  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 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.

Cosmology and Nongalactic Astrophysics