Modeling Emission-Line Surface Brightness in a Multiphase Galactic Wind: An O VI Case Study

Abstract: We present a fast and robust analytic framework for predicting surface brightness (SB) of emission lines in galactic winds as a function of radius up to $\sim 100$ kpc out in the circum-galactic medium. We model multi-phase structure in galactic winds by capturing emission from both the volume-filling hot phase (T $\sim 10^{6-7}$ K) and turbulent radiative mixing layers that host intermediate temperature gas at the boundaries of cold clouds (T $\sim 10^4$ K). Our multi-phase framework makes significantly different predictions of emission signatures compared to traditional single-phase models. We emphasize how ram pressure equilibrium between the cold clouds and hot wind in supersonic outflows, non-equilibrium ionization effects, and energy budgets other than mechanical energy from core-collapse supernovae affect our SB predictions and allow us to better match OVI observations in the literature. Our framework reveals that the optimal galactic wind properties that facilitate OVI emission observations above a detection limit of $\sim 10^{-18} \ \rm{erg \ s^{-1} \ cm^{-2} \ arcsec^{-2}}$ are star formation rate surface density $1 \lesssim \dot{\Sigma}_{\ast} \lesssim 20 \ M_{\odot}\ \rm{yr^{-1}\ kpc^{-2}}$, hot phase mass loading factor $\eta_{\rm M,hot} \sim 0.2 - 0.4$, and thermalization efficiency factor $\eta_{\rm E} \gtrsim 0.8$. These findings are consistent with existing observations and can help inform future target selections.