Diffuse Lyα haloes around galaxies at z=2.2-6.6: implications for galaxy formation and cosmic reionization

We present diffuse Ly alpha haloes (LAHs) identified in the composite Subaru narrow-band 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 artefacts mimicking LAHs that include a large-scale point-spread function made by instrumental and atmospheric effects. Based on our critical test with composite images of non-LAE samples whose narrow-band-magnitude and source-size distributions are the same as our LAE samples, we confirm that no artefacts can produce a diffuse extended feature similar to our LAHs. After this test, we measure the scalelengths of exponential profile for the LAHs estimated from our z = 2.2-6.6 LAE samples of L-Ly alpha greater than or similar to 2 x 10(42) erg s(-1). We obtain the scalelengths of similar or equal to 5-10 kpc at z = 2.2-5.7, and find no evolution of scalelengths 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 scalelength of our z=6.6 LAH is larger than 5-10 kpc just beyond the error bar, which is a hint that the scalelengths 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 scalelength change at z greater than or similar to 6 would be a signature of increasing fraction of neutral hydrogen scattering Ly alpha photons, due to cosmic reionization.