Signals from dark atom formation in halos

We consider indirect detection signals of atomic dark matter, with a massive dark photon which mixes kinetically with hypercharge. In significant regions of parameter space, dark matter remains at least partially ionized today, and dark atom formation can occur efficiently in dense regions, such as the centers of galactic halos. The formation of dark atoms is accompanied by emission of a dark photon, which can subsequently decay into Standard Model particles. We discuss the expected signal strength and compare it to that of annihilating dark matter. As a case study, we explore the possibility that dark atom formation can account for the observed 511 keV line and outline the relevant parameter space.