Absorption-based Circumgalactic Medium Line Emission Estimates
  Motivated by the integral field units on large aperture telescopes and
proposals for ultraviolet-sensitive space telescopes to probe circumgalactic
medium (CGM) emission, we survey the most promising CGM 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 HST/COS absorption
measurements of ions around z= 0.2 Milky Way mass halos and motivated models
for the density and temperature of gas, and we 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.
We comment on the implications of existing OIII and NII stacking measurements
by Zhang and coworkers (2018). OV 630 A and OVI 1032,1038 A are perhaps the
most promising ultraviolet lines, with motivated models predicting intensities
>100 $\gamma$ cm$^{-2}$ s$^{-1}$ sr$^{-1}$ in the inner 100 kpc of Milky
Way-like systems. A detection 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. Hydrogen n>2
Lyman-series lines are too weak to be detectable, and H$\alpha$ emission within
100 kpc of Milky Way-like galaxies is within the reach of current integral
field units even for the minimum signal from ionizing background fluorescence.
