We constrain the distribution of spatially offset Lyman-alpha emission ( Ly \alpha ) relative to rest-frame ultraviolet emission in \sim 300 high redshift ( 3 < z < 5.5 ) Lyman-break galaxies ( LBGs ) exhibiting Ly \alpha emission from VANDELS , a VLT/VIMOS slit-spectroscopic survey of the CANDELS Ultra Deep Survey and Chandra Deep Field South fields ( { \simeq 0.2 } ~ { } \mathrm { deg } ^ { 2 } total ) .
Because slit spectroscopy compresses two-dimensional spatial information into one spatial dimension , we use Bayesian inference to recover the underlying Ly \alpha spatial offset distribution .
We model the distribution using a 2D circular Gaussian , defined by a single parameter \sigma _ { r, \mathrm { Ly } \alpha } , the standard deviation expressed in polar coordinates .
Over the entire redshift range of our sample ( 3 < z < 5.5 ) , we find \sigma _ { r, \mathrm { Ly } \alpha } = 1.70 ^ { +0.09 } _ { -0.08 } kpc ( 68 \% conf .
) , corresponding to \sim 0 \farcs 25 at \langle z \rangle = 4.5 .
We also find that \sigma _ { r, \mathrm { Ly } \alpha } decreases significantly with redshift .
Because Ly \alpha spatial offsets can cause slit-losses , the decrease in \sigma _ { r, \mathrm { Ly } \alpha } with redshift can partially explain the increase in the fraction of Ly \alpha emitters observed in the literature over this same interval , although uncertainties are still too large to reach a strong conclusion .
If \sigma _ { r, \mathrm { Ly } \alpha } continues to decrease into the reionization epoch , then the decrease in Ly \alpha transmission from galaxies observed during this epoch might require an even higher neutral hydrogen fraction than what is currently inferred .
Conversely , if spatial offsets increase with the increasing opacity of the IGM , slit losses may explain some of the drop in Ly \alpha transmission observed at z > 6 .
Spatially resolved observations of Ly \alpha and UV continuum at 6 < z < 8 are needed to settle the issue .