing gas out to circum-galactic
scales comparable to the dark matter halo virial radius. To explore the
implications of such observations, we present a cosmological radiation
hydrodynamics simulation of a single galaxy, chosen to be typical of the Lyman
alpha-emitting galaxies detected by MUSE in deep fields. We use this simulation
to 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 of
material infalling for the first time towards the halo center, but with the
inner CGM also containing a comparable amount of mass that has moved past
first-pericentric passage, and is in the process of settling into a
rotationally supported configuration. Making the connection to Lyman alpha
emission, we find that the observed extended surface brightness profile is due
to a combination of three components: scattering of galactic Lyman alpha
emission in the CGM, in-situ emission of CGM gas (mostly infalling), and Lyman
alpha emission from small satellite galaxies. The weight of these contributions
vary with distance from the galaxy such that (1) scattering dominates the inner
regions (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 (or
SB ~10^-20). Our simulation fails to reproduce the characteristic observed
Lyman alpha spectral morphology that is red-shifted with respect to the
systemic velocity, with the implication that the simulation is missing an
important component of neutral outflowing gas.
