New constraints on Lyman-α opacity with a sample of 62 quasars at z > 5.7

Abstract

We present measurements of the mean and scatter of the IGM Lyman-{\\alpha}\nopacity at 4.9 < z < 6.1 along the lines of sight of 62 quasars at z > 5.7, the\nlargest sample assembled at these redshifts to date by a factor of two. The\nsample size enables us to sample cosmic variance at these redshifts more\nrobustly than ever before. The spectra used here were obtained by the SDSS,\nDES-VHS and SHELLQs collaborations, drawn from the ESI and X-Shooter archives,\nreused from previous studies or observed specifically for this work. We measure\nthe effective optical depth of Lyman-{\\alpha} in bins of 10, 30, 50 and 70 cMpc\nh-1, construct cumulative distribution functions under two treatments of upper\nlimits on flux and explore an empirical analytic fit to residual Lyman-{\\alpha}\ntransmission. We verify the consistency of our results with those of previous\nstudies via bootstrap re-sampling and confirm the existence of tails towards\nhigh values in the opacity distributions, which may persist down to z = 5.2.\nComparing our results with predictions from cosmological simulations, we find\nfurther strong evidence against models that include a spatially uniform\nionizing background and temperature-density relation. We also compare to IGM\nmodels that include either a fluctuating UVB dominated by rare quasars or\ntemperature fluctuations due to patchy reionization. Although both models\nproduce better agreement with the observations, neither fully captures the\nobserved scatter in IGM opacity. Our sample of 62 z > 5.7 quasar spectra opens\nmany avenues for future study of the reionisation epoch.\n