A novel method to trace the dark matter density profile around supermassive black holes with AGN reverberation mapping

Abstract: We propose a new method to determine the dark matter density profile in the vicinity of distant supermassive black holes (SMBH) using reverberation mapping measurements of active galactic nuclei (AGN). For some AGN, the mapping of multiple emission lines allows us to measure the enclosed mass within different radii from the central SMBH, which allows us to infer or constrain the dark matter density profile on sub-pc scales. We find that for five out of fourteen AGN in our identified sample, the observed enclosed mass grows with radii, hinting evidence for the presence of a dark matter component at the 1-2 $\sigma$ level. For these sources, we find global evidence for a universal dark matter profile with a preferred radial steepness of index $\gamma \sim 1.6$, consistent with the scenario expected for a dark matter spike mildly relaxed by stellar heating processes. For the remaining AGN in our sample, we are able to derive upper limits on the dark matter density profile parameters. Interestingly, the strongest evidence for a dark matter component arises from the farthest object considered, 3C 390. This new method allows us to measure or constrain for the first time the dark matter density in the vicinity of extragalactic SMBHs, which is crucial to our understanding of the dynamics and nature of dark matter.