Cold gas entrainment in hot AGN-driven outflows

Determine how cold gas becomes entrained and transported within hot, AGN-driven galactic outflows, reconciling the apparent conflict between cloud-crushing timescales that are shorter than entrainment timescales and the observations of cold gas moving at approximately 100 km s−1 in outflows.

Background

The paper highlights that cold gas clouds are observed in outflows traveling at about 100 km s−1, yet classical arguments suggest the cloud-crushing time is shorter than the entrainment time, implying rapid destruction of cold clouds before significant acceleration. Recent high-resolution simulations indicate that radiative cooling at cloud–wind mixing boundaries can enable cloud survival or even growth, but sensitivity to cloud density structure remains.

Resolving the formation, survival, and acceleration mechanisms of cold cloudlets in a wind remains critical to interpret multiphase outflows and their energetics, and to connect simulations with multiwavelength observations of AGN feedback.

References

Another open question is how cold gas becomes entrained in a hot outflow.

— AGN-driven outflows in clumpy media: multiphase structure and scaling relations  (2407.17593 - Ward et al., 2024) in Introduction (Section 1)

What is the ultimate fate of the \ clouds in the FBs?

— High Velocity Neutral Gas in the Fermi Bubbles: New Kinematic Limits and Spatial Structure  (2608.16754 - Lockman et al., 17 Aug 2026) in Section 8, “Summary and discussion”