Active Galactic Nucleus
Astronomers think that virtually every large galaxy in the Universe harbors a supermassive black hole at its center, with millions of times the mass of our Sun. The black hole itself cannot be observed with telescopes, but we can see the material that is approaching it before falling irretrievably into its interior.
The central black hole is not always feeding and there are times when nothing falls into it. But there are other times when a star or other object does pass close enough to be engulfed.
The nucleus of a galaxy is said to be active when the central black hole is devouring matter, which emits radiation just before hurtling past the black hole’s event horizon (the entrance boundary to the region where nothing can escape, not even light). It is this light that allows us to deduce that a gigantic black hole lives in that galaxy.
The unified AGN model
The most widely accepted theoretical model of the structure of an AGN today indicates that celestial objects that we call by different names are actually the same, but seen with one particular orientation or another.
The AGN consists of a supermassive black hole, surrounded by a disk of material called an accretion disk. It contains the material that is beginning to precipitate towards the center, rotating at high speed, heating up and emitting radiation that we can detect by its intense brightness. Some of the matter that falls towards the central object, possibly due to its own spin and interaction with the magnetic field, is ejected along with enormous amounts of energy from the poles of the system, forming powerful jets that can be seen from enormous distances.
Surrounding the disc is a thick ring, or torus, of dust.
The unified model indicates that (simplifying) three celestial objects are actually the same:
- Quasar: when from our point of view we can observe the entire structure of the AGN
- Seyfert Galaxy: when the AGN is positioned so that we only see the torus of dust edge-on and the jets
- Blazar: when one of the jets is aimed at us and we only see it and the torus
The 3D model
With this object we show the theoretical model of an active galactic nucleus, since none has yet been observed directly.
At the center is the supermassive black hole, represented by a sphere.
Two jets of material come out of the black hole’s surroundings towards the poles, which open up as they move away into space.
Surrounding the black hole is a spiral structure, material that is hurtling into it. It is the accretion disk.
Around the disk we locate the torus of dusty material that is in a stable orbit around the black hole and is therefore not falling towards it.
The physical sizes of the 3D model are chosen for ease of understanding, but they are not to scale: in reality the radius of the accretion disk is about 100-1000 times larger than that of the black hole’s event horizon, and the torus has a radius 1000-10,000 times larger than the disk. Jets, in turn, can reach distances millions of times greater.
This work has been carried out with a grant from the R&D&I ‘Proof of Concept’ 2022 (PDC2022) call for projects, financed by the European Union through the NextGenerationEU Recovery, Transformation and Resilience Plan. Project’s reference: PDC2022-133930-I00.