We study the projected spatial offset between the ultraviolet continuum and Ly \alpha emission for a sample of 65 lensed and unlensed galaxies in the Epoch of Reionization ( 5 \leq z \leq 7 ) , the first such study at these redshifts , in order to understand the potential for these offsets to confuse estimates of the Ly \alpha properties of galaxies observed in slit spectroscopy .
While we find that \sim 40 % of galaxies in our sample show significant projected spatial offsets ( | \Delta _ { Ly \alpha - UV } | ) , we find a relatively modest average projected offset of | \widetilde { \Delta } _ { Ly \alpha - UV } | = 0.61 \pm 0.08 proper kpc for the entire sample .
A small fraction of our sample , \sim 10 % , exhibit offsets in excess of 2 proper kpc , with offsets seen up to \sim 4 proper kpc , sizes that are considerably larger than the effective radii of typical galaxies at these redshifts .
An internal comparison and a comparison to a variety of other studies at lower redshift yielded no significant evidence of evolution of | \Delta _ { Ly \alpha - UV } | with redshift .
In our own sample , UV-bright galaxies ( \widetilde { L _ { UV } } / L ^ { \ast } _ { UV } = 0.67 ) showed offsets a factor of three greater than their fainter counterparts ( \widetilde { L _ { UV } } / L ^ { \ast } _ { UV } = 0.10 ) , 0.89 \pm 0.18 vs. 0.27 \pm 0.05 proper kpc , respectively .
We argue that offsets are likely not the result of merging processes , but are rather due to anisotropic processes internal to the galaxies as a result of stellar feedback , which is stronger in UV-brighter galaxies , and which facilitates Ly \alpha fluorescence and/or backscattering from nearby or outflowing gas .
The reduction in the Ly \alpha flux due to offset effects for various observational setups was quantified through mock observations of simple simulations .
It was found that the differential loss of Ly \alpha photons for galaxies with average offsets is not , if corrected for , a limiting factor for all but the narrowest slit widths ( < 0.4 \arcsec ) .
However , for the largest offsets , if such offsets are mostly perpendicular to the slit major axis , slit losses were found to be extremely severe in cases where slit widths of \leq 1 \arcsec were employed , such as those planned for James Webb Space Telescope /NIRSpec observations .