Absorption-based Circumgalactic Medium Line Emission Estimates

Abstract: Motivated by integral field units (IFUs) on large ground telescopes and proposals for ultraviolet-sensitive space telescopes to probe circumgalactic medium (CGM) emission, we survey the most promising emission lines and how such observations can inform our understanding of the CGM and its relation to galaxy formation. We tie our emission estimates to both HST/COS absorption measurements of ions around $z\approx 0.2$ Milky Way mass halos and models for the density and temperature of gas. We also provide formulas that simplify extending our estimates to other samples and physical scenarios. We find that OIII 5007 A and NII 6583 A, which at fixed ionic column density are primarily sensitive to the thermal pressure of the gas they inhabit, may be detectable with KCWI and especially IFUs on 30 m telescopes out to half a virial radius. OV 630 A and OVI $1032,1038$ A are perhaps the most promising ultraviolet lines, with models predicting intensities $>100~\gamma$ cm$^{-2}$ s$^{-1}$ sr$^{-1}$ in the inner 100 kpc of Milky Way-like systems. A detection of OVI would confirm the collisionally ionized picture and constrain the density profile of the CGM. Other ultraviolet metal lines constrain the amount of gas that is actively cooling and mixing. We find that CIII 978 A and CIV 1548 A may be detectable if an appreciable fraction of the observed OVI column is associated with mixing or cooling gas. H$\alpha$ emission within $100\;$kpc of Milky Way-like galaxies is within reach of current IFUs even for the minimum signal from ionizing background fluorescence, while Hydrogen $n>2$ Lyman-series lines are too weak to be detectable.