The signal of decaying dark matter with hydrodynamical simulations
  Dark matter particles may decay, emitting photons. Drawing on the EAGLE
family of hydrodynamic simulations of galaxy formation -- including the APOSTLE
and C-EAGLE simulations -- we assess the systematic uncertainties and scatter
on the decay flux from different galaxy classes, from Milky Way satellites to
galaxy clusters, and compare our results to studies of the 3.55~keV line. We
demonstrate that previous detections and non-detections of this line are
consistent with a dark matter interpretation. For example, in our simulations
the width of the the dark matter decay line for Perseus-analogue galaxy
clusters lies in the range 1300-1700~\kms. Therefore, the non-detection of the
3.55~keV line in the centre of the Perseus cluster by the {\it Hitomi}
collaboration is consistent with detections by other instruments. We also
consider trends with stellar and halo mass and evaluate the scatter in the
expected fluxes arising from the anisotropic halo mass distribution and from
object-to-object variations. We provide specific predictions for observations
with {\it XMM-Newton} and with the planned X-ray telescopes {\it XRISM} and
{\it ATHENA}. If future detections of unexplained X-ray lines match our
predictions, including line widths, we will have strong evidence that we have
discovered the dark matter.
