Lyman-alpha haloes in the aftermath of reionisation

We present a comparative study of łya haloes (LAHs) around low-luminosity (L_ Ly α łesssim 10^ 42 ,erg,s-1) Lyα-emitting galaxies (LAEs) at very high redshifts z and a reference sample at z∼ 3 covering a similar łya luminosity and host galaxy stellar mass range. Using data from the Multi-Unit Spectroscopic Explorer (MUSE) at the ESO Very Large Telescope, we extracted the samples such that at the different redshifts we obtained the same intrinsic surface brightness sensitivity while accounting for cosmological dimming. We detected extended łya emission around six out of 18 high-z LAEs in the MUSE eXtremely Deep Field (MXDF), more than doubling the known number of these objects at z We obtained only a slightly higher individual LAH detection fraction of 40% among the lower redshift comparison sample. Yet the typical exponential scale lengths at z are three times smaller than those at z Stacking the LAEs with undetected haloes also gives drastically different results for the two samples, with a highly significant halo detection at z∼ 3 but no trace of extended łya emission at z We also find the łya spectral line widths of the high-z sample to be $∼1.7 times 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 łya radiation, leaving only emission from the vicinity of the star-forming regions detectable. In an alternative scenario, the LAH properties might be linked more closely to the evolution of their host galaxies than previously thought.