Implications of the Lyα Emission Line from a Candidate z = 10 Galaxy

The z = 10 galaxy recently discovered by Pello and coworkers has a strong Lyα emission line that does not appear to have the expected asymmetry with more transmission on the red side. The blue wing of a Lyα line originating at high redshift should be strongly suppressed by resonant hydrogen absorption along the line of sight, an expectation borne out by the observed asymmetric shapes of the existing sample of Lyα-emitting sources at lower redshifts (3 < z < 6.7). The observed line is inconsistent with the galaxy being embedded in a fully neutral intergalactic medium (IGM) and having no receding peculiar velocity relative to the surrounding absorbing gas at the 95.0%-98.8% confidence level. Absorption on the blue side of the line of the Pello et al. source could be reduced if the IGM in the vicinity of the galaxy is highly ionized, but we show that this requires an unrealistically high ionizing emissivity. We suggest instead that the Lyα-emitting gas is receding relative to the surrounding gas with a velocity of ≳35 km s-1, which reduces the inconsistency confidence level to less than 76.0%-94.5%. We find that with this velocity shift, the observed strength and shape of the line is more consistent with the galaxy being surrounded by its own Strömgren sphere embedded in a fully neutral IGM. More generally, we predict that at any given redshift, the bright Lyα emitters with broader lines would exhibit stronger asymmetry than fainter ones. Bright galaxies with symmetric Lyα lines may be signposts for groups and clusters of galaxies, within which they can acquire random velocities comparable to or larger than their line widths.