Crucial Factors for Ly{\alpha} Transmission in the Reionizing
  Intergalactic Medium: Infall Motion, HII Bubble Size, and Self-shielded
  Systems
  Using the CoDa II simulation, we study the Ly$\alpha$ transmissivity of the
intergalactic medium (IGM) during reionization. At $z>6$, a typical galaxy
without an active galactic nucleus fails to form a proximity zone around itself
due to the overdensity of the surrounding IGM. The gravitational infall motion
in the IGM makes the resonance absorption extend to the red side of Ly$\alpha$,
suppressing the transmission up to roughly the circular velocity of the galaxy.
In some sight lines, an optically thin blob generated by a supernova in a
neighboring galaxy results in a peak feature, which can be mistaken for a blue
peak. Redward of the resonance absorption, the damping-wing opacity correlates
with the global IGM neutral fraction and the UV magnitude of the source galaxy.
Brighter galaxies tend to suffer lower opacity because they tend to reside in
larger HII regions, and the surrounding IGM transmits redder photons, which are
less susceptible to attenuation, owing to stronger infall velocity. The HII
regions are highly nonspherical, causing both sight-line-to-sight-line and
galaxy-to-galaxy variation in opacity. Also, self-shielded systems within HII
regions strongly attenuate the emission for certain sight lines. All these
factors add to the transmissivity variation, requiring a large sample size to
constrain the average transmission. The variation is largest for fainter
galaxies at higher redshift. The 68\% range of the transmissivity is similar to
or greater than the median for galaxies with $M_{\rm UV}\ge-21$ at $z\ge7$,
implying that more than a hundred galaxies would be needed to measure the
transmission to 10\% accuracy.
