Illuminating gas inflows/outflows in the MUSE deepest fields: Ly alpha nebulae around forming galaxies at z similar or equal to 3.3

We report the identification of extended Ly alpha nebulae at z similar or equal to 3.3 in the Hubble Ultra Deep Field (HUDF, similar or equal to 40 kpc 80 kpc) and behind the Hubble Frontier Field galaxy cluster MACSJ0416 (similar or equal to 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, disfavouring the possible presence of active galactic nuclei. 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 with the emitting gas. In both systems, the associated galaxies reveal possible intense rest-frame-optical nebular emissions lines [Omicron III] lambda lambda 4959, 5007+H beta with equivalent widths as high as 1500 angstrom 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 emitting ionizing radiation that induces 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.