2020ApJ...898...26G

Deep observations have detected extended Lyα emission nebulae surrounding tens of quasars at redshift 2-6. However, the metallicity of such extended halos 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 such as C IV λ1549 or He II λ1640 for about 20% of the sample, while C III] λ1909 is only marginally detected in a few objects. By stacking the cubes, we detect emission of C IV, He II, and C III] out to a radius of about 45 kpc. C IV and He II show a radial decline much steeper than Lyα, while C III] 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 halos, 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 circumgalactic medium that has much lower metallicity and is 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.