The Lifetimes of High-redshift Quasars Suggest Magnetic Disk Support

It has recently been suggested that a variety of data on active galactic nuclei (AGN) can be explained if AGN disks are supported against gravitational fragmentation by magnetic fields that are advected into the disk from the surrounding galaxy. Here we derive the maximum timescales over which accretion onto a black hole (BH) powering an AGN can be maintained at a given rate, both with and without magnetic disk support. We then compare these timescales to the lifetimes of episodes of sustained luminous accretion that are inferred from measurements of the photoionized proximity zones around high-redshift quasars. While some of the shortest inferred quasar lifetimes are consistent with pure gas pressure support, we find that some additional magnetic support is likely required to explain the longest inferred quasar lifetimes of > 10$^4$ yr. For these longest-lived AGN, we find that magnetic pressure in their disks can be up to a hundred times higher than the gas pressure. In addition, the lack of inferred quasar lifetimes that are definitively > 10$^6$ yr is consistent with gas pressure and advected magnetic fields being the principal sources of disk support. This adds to the body of evidence that magnetic fields play an important role in sustaining the rapid growth of supermassive BHs in the early universe.