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Cosmological simulations suggest that most of the matter in the Universe isdistributed along filaments connecting galaxies. Illuminated by the cosmic UVbackground (UVB), these structures are expected to glow in fluorescent Lymanalpha emission with a Surface Brightness (SB) that is well below currentobservational limits for individual detections. Here, we perform a stackinganalysis of the deepest MUSE/VLT data using three-dimensional regions(subcubes) with orientations determined by the position of neighbouring Lymanalpha galaxies (LAEs) at 3<z<4. Our method should increase the probability ofdetecting filamentary Lyman alpha emission, provided that these structures areLyman Limit Systems (LLSs). By stacking 390 oriented subcubes we reach a 2sigma sensitivity level of SB ~ 0.44e-20 erg/s/cm^2/arcsec^2 in an aperture of1 arcsec^2 x 6.25 Angstrom, which is three times below the expected fluorescentLyman alpha signal from the Haardt-Madau 2012 (HM12) UVB at z~3.5. Nodetectable emission is found on intergalactic scales, implying that at leasttwo thirds of our subcubes do not contain oriented LLSs for a HM12 UVB. On theother hand, significant emission is detected in the circum-galactic medium(CGM) of galaxies in the direction of the neighbours. The signal is strongerfor galaxies with a larger number of neighbours and appears to be independentof any other galaxy properties such as luminosity, redshift and neighbourdistance. We estimate that preferentially oriented satellite galaxies cannotcontribute significantly to this signal, suggesting instead that gas densitiesin the CGM are typically larger in the direction of neighbouring galaxies oncosmological scales.

Stacking the Cosmic Web in Fluorescent Lyman alpha Emission with MUSE