Inferences on the distribution of Ly-alpha emission of z 7 and z 8 galaxies

الأصل · EN

Spectroscopic confirmation of galaxies at z 7 and above has been extremely difficult, owing to a drop in intensity of Ly-alpha emission in comparison with samples at z 6. This crucial finding could potentially signal the ending of cosmic reionization. However it is based on small datasets, often incomplete and heterogeneous in nature. We introduce a flexible Bayesian framework, useful to interpret such evidence. Within this framework, we implement two simple phenomenological models: a smooth one, where the distribution of Ly-alpha is attenuated by a factor with respect to z 6; a patchy one where a fraction is absorbed/non-emitted while the rest is unabsorbed. From a compilation of 39 observed z 7 galaxies we find =0.69+-0.12 and =0.66+-0.16. The models can be used to compute fractions of emitters above any equivalent width W. For W>25Å, we find X²⁵z₌₇=0.37+-0.11 (0.14+-0.06) for galaxies fainter (brighter) than MUV=-20.25 for the patchy model, consistent with previous work, but with smaller uncertainties by virtue of our full use of the data. At z 8 we combine new deep (5-σflux limit 10⁻¹⁷ergs⁻¹cm⁻²) Keck-NIRSPEC observations of a bright Y-dropout identified by our BoRG Survey, with those of three objects from the literature and find that the inference is inconclusive. We compute predictions for future near-infrared spectroscopic surveys and show that it is challenging but feasible to constrain the distribution of Ly-alpha emitters at z 8 and distinguish between models.