We present diffuse Lyman-alpha halos (LAHs) identified in the composite Subaru narrowband images of 100-3600 Lyman-alpha emitters (LAEs) at z=2.2, 3.1, 3.7, 5.7, and 6.6. First, we carefully examine potential artifacts mimicking LAHs that include a l
arge-scale point-spread function (PSF) made by instrumental and atmospheric effects. Based on our critical test with composite images of non-LAE samples whose narrowband-magnitude and source-size distributions are the same as our LAE samples, we confirm that no artifacts can produce a diffuse extended feature similar to our LAHs. After this test, we measure the scale lengths of exponential profile for the LAHs estimated from our z=2.2-6.6 LAE samples of L(Lyman-alpha) > 2 x 10^42 erg s^-1. We obtain the scale lengths of ~ 5-10 kpc at z=2.2-5.7, and find no evolution of scale lengths in this redshift range beyond our measurement uncertainties. Combining this result and the previously-known UV-continuum size evolution, we infer that the ratio of LAH to UV-continuum sizes is nearly constant at z=2.2-5.7. The scale length of our z=6.6 LAH is larger than 5-10 kpc just beyond the error bar, which is a hint that the scale lengths of LAHs would increase from z=5.7 to 6.6. If this increase is confirmed by future large surveys with significant improvements of statistical and systematical errors, this scale length change at z > 6 would be a signature of increasing fraction of neutral hydrogen scattering Lyman-alpha photons, due to cosmic reionization.
We present a statistical study of velocities of Lya, interstellar (IS) absorption, and nebular lines and gas covering fraction for Lya emitters (LAEs) at z~2. We make a sample of 22 LAEs with a large Lya equivalent width (EW) of > 50A based on our de
ep Keck/LRIS observations, in conjunction with spectroscopic data from the Subaru/FMOS program and the literature. We estimate the average velocity offset of Lya from a systemic redshift determined with nebular lines to be dv_Lya=234+-9 km s-1. Using a Kolmogorv-Smirnov test, we confirm the previous claim of Hashimoto et al. (2013) that the average dv_Lya of LAEs is smaller than that of LBGs. Our LRIS data successfully identify blue-shifted multiple IS absorption lines in the UV continua of four LAEs on an individual basis. The average velocity offset of IS absorption lines from a systemic redshift is dv_IS=204+-27 km s-1, indicating LAEs gas outflow with a velocity comparable to typical LBGs. Thus, the ratio, R^Lya_ IS = dv_Lya/dv_IS of LAEs, is around unity, suggestive of low impacts on Lya transmission by resonant scattering of neutral hydrogen in the IS medium. We find an anti-correlation between Lya EW and the covering fraction, f_c, estimated from the depth of absorption lines, where f_c is an indicator of average neutral hydrogen column density, N_HI. The results of our study support the idea that N_HI is a key quantity determining Lya emissivity.
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 L
AEs 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.
