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ABSTRACT Rapid mass assembly, likely from mergers or smooth accretion, has been predicted to play a vital role in star formation in high-redshift Lyα emitters. Here, we predict the major merger, minor merger and smooth accreting Lyα emitter fraction from z ≈ 3 to 7 using a large dark matter simulation and a simple physical model that is successful in reproducing many observations over this large redshift range. The central tenet of this model, different from many of the earlier models, is that the star formation in Lyα emitters is proportional to the mass accretion rate rather than the total halo mass. We find that at z ≈ 3 nearly 35 per cent of the Lyα emitters accrete their mass through major (3:1) mergers, and this fraction increases to about 50 per cent at z ≈ 7. This implies that the star formation in a large fraction of high-redshift Lyα emitters is driven by mergers. Comparing our predictions with observations, we find that the model-predicted fractions are relatively larger. While there is discrepancy between the model predictions and observed merger fractions, some of this difference (˜15 per cent) can be attributed to the mass-ratio used to define a merger in the simulation. We predict that future, deeper observations which use a 3:1 definition of major mergers will find ≥30 per cent major merger fraction of Lyα emitters at redshifts ≥3. [ABSTRACT FROM AUTHOR]

The observed deficit of strongly Lyα emitting galaxies at z > 6.5 is attributed to increasing neutral hydrogen in the intergalactic medium (IGM) and/or to the evolving galaxy properties. To investigate this we have performed very deep near-IR spectroscopy of z ≳ 7 galaxies using MOSFIRE on the Keck-I Telescope. We measure the Lyα fraction at z ∼ 8 using two methods. First, we derived NLyα/Ntot directly, using extensive simulations to correct for incompleteness. Second, we used a Bayesian formalism (introduced by Treu et al.) that compares the z > 7 galaxy spectra to models of the Lyα equivalent width (WLyα) distribution at z ∼ 6. We explored two simple evolutionary scenarios: pure number evolution where Lyα is blocked in some fraction of galaxies (perhaps due to the IGM being opaque along only some fraction of sightlines) and uniform dimming evolution where Lyα is attenuated in all galaxies by a constant factor (perhaps owing to processes from galaxy evolution or a slowly increasing IGM opacity). The Bayesian formalism places stronger constraints compared with the direct method. Combining our data with that in the literature, we find that at z ∼ 8 the Lyα fraction has dropped by a factor of >3 (84% confidence interval) using both the dimming and number evolution scenarios, compared to the z ∼ 6 values. Furthermore, we find a tentative positive Bayesian evidence favoring the number evolution scenario over dimming evolution, extending trends observed at z ≲ 7 to higher redshift. A comparison of our results with theoretical models implies the IGM volume averaged neutral hydrogen fraction ≳ 0.3, suggesting that we are likely witnessing reionization in progress at z ∼ 8. [ABSTRACT FROM AUTHOR]