WHAT IS THE PHYSICAL ORIGIN OF STRONG Lyα EMISSION? I. DEMOGRAPHICS OF Lyα EMITTER STRUCTURES

We present the results of structure analyses for a large sample of 426 Ly alpha emitters (LAEs) at z similar to 2.2 that are observed with the Hubble Space Telescope/Advanced Camera for Surveys and WFC3-IR during 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 Ly alpha and stellar-continuum emission peaks, delta(Ly alpha), by similar to 2.5-4 kpc beyond our statistical errors. To uncover the physical origin of strong Ly alpha emission found in LAEs, we investigate the Ly alpha equivalent width (EW) dependences of three structural parameters: merger fraction, delta(Ly alpha), and ellipticity of stellar distribution in the range of EW(Lya) = 20-250 angstrom. Contrary to expectations, we find that the merger fraction does not significantly increase with Ly alpha EW. We reveal an anti-correlation between delta(Ly alpha) and EW(Ly alpha) using a Kolmogorov-Smirnov (K-S) test. There is a trend that the LAEs with a large Ly alpha EW have a small ellipticity. This is consistent with the recent theoretical claims that Ly alpha 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 K-S test indicates that this trend is not statistically significant. Our results of Ly alpha-EW dependence generally support the idea that an H I column density is a key quantity determining Ly alpha emissivity.