Metal enrichment in the circumgalactic medium and Lyα haloes around quasars at z $\sim$3

Abstract: Deep observations have detected extended Ly$\alpha$ 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 $z\sim3$ based on MUSE-VLT data. We find clear evidence of extended emission of the UV nebular lines such as CIV or HeII for about 20$\%$ 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 Ly$\alpha$, while CIII] shows a shallower profile similar to Ly$\alpha$ in the inner 45 kpc. We infer that the average metallicity of the circumgalactic gas within the central 30-50~kpc is $\sim$0.5 solar, or even higher. However, we also find evidence of a component of the Ly$\alpha$ haloes, which has much weaker metal emission lines relative to Ly$\alpha$. We 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.

• The paper demonstrates that quasar CGMs exhibit extended UV line emissions, with stacking techniques detecting metal lines out to ~45 kpc.
• The study finds a steep radial decline in emissions for CIV and [OIII] while HeII mirrors the Lyα profile, indicating varied ionization processes.
• The paper infers an average metallicity of about half solar in the central CGM, supporting simulation predictions of quasar-driven outflows and gas accretion.

Metal Enrichment in the Circumgalactic Medium and Ly$\alpha$ Halos around Quasars at $z\sim3$

The paper by Guo et al. investigates the metal enrichment in the circumgalactic medium (CGM) and the associated Ly$\alpha$ halos around quasars at redshift $z \sim 3$, utilizing data from the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT). The research focuses on understanding the distribution of metallicity within the CGM, which plays a crucial role in galaxy evolution by serving as a reservoir for gas that can potentially accrete onto galaxies.

The study leverages a comprehensive sample of 80 quasars and reveals the presence of extended UV nebular line emissions such as CIV, HeII, and CIII] around roughly 20% of the quasars, while the [OII] emission was marginally detected in a limited number of cases. By employing stacking techniques, the authors detect extended emission in several UV lines out to a radius of approximately 45 kpc. Their analysis shows a steep radial decline for CIV and [OIII] emissions, while the profile of HeII appears shallower and more comparable to the Ly$\alpha$ line, suggesting a differentiated spatial behavior possibly linked to varying ionization and enrichment processes.

Through detailed empirical assessment, the study infers an average metallicity of about half solar, or potentially higher, within the central 30-50 kpc of the CGM. Notably, the presence of a significant but weaker component of the Ly$\alpha$ halos—with much fainter metal emission lines—also emerges from the data. This dual nature of the CGM is postulated to result from substantial chemical pre-enrichment by past quasar-driven outflows, with the extended component potentially sourced from near-pristine gas accreted from the intergalactic medium (IGM).

The observations are found to be consistent with predictions from the FABLE cosmological simulations, supporting the theoretical model that proposes significant metal transport into the CGM through quasar-driven outflows. This concordance between observations and simulations highlights the ongoing interplay between inflows and outflows in determining the metal content of the CGM at high redshift.

The implications of these findings are manifold. Practically, the identified metallicity gradients and spatial ionization variations provide new constraints for theoretical models of galaxy evolution, specifically regarding feedback mechanisms. Theoretically, this work underscores the importance of considering highly resolved simulations to capture the complex, small-scale processes that dominate the CGM’s chemical evolution.

Future AI developments in astrophysics may enhance the capability of simulations and observational alignments by incorporating convolutional networks for pattern recognition in complex spectral data, enabling refined parameter estimation and, consequently, more precise models of CGM evolution. The insights from this study can lead to more sophisticated models that better capture the complex interplay between galactic feedback and the galactic environment at early cosmic times.