What is the Physical Origin of Strong Lya Emission? I. Demographics of
  Lya Emitter Structures
  We present the results of structure analyses for a large sample of 426 Lya
emitters (LAEs) at z~2.2 that are observed with HST/ACS and WFC3-IR by deep
extra-galactic legacy surveys. We confirm that the merger fraction and the
average ellipticity of LAE's stellar component are 10-30 % and 0.4-0.6,
respectively, that are comparable with previous study results. We successfully
identify that some LAEs have a spatial offset between Lya and stellar-continuum
emission peaks, d_Lya, by ~2.5-4 kpc beyond our statistical errors. To uncover
the physical origin of strong Lya emission found in LAEs, we investigate Lya
equivalent width (EW) dependences of these three structural parameters, merger
fraction, d_Lya, and ellipticity of stellar distribution in the range of
EW(Lya)=20-250A. Contrary to expectations, we find that merger fraction does
not significantly increase with Lya EW. We reveal an anti-correlation between
d_Lya and EW(Lya) by Kolmogorov-Smirnov (KS) test. There is a trend that the
LAEs with a large Lya EW have a small ellipticity. This is consistent with the
recent theoretical claims that Lya photons can more easily escape from face-on
disks having a small ellipticity, due to less inter-stellar gas along the line
of sight, although our KS test indicates that this trend is not statistically
significant. Our results of Lya-EW dependence generally support the idea that
an HI column density is a key quantity determining Lya emissivity.
