2007ApJ...671..118D

A new model is presented that explains the origin of the broad emission lines observed in the LINER/Seyfert nucleus of M81 in terms of a steady state spherically symmetric inflow, amounting to ∼1x10^–5 M_☉/yr, which is sufficient to explain the luminosity of the active galactic nucleus (AGN). The emitting volume has an outer radius of ∼1 pc, making it the largest broad-line region (BLR) yet to be measured, and it contains a total mass of ∼5x10^–2 M_☉ of dense, ∼10⁸/cm3, ionized gas, leading to a very low filling factor of ∼5x10^–9. The fact that the BLR in M81 is so large may explain why the AGN is unable to sustain the ionization seen there. Thus, the AGN in M81 is not simply a scaled-down quasar.