Deep observations have detected extended Lyα emission nebulae
surrounding tens of quasars at redshift 2 to 6. However, the metallicity of
such extended haloes is still poorly understood. We perform a detailed analysis
on a large sample of 80 quasars at z∼3 based on MUSE-VLT data. We find
clear evidence of extended emission of the UV nebular lines CIV and HeII for
about 18% of the sample, while CIII] is only marginally detected in a few
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 much steeper than
Lyα, while CIII] shows a shallower profile similar to Lyα in the
inner 45 kpc. We infer that the average metallicity of the circumgalactic gas
within the central 30-50~kpc is ∼0.5 solar, or even higher. However, we
also find evidence of a component of the Lyα haloes, which has much
weaker metal emission lines relative to Lyα. We suggest that the high
metallicity of the circumgalactic medium within the central 30-50 kpc is
associated with chemical pre-enrichment by past quasar-driven outflows and that
there is a more extended component of the CGM that has much lower metallicity
and likely associated with near-pristine gas accreted from the intergalactic
medium. We show that our observational results are in good agreement with the
expectations of the FABLE zoom-in cosmological simulations.ERC
STF