The CGM serves as the interface between galaxies and the larger structures within which they evolve. Thus, studying the environment of galaxies represents a crucial step in understanding the mechanisms governing their formation and evolution. Unfortunately, direct observation of the CGM is often quite difficult, since these regions are very faint. Using the unrivaled sensitivity of the MUSE/VLT instrument, we reported — in Leclercq et al. 2017 — the detection of cold hydrogen gas surrounding 145 low-mass (<1010 M⊙), faint (-15 ≥ MUV ≥ -22) and very distant (3 < z < 6) galaxies emitting Lya photons (forming 80% of the total galaxy sample tested). This sample represents the largest collection ever compiled of individually-detected Lya halos around normal star-forming galaxies observed in an epoch when the Universe was still forming. Our results (i) reinforce the idea that Lya haloes are ubiquitous around high-z galaxies, (ii) reveal a large diversity in the Lya halo properties which (iii) do not show strong and significant correlations with the UV properties of the host galaxies.

In this talk, I will present the main results from this Lya haloes study and introduce our new analysis of the Lya haloes spectral properties (Leclercq et al. in prep). In this work, we look at the evolution of the Lya line profile within the halo in order to better understand the information encoded in the shape of the Lya line. This information can help shed light on the origin of the Lya emission and therefore constrain the gas opacity and kinematics.