The MUSE Hubble Ultra Deep Field Survey VIII : Extended Lyman-alpha
  haloes around high-redshift star-forming galaxies
  We report the detection of extended Lyman-alpha (Lya) haloes around 145
individual star-forming galaxies at redshifts 3<z<6 in the Hubble Ultra Deep
Field observed with the Multi-Unit Spectroscopic Explorer at ESO-VLT. Our
sample consists of continuum-faint (-15> M_{UV}> -22) Lya emitters (LAEs).
Using a 2D, two-component decomposition of Lya emission assuming circular
exponential distributions, we measure scale lengths and luminosities of Lya
haloes. We find that 80% of our objects having reliable Lya halo measurements
show Lya emission that is significantly more extended than the UV continuum
detected by HST (by a factor ~4 to >20). The median exponential scale length of
the Lya haloes in our sample is ~4.5 kpc. By comparing the maximal detected
extent of the Lya emission with the predicted dark matter halo virial radii of
simulated galaxies, we show that the detected Lya emission of our selected
sample 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 be
significant HI reservoirs in the CGM and reinforces the idea that Lya haloes
are ubiquitous around high-redshift Lya emitting galaxies. Our characterization
of the Lya haloes indicates that the majority of the Lya flux comes from the
halo (~65%) and that their scale lengths seem to be linked to the UV properties
of the galaxies. We do not observe a significant Lya halo size evolution with
redshift. We also find that the Lya lines cover a large range of full width at
half maximum (FWHM) from 118 to 512 km/s. While the FWHM does not seem to be
correlated to the Lya scale length, most compact Lya haloes and those that are
not detected with high significance tend to have narrower Lya profiles.
Finally, we investigate the origin of the extended Lya emission but we conclude
that our data do not allow us to disentangle the possible processes.
