When are galactic winds molecular?

TL;DR

This paper develops an analytical model to determine when galactic winds contain significant molecular gas, revealing that strong starbursts are more likely to have molecular outflows, unlike normal galaxies.

Contribution

It introduces a model based on dimensionless parameters to predict molecular survivability in galactic winds, bridging observational and simulation limitations.

Findings

Molecular content depends on outflow strength and radiation field.

Normal galaxies likely destroy molecules near the disc.

Starburst galaxies can host molecular-dominated outflows.

Abstract

The molecular phase of supernova-driven outflows originates from the cold, molecular gas in the disc of a star-forming galaxy, and may carry a substantial fraction of the wind mass flux in some galaxies, but it remains poorly understood. Observations of this phase come mostly from very nearby galaxies due its low surface brightness and covering fraction, and simulations often lack the spatial resolution necessary to resolve it. Here we analytically estimate the survivability of this phase in order to understand under what conditions an galactic wind can contain a significant molecular phase. We show that the molecular content of outflows is primarily determined by two dimensionless numbers: a generalised Eddington ratio describing the strength of the outflow and an ionisation parameter-like quantity describing the strength of the radiation field per baryon. We apply this model to a sample of galaxies and show that, while any molecules entrained in the winds of normal star-forming galaxies should be destroyed close to the galactic disc, the outflows of strong starburst should become increasingly dominated by molecules.