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The intensity of the Cosmic UV background (UVB), coming from all sources ofionising photons such as star-forming galaxies and quasars, determines thethermal evolution and ionization state of the intergalactic medium (IGM) andis, therefore, a critical ingredient for models of cosmic structure formation.Most of the previous estimates are based on the comparison between observed andsimulated Lyman-$\alpha$ forest. We present the results of an independentmethod to constrain the product of the UVB photoionisation rate and thecovering fraction of Lyman limit systems (LLSs) by searching for thefluorescent Lyman-$\alpha$ emission produced by self-shielded clouds. Becausethe expected surface brightness is well below current sensitivity limits fordirect imaging, we developed a new method based on three-dimensional stackingof the IGM around Lyman-$\alpha$ emitting galaxies (LAEs) between 2.9<z<6.6using deep MUSE observations. Combining our results with covering fractions ofLLSs obtained from mock cubes extracted from the EAGLE simulation, we obtainnew and independent constraints on the UVB at z>3 that are consistent withprevious measurements, with a preference for relatively low UVB intensities atz=3, and which suggest a non-monotonic decrease of $\Gamma$HI with increasingredshift between 3<z<5. This could suggest a possible tension between some UVBmodels and current observations which however require deeper and widerobservations in Lyman-$\alpha$ emission and absorption to be confirmed.Assuming instead a value of UVB from current models, our results constrain thecovering fraction of LLSs at 3<z<4.5 to be less than 25% within 150kpc fromLAEs.

Constraining the cosmic UV background at z>3 with MUSE Lyman-{\alpha} emission observations