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The origin of the 3.55 keV excess reported in various galaxies and galaxyclusters remains unknown. Non-observations of the line in dwarf spheroidalgalaxies (dSphs) create tension with galaxy cluster observations whenconsidering a direct Dark Matter (DM) $\rightarrow \gamma$ decay. Thisdiscrepancy could be explained by a DM $\rightarrow a \rightarrow \gamma$decay, where the DM particles decay to axion-like particles (ALPs), which thenconvert to photons in a magnetic field. The 3.55 keV line from a decay to ALPstherefore has very different morphology from a direct decay to photons, as itwould only appear in environments with substantial magnetic fields between thesource and us. To test this hypothesis we consider targets where the linestrength would be enhanced compared to the DM $\rightarrow \gamma$ model. Thegiant lobes of radio galaxies could represent good environments, havingmagnetic fields with strength $\mathcal{O}(\mu\rm{G})$ that extend for hundredsof kiloparsecs, and minimal X-ray background. In the case of Centaurus A, alarge population of dSphs are associated with the galaxy, some of which arepotentially in or behind its lobes. In this paper we estimate the flux of a3.55 keV DM $\rightarrow a \rightarrow \gamma$ line from these sources, andcompare it to the flux from the DM halo of Centaurus~A. We comment on thepotential to detect such a signal with current satellites (such as XMM-Newtonand Chandra) and future satellites (such as Athena).

Estimates for a DM $\rightarrow a \rightarrow \gamma$ 3.55 keV line in the radio lobes of Centaurus A