Physical origin of the dense CGM component

Determine the physical origin of the dense gas component responsible for the recombination emission in the circumgalactic medium of luminous quasars, distinguishing among condensation, transport, turbulent mixing, compression, disruption, and other multiphase processes.

Background

The observed He II/Hα ratios require a broad density distribution in which the emission is dominated by a relatively small fraction of dense gas embedded in a more diffuse circumgalactic medium. The inferred density distribution is therefore an emissivity-weighted diagnostic rather than a unique, volume-filling halo-wide density model.

The paper discusses several possible mechanisms for generating this dense component, including condensation, transport, turbulent mixing, compression, disruption, chaotic cold accretion, cold streams, satellite interactions, and interface compression. However, the observations and modeling do not distinguish uniquely among these mechanisms, leaving the physical origin of the dense emitting phase unresolved.

References

While our analysis cannot uniquely determine the origin of this dense component, it provides strong observational evidence that the CGM around luminous quasars is highly inhomogeneous and dynamically multiphase.

— Constraining the density distribution of the cold Circumgalactic Medium of quasars at z>2 through Lya, HeII and Ha emission  (2610.01454 - Travascio et al., 1 Oct 2026) in Section 5.1, “Turbulence-driven condensation and CGM weather”