Assess shock heating as an alternative source of extended C IV and He II emission

Investigate whether shock heating, together with resonance scattering, is required to reproduce the observed spatial extents and line ratios of C IV and He II around quasars without relying exclusively on an external photoionizing field.

Background

The paper finds that photoionization alone does not naturally reproduce the observed relative extent and brightness of C IV and He II halos, while resonance scattering can redistribute C IV photons to larger radii. It identifies shock heating as an alternative or additional mechanism capable of collisionally ionizing gas to temperatures of roughly 105–106 K, but leaves unresolved whether both shock heating and resonance scattering are needed for a complete explanation.

References

Beyond these methodological extensions, alternative powering mechanisms such as shock heating, which can collisionally ionize and heat the CGM to $T \sim 105$--$106\,$K and produce \CIV and \HeII emission without an external photoionizing field \citep{Allen2008, Cabot2016}, deserve exploration. Fully reproducing the observed spatial extent and line ratios of \CIV and \HeII around quasars may require both resonance scattering and shock heating.

— Tracing Warm Gas through C IV Radiative Transfer  (2609.11487 - Lim et al., 10 Sep 2026) in Section Conclusions