The warm-hot circumgalactic medium around EAGLE-simulation galaxies and
  its detection prospects with X-ray line emission
  We estimate the detectability of X-ray metal-line emission from the
circumgalactic medium (CGM) of galaxies over a large halo mass range
($\mathrm{M}_{\mathrm{200c}} =10^{11.5}$-$10^{14.5}\,\mathrm{M}_{\odot}$) using
the EAGLE simulations. With the XRISM Resolve instrument, a few bright
(K-$\alpha$ or Fe L-shell) lines from $\mathrm{M}_{\mathrm{200c}} \gtrsim
10^{13}\,\mathrm{M}_{\odot}$ haloes should be detectable. Using the Athena
X-IFU or the Lynx Main Array, emission lines (especially from O$\,$VII and
O$\,$VIII) from the inner CGM of $\mathrm{M}_{\mathrm{200c}}
\gtrsim10^{12.5}\,\mathrm{M}_{\odot}$ haloes become detectable, and intragroup
and intracluster gas will be detectable out to the virial radius. With the Lynx
Ultra-high Resolution Array, the inner CGM of haloes hosting $\mathrm{L}_{*}$
galaxies is accessible. These estimates do assume long exposure times ($\sim
1\,$Ms) and large spatial bins ($\sim1$-$10\,\mathrm{arcmin}^{2}$). We also
investigate the properties of the gas producing this emission. CGM emission is
dominated by collisionally ionized (CI) gas, and tends to come from halo
centres. The gas is typically close to the maximum emissivity temperature for
CI gas ($\mathrm{T}_\mathrm{peak}$), and denser and more metal-rich than the
bulk of the CGM at a given distance from the central galaxy. However, for the
K-$\alpha$ lines, emission can come from hotter gas in haloes where the
virialized, volume-filling gas is hotter than $\mathrm{T}_\mathrm{peak}$.
Trends of emission with halo mass can largely be explained by differences in
virial temperature. Differences between lines generally result from the
different behaviour of the emissivity as a function of temperature of the
K-$\alpha$, He-$\alpha$-like, and Fe~L-shell lines. We conclude that upcoming
X-ray missions will open up a new window onto the hot CGM.
