Mock HUBS observations of hot gas with IllustrisTNG
  The lack of adequate X-ray observing capability is seriously impeding the
progress in understanding the hot phase of circumgalactic medium (CGM), which
is predicted to extend to the virial radius of a galaxy or beyond, and thus in
acquiring key boundary conditions for studying galaxy evolution. To this end,
the Hot Universe Baryon Surveyor (\textit{HUBS}) is proposed. \textit{HUBS} is
designed to probe hot CGM by detecting its emission or absorption lines with a
non-dispersive X-ray spectrometer of high resolution and high throughput. The
spectrometer consists of a $60\times60$ array of microcalorimeters, with each
detector providing an energy resolution of $2~\mathrm{eV}$, and is placed in
the focal plane of an X-ray telescope of $1^{\circ}$ field-of-view. With such a
design, the spectrometer is also expected to enable studies of intra-group
medium (IGrM) and the outer region of intra-cluster medium (ICM). To assess the
scientific potential of \textit{HUBS}, we created mock observations of
galaxies, groups, and clusters at different redshifts with the \tng simulation.
Focusing exclusively on emission studies in this work, we took into account the
effects of light cone, Galactic foreground emission, and background AGN
contribution in the mock observations. From the observations, we made mock
X-ray images and spectra, analyzed them to derive the properties of the
emitting gas in each case, and compared the results with the input parameters
from the simulation. The results show that \textit{HUBS} is well suited for
studying hot CGM at low redshifts. The redshift range is significantly extended
for measuring IGrM and ICM.
