The MUSE Hubble Ultra Deep Field Survey XIII. Spatially resolved
  spectral properties of Lyman alpha haloes around star-forming galaxies at z >
  3
  We present spatially resolved maps of six individually-detected Lyman alpha
haloes (LAHs) as well as a first statistical analysis of the Lyman alpha (Lya)
spectral signature in the circum-galactic medium of high-redshift star-forming
galaxies using MUSE. Our resolved spectroscopic analysis of the LAHs reveals
significant intrahalo variations of the Lya line profile. Using a
three-dimensional two-component model for the Lya emission, we measure the full
width at half maximum (FWHM), the peak velocity shift and the asymmetry of the
Lya line in the core and in the halo of 19 galaxies. We find that the Lya line
shape is statistically different in the halo compared to the core for ~40% of
our galaxies. Similarly to object-by-object based studies and a recent resolved
study using lensing, we find a correlation between the peak velocity shift and
the width of the Lya line both at the interstellar and circum-galactic scales.
While there is a lack of correlation between the spectral properties and the
spatial scale lengths of our LAHs, we find a correlation between the width of
the line in the LAH and the halo flux fraction. Interestingly, UV bright
galaxies show broader, more redshifted and less asymmetric Lya lines in their
haloes. The most significant correlation found is for the FWHM of the line and
the UV continuum slope of the galaxy, suggesting that the redder galaxies have
broader Lya lines. The generally broad and red line shapes found in the halo
component suggests that the Lya haloes are powered either by scattering
processes through an outflowing medium, fluorescent emission from outflowing
cold clumps of gas, or a mix of both. Considering the large diversity of the
Lya line profiles observed in our sample and the lack of strong correlation,
the interpretation of our results is still broadly open and underlines the need
for realistic spatially resolved models of the LAHs.
