Spectral variations of Lyman α emission within strongly lensed sources observed with MUSE

We present an analysis of H I Lyman α emission in deep VLT/MUSE observations of two highly magnified and extended galaxies at z=3.5 and 4.03, including a newly discovered, almost complete Einstein ring. While these Lyman α haloes are intrinsically similar to the ones typically seen in other MUSE deep fields, the benefits of gravitational lensing allow us to construct exceptionally detailed maps of Lyman α line properties at sub-kpc scales. By combining all multiple images, we are able to observe complex structures in the Lyman α emission and uncover small (∼120 km s^-1 in Lyman α peak shift), but significant at > 4 σ, systematic variations in the shape of the Lyman α line profile within each halo. Indeed, we observe a global trend for the line peak shift to become redder at large radii, together with a strong correlation between the peak wavelength and line width. This systematic intrahalo variation is markedly similar to the object-to-object variations obtained from the integrated properties of recent large samples. Regions of high surface brightness correspond to relatively small line shifts, which could indicate that Lyman α emission escapes preferentially from regions where the line profile has been less severely affected by scattering of Lyman α photons.