# Emission line tracers of galactic outflows driven by stellar feedback in simulations of isolated disk galaxies

**Authors:** Elliot L. Howatson, Alexander J. Richings, Elke Roediger, Claude-Andre Faucher-Giguere, Tom Theuns, Yuankang Liu, Tsang Keung Chan, Oliver Thompson, Cody Carr, Daniel Angles-Alcazar  
**arXiv:** [2509.21295](https://arxiv.org/abs/2509.21295)  
**Date:** 2025-09-25  
**Summary page:** https://www.summarizepaper.com/en/paper/2509.21295v1/

## Key points

- Hydrodynamic simulations are crucial for studying galactic outflows and connecting observable characteristics to physical conditions of outflowing gas.
- Simulations of isolated disk galaxies using FIRE-2 subgrid models explored multiphase galactic outflows across different mass ranges.
- Synthetic spectra generated for common outflow tracers allowed for measurement of properties like mass outflow rate, kinetic power, and momentum flux.
- Observations showed good alignment with actual data in [CII]$_{158\rm{\mu m}}$ but significant discrepancies in $\mathrm{CO}_{J(1-0)}$ due to uncertainties in electron density measurements.
- Molecular outflows were found to be sensitive to the conversion factor between CO luminosity and H2 mass, causing variations in outflow rates especially in lower-mass galaxies.

## Summary

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In the study of galactic outflows, hydrodynamic simulations play a crucial role in connecting observable characteristics of outflows to the physical conditions of the outflowing gas. This particular investigation conducted simulations of isolated disk galaxies using FIRE-2 subgrid models to explore multiphase galactic outflows across a range of dwarf to Milky Way masses. By employing the CHIMES non-equilibrium chemistry module, synthetic spectra were generated for common outflow tracers such as [CII]$_{158\rm{\mu m}}$, $\mathrm{CO}_{J(1-0)}$, H$\alpha$, and $[\mathrm{OIII}]_{5007\text{A}}$. The analysis of these synthetic spectra allowed for the measurement of various outflow properties using observational techniques, including mass outflow rate, kinetic power, and momentum flux. Notably, in [CII]$_{158\rm{\mu m}}$ observations, outflow rates aligned reasonably well with actual observations across a star formation rate (SFR) range. However, a significant discrepancy was found in $\mathrm{CO}_{J(1-0)}$ where simulations fell approximately 1 dex below observational data. Further investigation revealed that uncertainties in measurements of electron density could lead to overestimations by up to 2 dex, resulting in changes in mass outflow rates by up to 4 dex. Additionally, molecular outflows were found to be particularly sensitive to the conversion factor between CO luminosity and H2 mass, causing variations in outflow rates by up to 4 dex in lower-mass galaxies. Comparisons between derived properties from synthetic spectra and those directly from simulations indicated that [CII]$_{158\rm{\mu m}}$ probes outflows at greater distances from the disk. Interestingly, it was observed that molecular gas did not survive at large distances within the modestly star-forming disk galaxies simulated in this study. Expanding on existing literature regarding galactic outflows simulation studies reveals a diverse range of approaches, including cosmological large-volume simulations like EAGLE and TNG50, zoom-in simulations such as FIRE-2 and AURIGA, and idealized isolated galaxy simulations like SILCC and TIGRESS. These various simulation approaches contribute significantly towards unraveling the complexities surrounding galactic outflows driven by stellar feedback mechanisms.