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

ABSTRACT Galactic outflows strongly influence galactic evolution and have been detected in a range of observations. Hydrodynamic simulations can help interpret these by connecting direct observables to the physical conditions of the outflowing gas. Here we use simulations of isolated disc galaxies ranging from dwarf mass ($M_{200} = 10^{10}\, \mathrm{M}_{\odot }$) to Milky Way mass ($M_{200} = 10^{12}\, \mathrm{M}_{\odot }$), based on the FIRE-2 subgrid models to investigate multiphase galactic outflows. We use the chimes non-equilibrium chemistry module to create synthetic spectra of common outflow tracers ([C ii]$_{158\, \mu\rm m}$, $\mathrm{CO}_{J(1-0)}$, H$\alpha$ and $[\mathrm{O}{\small III}]_{5007\, \rm{\mathring{\rm A}}}$). Using our synthetic spectra we measure the mass outflow rate, kinetic power and momentum flux using observational techniques. In [C ii]$_{158\, \mu\rm m}$ we measure outflow rates of $10^{-4}$ to 1 $\mathrm{\, {\rm M}_{\odot }\, \rm yr^{-1}}$ across an SFR range of $10^{-3}$ to 1 $\text{M}_{\odot }\text{yr}^{-1}$, which is in reasonable agreement with observations. The significant discrepancy is in $\mathrm{CO}_{J(1-0)}$, with the simulations lying $\approx 1$ dex below the observational sample. We test observational assumptions used to derive outflow properties from synthetic spectra. We find the greatest uncertainty lies in measurements of electron density, as estimates using the SII doublet can overestimate the actual electron density by up to 2 dex, which changes mass outflow rates by up to 4 dex. We also find that molecular outflows are especially sensitive to the conversion factor between CO luminosity and H2 mass, with outflow rates changing by up to 4 dex in our least massive galaxy. Comparing the outflow properties derived from the synthetic spectra to those derived directly from the simulation, we find that [C ii]$_{158\, \mu\rm m}$ probes outflows at greater distances from the disc, whilst we find that molecular gas does not survive at large distances within outflows within our modestly star-forming disc galaxies simulated in this work.