3.55 keV photon line and its morphology from a 3.55 keV axionlike particle line

Galaxy clusters can efficiently convert axionlike particles (ALPs) to photons. We propose that the recently claimed detection of a 3.55–3.57 keV line in the stacked spectra of a large number of galaxy clusters and the Andromeda galaxy may originate from the decay of either a scalar or fermionic 7.1 keV dark matter species into an ALP of mass ma≲6×10^−11  eV, which subsequently converts to a photon in the cluster magnetic field. In contrast to models in which the photon line arises directly from dark matter decay or annihilation, this can explain the anomalous line strength in the Perseus cluster. As cool-core clusters have high central magnetic fields and axion-photon conversion scales as B^2, this model can also explain the observed peaking of the line emission in the cool cores of the Perseus, Ophiuchus, and Centaurus clusters, as opposed to the much larger dark matter halos. We describe distinctive predictions of this scenario for future observations.