The MUSE Hubble Ultra Deep Field Survey. VIII. Extended Lyman-α haloes around high-z star-forming galaxies

We report the detection of extended Ly α 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 (MUSE) at ESO-VLT. Our sample consists of continuum-faint (− 15 ≥ M UV ≥ −22) Ly α emitters (LAEs). Using a 2D, two-component (continuum-like and halo) decomposition of Ly α emission assuming circular exponential distributions, we measure scale lengths and luminosities of Ly α haloes. We find that 80% of our objects having reliable Ly α halo measurements show Ly α 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 Ly α haloes in our sample is ≈4.5 kpc with a few haloes exceeding 10 kpc. By comparing the maximal detected extent of the Ly α emission with the predicted dark matter halo virial radii of simulated galaxies, we show that the detected Ly α emission of our selected sample of Ly α emitters probes a significant portion of the cold circum-galactic medium of these galaxies (>50% in average). This result therefore shows that there must be significant HI reservoirs in the circum-galactic medium and reinforces the idea that Ly α haloes are ubiquitous around high-redshift Ly α emitting galaxies. Our characterization of the Ly α haloes indicates that the majority of the Ly α flux comes from the halo (≈65%) and that their scale lengths seem to be linked to the UV properties of the galaxies (sizes and magnitudes). We do not observe a significant Ly α halo size evolution with redshift, although our sample for z > 5 is very small. We also explore the diversity of the Ly α line profiles in our sample and we find that the Ly α lines cover a large range of full width at half maximum (FWHM) from 118 to 512 km s -1 . While the FWHM does not seem to be correlated to the Ly α scale length, most compact Ly α haloes and those that are not detected with high significance tend to have narrower Ly α profiles ( -1 ). Finally, we investigate the origin of the extended Ly α emission but we conclude that our data do not allow us to disentangle the possible processes, i.e. scattering from star-forming regions, fluorescence, cooling radiation from cold gas accretion, and emission from satellite galaxies.