Lyman-alpha haloes in the aftermath of reionisation

We present a comparative study of Ly alpha haloes (LAHs) around low-luminosity (L Ly alpha 10 42 erg s-1) Ly alpha-emitting galaxies (LAEs) at very high redshifts z at least6 and a reference sample at z about 3 covering a similar Ly alpha luminosity and host galaxy stellar mass range. Using data from the Multi-Unit Spectroscopic Explorer (MUSE) at the ESO VLT, we extracted the samples such that at the different redshifts we obtain the same intrinsic surface brightness sensitivity, accounting for cosmological dimming. We detect extended Ly alpha emission around 6 out of 18 high- z LAEs in the MUSE eXtremely Deep Field (MXDF), more than doubling the number of known such objects at z at least6. We obtain an only slightly higher individual LAH detection fraction of 40% among the lower redshift comparison sample. Yet the typical exponential scale lengths at z at least6 are three times smaller than those at z about3. Stacking the LAEs with undetected haloes gives again drastically different results for the two samples, with a highly significant halo detection at z about 3 but no trace of extended Ly alpha emission at z at least6. We also find the Ly alpha spectral line widths of the high- z sample to be about 2.5 smaller in comparison to the lower redshift objects. We discuss the potential mechanisms driving such strong changes. In a reionisation-driven scenario the higher neutral fraction in the intergalactic and circumgalactic media might lead to substantial scattering losses of escaping Ly alpha radiation, leaving detectable only emission from the vicinity of the star-forming regions. In an alternative scenario the LAH properties might be linked more closely to the evolution of their host galaxies than previously thought.