Abstract
We present diffuse Ly$\alpha$ halos (LAHs) identified in the composite Subaru
narrowband images of $100-3600 $ Ly$\alpha$ 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 $L_Ly\alpha210^42$ 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 $z=2.2-5.7$. On the other hand, the scale length of
our $z=6.6$ LAH is larger than $5-10$ kpc, and there is a hint that the scale
lengths of LAHs would increase from $z=5.7$ to $6.6$. This may be a signature
of increasing fraction of neutral hydrogen scattering Ly\alpha$ photons, due to
cosmic reionization, although larger data are needed for a definitive
conclusion.
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