SMBH located at the center of a active galactic nuclei and this SMBH gives power to the AGN. but SMBH is surrounded by Accretion Disk. In this Disk, gas and dust are present and these gas revolving around the SMBH in accretion disk plane. due to high gravity of SMBH, these gas and dust get attracted and fall into the SMBH. SMBH absorb these matters and emit lots of matter and energy in the surrounding atmosphere that affect the start formation rate. This accretion disk is glowing because of the dust and gas are moving with very high speed and producing intense radiation.
This Accretion disk is surrounded by donuts shaped cloud of hot dust and gas and this surrounded region is called dusty torus. This dusty tours region absorb those lights, who came from the accretion disk, after absorbing light, this torus re-emit in infrared radiation.
Source: https://fermi.gsfc.nasa.gov/science/eteu/agn/
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The SMBH at the center of the galaxy is surrounded by dust, like a lamp covered in fog. Just like we can't always see it directly using visible light, so we can't study SMBH using optical light because optical light cannot penetrate through this dust. But what if we study through the heat emitted by the lamp instead of optical light, similarly we can sense the heat of the gas and dust present in the AGN so that we can know about the SMBH. To study this heat, we need infrared (IR) light - a type of light that can penetrate dust and gas and reach us. And this will help us to find and study the AGN.
IR Spectrum Regions For clarity, they split IR into:
- Near-IR (NIR 1–3 μm): starlight dominates.
- Mid-IR (MIR 3–50 μm): the best for detecting AGN!
- Far-IR (FIR 50–500 μm): dominated by star formation in the galaxy.
Earth’s atmosphere blocks mid-IR, we use space telescopes like: Spitzer WISE AKARI.
Source: https://www.researchgate.net/figure/The-geometry-of-the-AGN-assumed-in-unified-models-where-the-classifica-tion-of-the_fig4_237788999
In Simple way, Sources (origin) of IR radiation in AGN:
- Dusty Torus: The UV/optical light emitted from the accretion disk is absorbed by the dust present in the torus and re-emitted in IR light. it's tells us counting or seeing AGN hidden by dust (obscuration), and the size/temperature of the torus.
- Polar Dust Winds: Dusty Winds: The radiation pressure that comes from the accretion disk pushes dust particles upwards, i.e., in the direction of the pole. This dusty gas is called dusty polar wind. It shines in IR light because the dust is heated. Both dusty polar winds and jets emanate from the poles, but jets are like laser beams, and dusty winds are like smoke rising — softer, slower, and infrared bright. Some areas also have dusty winds that emanate from the poles, except dusty Torus. it's tells us that the dusty polar wind provides proof that other dust structures exist besides the torus.
- Host galaxy ke dust clouds: Star formation dust or galactic dust means both dusts also produce IR. it's tells us Properties of the AGN's host galaxy, such as star formation rate.
- and other things.
- Obscuration level: Is there dust in the center of the AGN that blocks optical/x-ray light?
- What is the structure of a torus?
- Can IR help detect Type 1 vs Type 2 AGN, since IR can pass through dust?
- it can tells about Luminosity of AGN.
- Spectrum analysis can determine whether the light is coming from AGN or from star formation in the galaxy. what is rate star formation?
- High redshift AGN: We can find out about distant AGN whose optical light gets redshifted and becomes visible in IR.
- Some special emission lines are also found in IR. These lines tell us: whether the AGN is active or not ? how much ionization is taking place ? whether there is star formation in the host galaxy or not ?
- many other things.
Eddington ratio: The Eddington ratio tells us how actively a black hole is absorbing energy.
Eddington ratio: Actual brightness of the AGN/Maximum possible brightness.
So: High Eddington ratio → black hole is feeding fast → very bright AGN.
Low Eddington ratio → black hole is feeding slowly → fainter AGN.
IR surveys are better at finding bright AGN but miss many faint AGN that are still real, just less active. This is called a selection bias. It means that when we use IR to find AGN, we mostly detect: Big, powerful black holes.
Infrared (IR) surveys often only detect AGNs that are very bright – they are mostly in large, massive galaxies where a black hole is actively feeding the dust. But there are also many AGNs in the universe that are in smaller galaxies or where a black hole is feeding slowly – these are called low-mass galaxies and low-accretion AGNs. These AGNs don’t shine very brightly in the IR, so surveys miss them. So it is said that we are only seeing the “tip of the iceberg” – a large part of the real AGN population is hidden.
The other thing is that previously scientists thought that the dust surrounding AGNs was simply contained in a doughnut-shaped torus. But now new evidence suggests that some of the dust is not in the torus, but is coming out in the direction of the pole – like smoke rising from a chimney. We call this the polar dusty outflow. This discovery shows that the structure of AGN is more complex than previously thought – not just a torus, but also a cone-shaped outflow from which dust emits IR light.
Comparison with light:
- IR is better than optical light at detecting obscured AGN (because IR can pass through dust).
- But X-rays are better for heavily obscured AGN, and IR may miss dim or dustless AGN.
- Sometimes, some are not AGNs; they look like AGNs in the IR.
- IR light helps us discover the earliest AGNs in the universe!
Telescope:
- Ground-based Giants: GMT, TMT, ELT, TAO will see deeper into the IR from Earth.
- Space Missions: JWST, SPICA, Euclid & WFIRST