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

Abstract

We present diffuse Lyman-alpha halos (LAHs) identified in the composite\nSubaru narrowband images of 100-3600 Lyman-alpha emitters (LAEs) at z=2.2, 3.1,\n3.7, 5.7, and 6.6. First, we carefully examine potential artifacts mimicking\nLAHs that include a large-scale point-spread function (PSF) made by\ninstrumental and atmospheric effects. Based on our critical test with composite\nimages of non-LAE samples whose narrowband-magnitude and source-size\ndistributions are the same as our LAE samples, we confirm that no artifacts can\nproduce a diffuse extended feature similar to our LAHs. After this test, we\nmeasure the scale lengths of exponential profile for the LAHs estimated from\nour z=2.2-6.6 LAE samples of L(Lyman-alpha) > 2 x 10^42 erg s^-1. We obtain the\nscale lengths of ~ 5-10 kpc at z=2.2-5.7, and find no evolution of scale\nlengths in this redshift range beyond our measurement uncertainties. Combining\nthis result and the previously-known UV-continuum size evolution, we infer that\nthe ratio of LAH to UV-continuum sizes is nearly constant at z=2.2-5.7. The\nscale length of our z=6.6 LAH is larger than 5-10 kpc just beyond the error\nbar, which is a hint that the scale lengths of LAHs would increase from z=5.7\nto 6.6. If this increase is confirmed by future large surveys with significant\nimprovements of statistical and systematical errors, this scale length change\nat z > 6 would be a signature of increasing fraction of neutral hydrogen\nscattering Lyman-alpha photons, due to cosmic reionization.\n