AGN: The Cosmic Monster That Feeds on Galaxies — From Black Holes to Feedback

 

Source: https://www.researchgate.net/figure/The-anatomy-of-an-AGN-as-according-to-the-standard-unified-model-Figure-from-Urry_fig2_269116761

 

AGN: The Cosmic Monster That Feeds on Galaxies:

AGN is like a cosmic monster that lives in the center of the galaxy. It eats up gas and dust, and when it does it releases a lot of energy — which affects the entire galaxy: the life of the galaxy, its growth, or the formation of new stars all depend on the behavior of this monster.

At the center of every active galaxy (AGN) is a supermassive black hole (SMBH), the mass equivalent to millions or billions of Suns, but unless gas or dust falls into it, due to gravity, the black hole cannot become an AGN. When matter or dust falls into it, it will release energy and matter and due to the release of energy and matter, that black hole is called an active black hole and that system is called an AGN. When the black hole throws matter and energy into the environment, it will influence the environment or we can say that that matter and energy will feed into the environment of the galaxy with matter and energy and this process is called AGN feedback. This feeding will affect the rate of star formation.

AGN feedback:

• Positive feedback (rare): Sometimes jets compress gas clouds, which increases star formation.
• Negative feedback (common): Radiation, winds or jets expel the gas from the galaxy, then there is no gas left so star formation stops. This causes the galaxy to "quench out" (no new stars are formed).

Black Hole + Accretion disc+ brightness+ high jets+ nearby gas and dust == AGN

A black hole is surrounded by gas, dust and stars. These gases, dust and stars orbit the SMBH and fall into it. The material orbiting the SMBH is shaped like a disk and this shape is called an accretion disk. The spinning disk gets very hot due to friction and gravity.

Main components of AGN:

1. Accretion Disk: main source of Optical/UV light
2.  Corona: emits X-ray radiation
3.  Broad Line Region (BLR): Gas clouds that near to the SMBH and move fast and emits broad emission lines.
4.  Dusty Torus: absorbs the light of the accretion disk and emits in IR
5.  Narrow Line Region (NLR): Gas clouds that are slightly far away from SMBH and move slowly and narrow emission lines.
6.  Jets: Some AGN, like radio galaxies, also form high-speed jets. but not in all AGN.  

Source: https://en.wikipedia.org/wiki/Active_galactic_nucleus

Q.1 But just having a black hole doesn’t make an AGN active. So what does it take for an AGN to be active?

AGNs are not active just because they have a black hole at their center. If no matter falls into the SMBH, the SMBH will remain in inactive/sleeping mode. For it to remain active, matter must continue to fall into the SMBH from the accretion disk. There are also AGNs that have black holes or super massive black holes at their cores, but they do not have enough gas to accrete or if they do, the matter falls into the accretion disk at a very low rate, so they produce very little light/energy. These types of black holes are called inactive super massive black holes.

Our galaxy has a super massive black hole called Sagittarius A. This SMBH is now mostly inactive. This is because the accretion rate of this SMBH is very low.

Accretion rate: How much matter (gas, dust, etc.) flows into an object like a star or black hole over time.

Q. 2 Where does AGN get its power from? / Where does the energy come from?

Ans.: The AGN gets its power from its SMBH because the SMBH consumes matter from the accretion disk and powers the AGN. 

A supermassive black hole does not emit light directly, but the material falling into it releases a huge amount of energy - which lights up the entire AGN system. This energy escapes the core of the AGN in three main forms:

1. Radiation(X-rays, UV, IR): Which radiates in all directions from the accretion disk and ionizes the gas, heating the center of the galaxy; 
2. Relativistic jets: Which emerge as high-speed beams from the poles of the SMBH, mostly seen in radio-loud AGNs, and reach the intergalactic medium.
3. Winds and outflows: Which escape from the disk and torus in spherical or conical shapes, pushing gas out of the galaxy, which stops star formation. Whether this energy is bad or not plays an important role in the evolution of the AGN system and the galaxy.

When the gas and dust particles in this accretion disk heat up while revolving around the black hole, this disk shines in UV and optical light and X-rays and gamma rays are emitted from this disk and some of its radiation is also emitted in the IR when the dust absorbs this energy and re-emits it. That is why we have to see AGN in the entire EM spectrum. We cannot study it completely with any one light.

 

Table 1: AGN Classification Based on Orientation, Jet Morphology, and Optical Properties (Unified Model Framework)

AGN Type	FR Class	Excitation Class	Jet Orientation	Optical Signature	Obscuration	Unified With
Radio Quasars	FR II	HEG	Small angle (≲ 45°)	Strong, broad lines	High	FR II/HEG RGs
Radio Galaxies	FR I/II	HEG/LEG	Large angle	Narrow or no lines	High (HEG), Low (LEG)	Quasars or BL Lacs
BL Lacs	Core-dominated	LEG	Small angle	Weak/none	Very low	FR I/LEG RGs
Seyferts/Quasars	—	HEG	Type 1 AGN	Strong, broad lines	—	—

Table 2: AGN Classification Based on Multiwavelength Observations and Physical Characteristics

AGN Type	Main Features	Strongly Detected In	Key Physical Indicators
Seyfert Galaxies	Bright nuclei, strong emission lines (Type 1: broad + narrow, Type 2: narrow only)	Optical, IR, X-ray	Presence of obscuring torus; orientation-based differences
Quasars (QSOs)	Extremely luminous AGN, can outshine host galaxies	Optical, UV, X-ray, IR	High accretion rates; seen at high redshifts
Blazars	AGN with jet pointed nearly at Earth; highly variable and polarized	Radio, γ-ray, X-ray, Optical	Strong relativistic jets; fast variability
Radio Galaxies	AGN with strong radio jets/lobes	Radio, Optical	Jet emission; radio-loud systems
LINERs	Low-ionization nuclear emission-line regions	Optical	Weakly ionized emission lines; debated AGN status
Obscured AGN	Hidden by dust; weak optical but bright in IR/X-rays	IR, X-ray	Strong absorption; often missed in optical surveys
X-ray AGN	Strong X-ray sources, including obscured and unobscured types	X-ray	Emission from corona; useful for finding hidden AGNs
γ-ray AGN	AGN with strong high-energy emission (mostly blazars)	γ-ray	Relativistic jet physics; extreme acceleration processes
Infrared AGN	Detected via dust-reprocessed light in IR	Infrared	Obscured accretion; star formation may contaminate signal
Variability-Selected AGN	Identified by rapid or long-term changes in brightness	Optical, IR, X-ray	Small size of emitting region; probes accretion behavior

Table 3: Fanaroff-Riley (FR) and Related Radio Galaxy Morphological Classification

Type	Brightness Location	Jet Structure	Radio Power	Notes
FR I	Core-brightened	Jets fade outward	Lower	Common in clusters
FR II	Edge-brightened	Bright hotspots at ends	Higher	Strong, well-defined lobes
FR 0	Core-only	No extended emission	Lower	Compact, very core-dominated
WAT/NAT/X	Varying	Bent or X-shaped jets	Varies	Environmental effects

Source: Active Galactic Nuclei: what's in a name?