  We report on the discovery of extended Ly-alpha nebulae at z~3.3 in the
Hubble Ultra Deep Field (HUDF, ~ 40 kpc X 80 kpc) and behind the Hubble
Frontier Fields galaxy cluster MACSJ0416 (~ 40kpc), 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 two 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
[OIII]4959-5007+Hbeta with equivalent widths as high as 1500A 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.
