Supermassive black holes are thought to grow in heavily obscured environments. A new study now suggests that many of the brightest supermassive black holes around us may be escaping our detection as they hide in these environments.
The geometric dependence of AGN types in the unified AGN model. Type 1 AGN are viewed from an angle where the central engine is visible. In Type 2 AGN, the dusty torus obscures the central engine from view. [Urry & Padovani, 1995]
A Torus Puzzle
The centers of galaxies with bright, actively accreting supermassive black holes are known as active galactic nuclei, or AGN. According to a commonly accepted model for AGN, these rapidly growing black holes and their accretion disks are surrounded by a thick torus of dust. From certain angles, the torus can block our direct view of the central engines, changing how the AGN appears to us. AGN for which we can see the central engine are known as Type 1 AGN, whereas those with an obscured central region are classified as Type 2.
Oddly, the fraction of AGN classified as Type 2 decreases substantially with increasing luminosity; brighter AGN seem to be more likely to be unobscured. Why? One ...