  The Astro2020 Decadal Survey has identified the mapping of the circumgalactic
medium (CGM, gaseous plasma around galaxies) as a key objective. We explore the
prospects for characterizing the CGM in and around nearby galaxy halos with a
future, large grasp X-ray microcalorimeter. We create realistic mock
observations from hydrodynamical simulations (EAGLE, IllustrisTNG, and Simba)
that demonstrate a wide range of potential measurements, which will address the
open questions in galaxy formation and evolution. By including all background
and foreground components in our mock observations, we show why it is
impossible to perform these measurements with current instruments, such as
X-ray CCDs, and only microcalorimeters will allow us to distinguish the faint
CGM emission from the bright Milky Way (MW) foreground emission lines. We find
that individual halos of MW mass can, on average and depending on star
formation rate, be traced out to large radii, around R500, and for larger
galaxies even out to R200, using prominent emission lines, such as OVII, or
OVIII. Furthermore, we show that emission line ratios for individual halos can
reveal the radial temperature structure. Substructure measurements show that it
will be possible to relate azimuthal variations to the feedback mode of the
galaxy. We demonstrate the ability to construct temperature, velocity, and
abundance ratio maps from spectral fitting for individual galaxy halos, which
reveal rotation features, AGN outbursts, and enrichment.
