Charge exchange emission and cold clumps in multi-phase galactic outflows

TL;DR

This paper investigates the structure and evolution of cold dense clumps in galactic outflows, revealing their flattened shapes, their interaction with ionised fluid, and their role in galactic material recycling.

Contribution

It provides observational evidence and analysis of cold clump morphology and dynamics in galactic outflows, supporting models of clump compression, ablation, and recycling.

Findings

Cold dense clumps are flattened, pancake-like structures.

Filamentary features are stripped material, not original clumps.

Some clumps survive and seed circumgalactic medium.

Abstract

Large-scale outflows from starburst galaxies are multi-phase, multi-component fluids. Charge-exchange lines which originate from the interfacing surface between the neutral and ionised components are a useful diagnostic of the cold dense structures in the galactic outflow. From the charge-exchange lines observed in the nearby starburst galaxy M82, we conduct surface-to-volume analyses and deduce that the cold dense clumps in its galactic outflow have flattened shapes, resembling a hamburger or a pancake morphology rather than elongated shapes. The observed filamentary H$\alpha$ features are therefore not prime charge-exchange line emitters. They are stripped material torn from the slow moving dense clumps by the faster moving ionised fluid which are subsequently warmed and stretched into elongated shapes. Our findings are consistent with numerical simulations which have shown that cold dense clumps in galactic outflows can be compressed by ram pressure, and also progressively ablated and stripped before complete disintegration. We have shown that some clumps could survive their passage along a galactic outflow. These are advected into the circumgalactic environment, where their remnants would seed condensation of the circumgalactic medium to form new clumps. The infall of these new clumps back into the galaxy and their subsequent re-entrainment into the galactic outflow form a loop process of galactic material recycling.