Introduction to Active Galactic Nuclei

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Digital artist's impression of an active galactic nuclei (Source -

Active galactic nuclei (AGN) are some of the most energetic, luminous, peaceful sources in the universe, giving out persistent radiation. Peaceful would be a generous adjective - they are relatively non-violent; they do not explode as most things in the universe tend to do! We generally identify AGN with large plume-like jets seen on either side of galaxies. AGN are powered by violent accretion of matter onto supermassive black holes (SMBH) in the galactic centers. The radiation from such a process far outshines the combined starlight from a galaxy. Some of the brightest sources of radiation in the universe are AGN and thus can be “seen” upto incredible distances. Infact the majority of the farthest (hence oldest) known objects are AGN. Therefore they can be used to map out the universe at the earliest epochs. Indeed, not all galaxies are active; and not all active galaxies have bright and dazzling jets. Infact the majority of AGN can hardly be distinguished from regular galaxies. But even these ordinary-looking AGN hold clues to solving intricate problems, and thus makes the life of AGN researchers devoid of boredom!

Components of AGN

Even after decades of research, our understanding of AGN is still in primitive stagges. Optical, UV, and IR signals undergo severe losses due to intergalactic medium and our atmosphere. Radio observations for long have been our best bet to observe the extragalactic sources. But in the vicinity of the central engine, even radio waves fail. The density of matter near the core (still much farther away from the event horizon) is so large that the radio photons emitted are absorbed by the source itself, leading to a complete information blackout. Thus, we are dependant on spectrometry, as well as on reverse engineering the observational signatures to understand the composition of AGN.

1. SMBH And The Accretion Disk

The fundamental element to all AGN is the central SMBH. It is surrounded by a geometrically thin, flat, fast-spinning accretion disk formed by cold diffuse plasma. Gravitational stress and dynamic friction compress the disk inwards, heating it up. In conjunction with magnetic forces, energy and angular momentum are extracted from the disk, and the matter is sucked into the black hole. The heating of the accretion disk produces thermal emission, which peaks in optical and UV wavelengths. The accretion disk is surrounded by a blanket of hot, ionized corona, pretty much similar to our sun. Inverse-Compton scattering of the accretion disk photons by the corona leads to the production of X-ray wavelengths.

2. The Dusty Torus

The SMBH-accretion disk system is hidden from our vision by a toroidal envelope of gas and dust. Upon excitation by the optical/UV radiation from the accretion disk, this dusty torus emits Infrared radiation. The torus, has been resolved only a couple of times. Several theoretical models have been contructed, but its actual geometry and physical properties areyet to be conclusively understood.

3. AGN Jets And Outflows

Plume like jets coming out of both sides of a galaxy, that is probably the image conjured in one’s mind when AGN are mentioned. Jets are easily the most striking visual feature, and it is a treat to observe and image jetted AGN. But even of the jets, we understand very little. 

Jet creation process occurs in the vicinity of the central engine and can be broken down essentially into three main parts - extraction of plasma packets from the disk by the disk magnetic field, accumulation of the plasma packets along the axis, and acceleration of jets outwards away from the disk.

AGN structure and composition.
Figure : Different features of an AGN. (Credit: Aurore Simonnet, Sonoma State University)
Jets give us an approximate idea of the power of the central engine - stronger the engine, larger the jets. Near the center, these jets are relativistic and often appear single sided courtesy doppler shifts. As these jets move out, they are slowed down by the intergalactic medium. Low power Seyfert AGNs have tiny bubble like jets, moderate Fanaroff Riley I (FR I) classes have large structures jets. Largest jets seen are those in quasars, like the one pictured at the center of the page (Hercules A). Jets are made of magnetized plasma, and acceleration of charged particles give rise to radio emission (synchrotron mechanism, class XII stuff!). Jets dominate AGN emission in the γ-rays and often in the radio band. 

The AGN Zoo

As I have already mentioned, AGN radiation spans the entire electromagnetic spectrum, each wavelength consisting of unique observational signatures, telling us different stories. Reverse engineering these signatures has led to the identification of several distinct features - leading to simply too many classes of AGN. There are so many types of AGN that regular words like group, herd, etc., do not do justice. Currently we know of over 50 classes of AGN. There are several ways of classification – based on the observing frequencies (such as Radio galaxies), nuclear activity (eg., LINERs), brightness and luminosity (LLAGNs), variability (Blazars), difference in emission lines (Seyferts), and so on. All these classes are based on observational attributes or selection bias, and so they overlap frequently. A source might appear to be a Seyfert in optical observations and then behave like an FR in radio wavelengths. Being such a chaotic and confusing area of research, this has been aptly named a zoo. The confusion is inevitable, and anyone claiming to completely understand this zoo is deceiving themselves.


  • AGN do not continously eat matter and spewjets. AGN go through large phases of inactivity. Such AGN are said to have been switched-off.
  • Our Milky Way is said to have hosted an active nuclei thousands of years ago. With a collision with Andromeda, we may see revival of the AGN.
  • AGN are bad for health! An active galaxy will not support life due to the presence of unsurvivable amounts of radiation.
  • The oldest observed AGN is ULAS J1120+0641. Its light takes 12.9 billion years to reach us. Thus when we look at it, we are seeing the skies just 770 million years after the big bang!

AGN - The Key to Many Puzzles

Active galactic nuclei are crucial to our understanding of the life and evolution of galaxies, and of the early history of universe. Due to their immense power, AGN can be seen upto incredible redshifts (higher the redshift, older is the structure.) AGN observations thus hold clues about the early stages of the universe and the very first objects formed. AGN also influence galaxy observations, and vice versa. It has been observed that the largest of AGN are found in old elliptical galaxies; young and dimmer AGN in newer spirals. On one hand, shocks from AGN act as catalyst to star formation, while on the other hand, AGN are known to heat up the star forming gases, thus quenching star formation. Further, AGN provide a window to understanding black-hole physics - we can now calculate critical black hole param- eters by studying AGN spectral information.

In itself AGN has remained quite a mystery thus far. The very structure and composition of active galaxies has remained a matter of conjecture. Almost all galaxies host SMBH at the center, yet a very meagre percentage are active. To understand the evolutinary path of AGN, we are energetically conducting cutting-edge observations. With the advent of Very Large Baseline Interferometry (VLBI; they are simply the largest telescopes, spanning multiple continents), we are finally in a position to get out of the blind and observe the vicinity of the black hole.

Dramatic increases in our computational power have made AGN a test bed for relativistic simulations. AGN, hosting the largest of SMBHs, are the primary targets for gravitational wave astronomy. A significant amount of research on AGN has also been achieved using the Indian facilities such as the GMRT, ASTROSAT and the Himalayan Chandra Telescope (HCT). The future holds more dedicated studies of AGN which will strengthen our understanding and help solve the mysteries of the universe.

  Tags:   agn galaxies research

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