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Deep observations have detected extended Ly$\alpha$ emission nebulaesurrounding tens of quasars at redshift 2 to 6. However, the metallicity ofsuch extended haloes is still poorly understood. We perform a detailed analysison a large sample of 80 quasars at $z\sim3$ based on MUSE-VLT data. We findclear evidence of extended emission of the UV nebular lines such as CIV or HeIIfor about 20$\%$ of the sample, while CIII] is only marginally detected in afew objects. By stacking the cubes we detect emission of CIV, HeII and CIII]out to a radius of about 45 kpc. CIV and HeII show a radial decline muchsteeper than Ly$\alpha$, while CIII] shows a shallower profile similar toLy$\alpha$ in the inner 45 kpc. We infer that the average metallicity of thecircumgalactic gas within the central 30-50~kpc is $\sim$0.5 solar, or evenhigher. However, we also find evidence of a component of the Ly$\alpha$ haloes,which has much weaker metal emission lines relative to Ly$\alpha$. We suggestthat the high metallicity of the circumgalactic medium within the central 30-50kpc is associated with chemical pre-enrichment by past quasar-driven outflowsand that there is a more extended component of the CGM that has much lowermetallicity and likely associated with near-pristine gas accreted from theintergalactic medium. We show that our observational results are in goodagreement with the expectations of the FABLE zoom-in cosmological simulations.

Metal enrichment in the circumgalactic medium and Ly{\alpha} haloes around quasars at z $\sim$3