  Major mergers between galaxy clusters can produce large turbulent and bulk
flow velocities in the intra-cluster medium and thus imprint diagnostic
features in X-ray spectral emission lines from heavy ions. As demonstrated by
Hitomi in observations of the Perseus cluster, measurements of gas velocities
in clusters from high-resolution X-ray spectra will be achievable with upcoming
X-ray calorimeters like those on board XRISM, Athena, or a Lynx like mission.
We investigate this possibility for interesting locations across a major
cluster merger from a hydrodynamical simulation, via X-ray synthetic spectra
with a few eV energy resolution. We observe the system from directions
perpendicular to the plane of the merger and along the merger axis. In these
extreme geometrical configurations, we find clear non-Gaussian shapes of the
iron He-like K_alpha line at 6.7keV. The velocity dispersion predicted from the
simulations can be retrieved for the brightest 100ks pointings with XRISM
Resolve, despite some discrepancy related to the complex non-Gaussian line
shapes. Measurements in faint regions require however high S/N and the larger
collecting area of the Athena X-IFU calorimeter is thus needed. With the
latter, we also investigate the gas temperature and velocity gradient across
the merger bow shock edge, from 20"-wide annuli extracted from a single 1Ms
X-IFU pointing. We find best-fit temperature and velocity dispersion values
that are consistent with predictions from the simulations within 1-sigma, but
the uncertainties on the inferred velocity dispersion are too large to place
any stringent constraints on the shallow gradient downstream of the shock. We
also present simulated images of the thermal and kinetic Sunyaev-Zeldovich
effects, using the above viewing configurations, and compare the results at
angular resolutions appropriate for future observatories such as CMB-S4 and
AtLAST.
