Key points are not available for this paper at this time.
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 large-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 10⁴2 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.
Momose et al. (2014) studied this question.