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

Abstract: Hydrodynamic simulations can connect outflow observables to the physical conditions of outflowing gas. Here, we use simulations of isolated disk 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 ([CII]$_{158\rm{\mu m}}$, $\mathrm{CO}_{J(1-0)}$, H$\alpha$ and $[\mathrm{OIII}]_{5007\text{A}}$). Using our synthetic spectra we measure the mass outflow rate, kinetic power and momentum flux using observational techniques. In [CII]$_{158\rm{\mu m}}$ we measure outflow rates of $10^{-4}$ to $1$ $\mathrm{M_{\odot}yr^{-1}}$ 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 $\approx1$ 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 [CII]$_{158\rm{\mu m}}$ probes outflows at greater distances from the disk, whilst we find that molecular gas does not survive at large distances within outflows within our modestly star-forming disk galaxies simulated in this work.