2019ApJ...875L..24L

The nearby Perseus galaxy cluster is a key target for indirect detection searches for decaying dark matter. We use the C-EAGLE simulations of galaxy clusters to predict the flux, width, and shape of a dark matter decay line, paying particular attention to the unexplained 3.55 keV line detected in the spectra of some galaxies and clusters, and the upcoming XRISM X-ray observatory mission. We show that the line width in C-EAGLE clusters similar to Perseus is typically [600-800] kms^–1, and therefore narrower than the amplitude of the velocity dispersion of galaxies in the cluster. Halos that are significantly disturbed can, however, exhibit galaxy velocity dispersions higher than 1000 kms^–1, and in this case will show a large difference between the line profiles of on- and off-center observations. We show that the line profile is likely to be slightly asymmetric, but still well approximated by a Gaussian at the 10% level, and that the halo asymmetry can lead to fluxes that vary by a factor of two. In summary, we predict that, if the previously reported 3.55 keV line detections do originate from dark matter decay, the XRISM mission will detect a line with a roughly Gaussian profile at a rest-frame energy of 3.55 keV, with a width >600 kms^–1 and flux approximately in the range [4–9]×10^–8countss^–1cm^–2.