Catastrophic Cooling in Superwinds. II. Exploring the Parameter Space

TL;DR

This study investigates the conditions under which superwinds in starburst galaxies undergo catastrophic cooling, revealing that high mass-loading and reduced heating are key factors, and explores observable signatures of these cooling regimes.

Contribution

It introduces a comprehensive grid of hydrodynamic and photoionization models to analyze cooling modes in superwinds, emphasizing the roles of metallicity, mass-loading, and ambient density.

Findings

Catastrophic cooling is more sensitive to mass-loading and heating efficiency than metallicity.

Hot superbubbles do not necessarily indicate adiabatic outflows.

Line emission diagnostics alone cannot definitively identify cooling regimes.

Abstract

Superwinds and superbubbles driven by mechanical feedback from super star clusters (SSCs) are common features in many star-forming galaxies. While the adiabatic fluid model can well describe the dynamics of superwinds, several observations of starburst galaxies revealed the presence of compact regions with suppressed superwinds and strongly radiative cooling, i.e., catastrophic cooling. In the present study, we employ the non-equilibrium atomic chemistry and cooling package MAIHEM, built on the FLASH hydrodynamics code, to generate a grid of models investigating the dependence of cooling modes on the metallicity, SSC outflow parameters, and ambient density. While gas metallicity plays a substantial role, catastrophic cooling is more sensitive to high mass-loading and reduced kinetic heating efficiency. Our hydrodynamic simulations indicate that the presence of a hot superbubble does not necessarily imply an adiabatic outflow, and vice versa. Using CLOUDY photoionization models, we predict UV and optical line emission for both adiabatic and catastrophic cooling outflows, for radiation-bounded and partially density-bounded models. Although the line ratios predicted by our radiation-bounded models agree well with observations of star-forming galaxies, they do not provide diagnostics that unambiguously distinguish the parameter space of catastrophically cooling flows. Comparison with observations suggests the possibility of minor density bounding, non-equilibrium ionization, and/or observational bias toward the central outflow regions.