Active galactic nucleus
An active galactic nucleus, often abbreviated as AGN (Active Galactic Nucleus), refers to the center of a galaxy where a supermassive black hole is actively accreting matter. This accretion process produces considerable amounts of energy, often exceeding the total luminosity of all the stars in the galaxy. AGNs manifest themselves in different forms depending on their orientation and power.
A unified model
Astronomers have gradually come to understand that quasars, radio galaxies, blazars, and Seyfert galaxies are in fact manifestations of the same physical phenomenon— a supermassive black hole in the process of accretion—viewed from different angles. This unified model of AGNs states that a torus of gas and dust surrounds the active nucleus, and that the appearance of theobject depends on the angle from which it is observed.
The different types of AGNs
When the torus is viewed head-on and the jets are pointing toward us, we observe a blazar— extreme in both luminosity and variability. If the accretion disk is seen directly, it is a quasar. Seyfert galaxies are less luminous AGNs, with a bright nucleus visible within a recognizable spiral galaxy. Radio galaxies are AGNs viewed from the side, with intense radio jets.
The impact on the host galaxy
An active AGN can profoundly influence its host galaxy. The jets and winds it produces heat and expel interstellar gas, slowing or halting star formation. Conversely, shock waves can sometimes compress the gas and trigger new star formation. This feedback between the central black hole and its host galaxy is an active area of research in astrophysics.
Did you know?
Even the Milky Way likely went through an AGN phase in the past. Traces of intense activity from Sagittarius A* have been detected in the form of hot gas bubblesspanning thousands of light-years above and below the galactic plane: the Fermi bubbles.
Frequently Asked Questions
What is the difference between an AGN and a quasar?
A quasar is a particularly luminous type of AGN, observed mainly in the distant universe. AGN is the generic term that encompasses all active galactic nuclei, from the most modest (Seyfert galaxies) to the most powerful (quasars and blazars).
Are all supermassive black holes AGNs?
No. A supermassive black hole becomes an AGN only if it has matter to actively accrete. Sagittarius A* is nearly inactive today, so the Milky Way is not classified as an active galaxy.
Can an AGN be observed from Earth?
Yes. Several AGNs are visible with an amateur telescope: Markarian 231, NGC 1068, and M77. The most distant and brightest ones (quasars) are also detectable despite their considerable distances.