Ly$α$ emission in galaxies at $zsimeq5-6$: new insight from JWST into the statistical distributions of Ly$α$ properties at the end of reionization

Kavli Affiliate: Roberto Maiolino

| First 5 Authors: Mengtao Tang, Daniel P. Stark, Richard S. Ellis, Fengwu Sun, Michael Topping

| Summary:

JWST has recently sparked a new era of Lya spectroscopy, delivering the first
measurements of the Lya escape fraction and velocity profile in typical
galaxies at z~6-10. These observations offer new prospects for insight into the
earliest stages of reionization. But to realize this potential, we need robust
intrinsic models of Lya properties in galaxies at z~5-6 when the IGM is mostly
ionized. Here we use new JWST observations from the JADES and FRESCO surveys to
characterize statistical distributions of Lya velocity offsets, escape
fractions, and EWs in z~5-6 galaxies that will be applicable to growing
datasets at z>6. We find that galaxies with large Lya escape fractions (>0.2)
are common at z~5-6, comprising 30% of Lyman break selected samples. Comparing
to literature studies, our census suggests that Lya becomes more prevalent in
the galaxy population toward higher redshift from z~3 to z~6, although we find
that this evolution slows considerably between z~5 and z~6, consistent with
modest attenuation from residual HI in the mostly ionized IGM at z~5-6. We find
significant evolution in Lya velocity profiles between z~2-3 and z~5-6. At
lower redshifts, the strongest Lya emitters often have line profiles peaking
near the systemic redshift, reflecting escape through low HI density channels.
At z~5-6, the strongest Lya emitters have profiles with flux emerging at
typical redshifted velocities ~230km/s. The rarity of Lya emitters with peak
flux near the systemic redshift at z~5-6 may reflect the influence of resonant
scattering from residual HI in the IGM. This effect will make it challenging to
use Lya peak offsets as a probe of Lyman continuum leakage at z~5-6. We use our
z~5-6 Lya distributions to make predictions for typical Lya properties at z>8
and discuss implications of a recently-discovered Lya emitter at z=8.5 with a
small peak velocity offset (156km/s).

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