Empirically Constrained Predictions for Metal-Line Emission from the
  Circumgalactic Medium
  The circumgalactic medium (CGM) remains one of the least constrained
components of galaxies and as such has significant potential for advancing
galaxy formation theories. In this work, we vary the extragalactic ultraviolet
background for a high-resolution cosmological simulation of a Milky Way-like
galaxy and examine the effect on the absorption and emission properties of
metals in the CGM. We find that a reduced quasar background brings the column
density predictions into better agreement with recent data. Similarly, when the
observationally derived physical properties of the gas are compared to the
simulation, we find that the simulation gas is always at temperatures
approximately 0.5 dex higher. Thus, similar column densities can be produced
from fundamentally different gas. However, emission maps can provide
complementary information to the line-of-sight column densities to better
derive gas properties. From the simulations, we find that the brightest
emission is less sensitive to the extragalactic background and that it closely
follows the fundamental filamentary structure of the halo. This becomes
increasingly true as the galaxy evolves from z=1 to z=0 and the majority of the
gas transitions to a hotter, more diffuse phase. For the brightest ions (CIII,
CIV, OVI), detectable emission can extend as far as 120 kpc at z=0. Finally,
resolution is a limiting factor for the conclusions we can draw from emission
observations but with moderate resolution and reasonable detection limits,
upcoming instrumentation should place constraints on the physical properties of
the CGM.
