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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 Galaxy formation and evolution are regulated by the feedback from galactic winds. Absorption lines provide the most widely available probe of winds. However, since most data only provide information integrated along the line of sight, they do not directly constrain the radial structure of the outflows. In this paper, we present a method to directly measure the gas electron density in outflows (n_e), which in turn yields estimates of outflow cloud properties (e.g., density, volume filling factor, and sizes/masses). We also estimate the distance (r_n) from the starburst at which the observed densities are found. We focus on 22 local star-forming galaxies primarily from the COS Legacy Archive Spectroscopic SurveY (CLASSY). In half of them, we detect absorption lines from fine-structure excited transitions of Siii(i.e., Siii*). We determinen_efrom relative column densities of Siiiand Siii*, given Siii* originates from collisional excitation by free electrons. We find that the derivedn_ecorrelates well with the galaxy’s star formation rate per unit area. From photoionization models or assuming the outflow is in pressure equilibrium with the wind fluid, we getr_n∼ 1–2r_*or ∼5r_*, respectively, wherer_*is the starburst radius. Based on comparisons to theoretical models of multiphase outflows, nearly all of the outflows have cloud sizes large enough for the clouds to survive their interaction with the hot wind fluid. Most of these measurements are the first ever for galactic winds detected in absorption lines and, thus, will provide important constraints for future models of galactic winds. 

Abstract Lyαline profiles are a powerful probe of interstellar medium (ISM) structure, outflow speed, and Lyman-continuum escape fraction. In this paper, we present the Lyαline profiles of the Cosmic Origins Spectrograph (COS) Legacy Archive Spectroscopic SurveY, a sample rich in spectroscopic analogs of reionization-era galaxies. A large fraction of the spectra show a complex profile, consisting of a double-peaked Lyαemission profile in the bottom of a damped, Lyαabsorption trough. Such profiles reveal an inhomogeneous ISM. We successfully fit the damped Lyαabsorption and the Lyαemission profiles separately, but with complementary covering factors, a surprising result because this approach requires no Lyαexchange between high-N_Hiand low-N_Hipaths. The combined distribution of column densities is qualitatively similar to the bimodal distributions observed in numerical simulations. We find an inverse relation between Lyαpeak separation and the [Oiii]/[Oii] flux ratio, confirming that the covering fraction of Lyman-continuum-thin sightlines increases as the Lyαpeak separation decreases. We combine measurements of Lyαpeak separation and Lyαred peak asymmetry in a diagnostic diagram, which identifies six Lyman-continuum leakers in the COS Legacy Archive Spectrocopy SurveY (CLASSY) sample. We find a strong correlation between the Lyαtrough velocity and the outflow velocity measured from interstellar absorption lines. We argue that greater vignetting of the blueshifted Lyαpeak, relative to the redshifted peak, is the source of the well-known discrepancy between shell-model parameters and directly measured outflow properties. The CLASSY sample illustrates how scattering of Lyαphotons outside the spectroscopic aperture reshapes Lyαprofiles because the distances to these compact starbursts span a large range.