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We report the detection of extended Lyman-alpha (Lya) haloes around 145individual star-forming galaxies at redshifts 3<z<6 in the Hubble Ultra DeepField observed with the Multi-Unit Spectroscopic Explorer at ESO-VLT. Oursample consists of continuum-faint (-15> M_{UV}> -22) Lya emitters (LAEs).Using a 2D, two-component decomposition of Lya emission assuming circularexponential distributions, we measure scale lengths and luminosities of Lyahaloes. We find that 80% of our objects having reliable Lya halo measurementsshow Lya emission that is significantly more extended than the UV continuumdetected by HST (by a factor ~4 to >20). The median exponential scale length ofthe Lya haloes in our sample is ~4.5 kpc. By comparing the maximal detectedextent of the Lya emission with the predicted dark matter halo virial radii ofsimulated galaxies, we show that the detected Lya emission of our selectedsample of LAEs probes a significant portion of the cold circum-galactic medium(CGM) of these galaxies (>50% in average). This result shows that there must besignificant HI reservoirs in the CGM and reinforces the idea that Lya haloesare ubiquitous around high-redshift Lya emitting galaxies. Our characterizationof the Lya haloes indicates that the majority of the Lya flux comes from thehalo (~65%) and that their scale lengths seem to be linked to the UV propertiesof the galaxies. We do not observe a significant Lya halo size evolution withredshift. We also find that the Lya lines cover a large range of full width athalf maximum (FWHM) from 118 to 512 km/s. While the FWHM does not seem to becorrelated to the Lya scale length, most compact Lya haloes and those that arenot detected with high significance tend to have narrower Lya profiles.Finally, we investigate the origin of the extended Lya emission but we concludethat our data do not allow us to disentangle the possible processes.

The MUSE Hubble Ultra Deep Field Survey VIII : Extended Lyman-alpha haloes around high-redshift star-forming galaxies