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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. 

ABSTRACT Observations indicate that a continuous supply of gas is needed to maintain observed star formation rates in large, discy galaxies. To fuel star formation, gas must reach the inner regions of such galaxies. Despite its crucial importance for galaxy evolution, how and where gas joins galaxies is poorly constrained observationally and rarely explored in fully cosmological simulations. To investigate gas accretion in the vicinity of galaxies at low redshift, we analyse the FIRE-2 cosmological zoom-in simulations for 4 Milky Way mass galaxies (Mhalo ∼ 1012M⊙), focusing on simulations with cosmic ray physics. We find that at z ∼ 0, gas approaches the disc with angular momentum similar to the gaseous disc edge and low radial velocities, piling-up near the edge and settling into full rotational support. Accreting gas moves predominately parallel to the disc and joins largely in the outskirts. Immediately prior to joining the disc, trajectories briefly become more vertical on average. Within the disc, gas motion is complex, being dominated by spiral arm induced oscillations and feedback. However, time and azimuthal averages show slow net radial infall with transport speeds of 1–3 km s−1 and net mass fluxes through the disc of ∼M⊙ yr−1, comparable to the galaxies’ star formation rates and decreasing towards galactic centre as gas is sunk into star formation. These rates are slightly higher in simulations without cosmic rays (1–7 km s−1, ∼4–5 M⊙ yr−1). We find overall consistency of our results with observational constraints and discuss prospects of future observations of gas flows in and around galaxies.