We report on the discovery of extended Ly \alpha nebulae at z \simeq 3.3 in the Hubble Ultra Deep Field ( HUDF , \simeq 40 kpc \times 80 kpc ) and behind the Hubble Frontier Field galaxy cluster MACSJ0416 ( \simeq 40 kpc ) , spatially associated with groups of star-forming galaxies .
VLT/MUSE integral field spectroscopy reveals a complex structure with a spatially-varying double peaked Ly \alpha emission .
Overall , the spectral profiles of the two Ly \alpha nebulae are remarkably similar , both showing a prominent blue emission , more intense and slightly broader than the red peak .
From the first nebula , located in the HUDF , no X-ray emission has been detected , disfavoring the possible presence of AGNs .
Spectroscopic redshifts have been derived for 11 galaxies within 2 \arcsec from the nebula and spanning the redshift range 1.037 < z < 5.97 .
The second nebula , behind MACSJ0416 , shows three aligned star-forming galaxies plausibly associated to the emitting gas .
In both systems , the associated galaxies reveal possible intense rest-frame-optical nebular emissions lines [ O iii ] \lambda \lambda 4959 , 5007 +H \beta with equivalent widths as high as 1500Å rest-frame and star formation rates ranging from a few to tens of solar masses per year .
A possible scenario is that of a group of young , star-forming galaxies sources of escaping ionising radiation that induce Ly \alpha fluorescence , therefore revealing the kinematics of the surrounding gas .
Also Ly \alpha powered by star-formation and/or cooling radiation may resemble the double peaked spectral properties and the morphology observed here .
If the intense blue emission is associated with inflowing gas , then we may be witnessing an early phase of galaxy or a proto-cluster ( or group ) formation .