ASTRO THREAD : Part 1 - Active Galaxies

Disclaimer: Not an expert thread. Just an undergrad passionately sharing about things that get them excited.

That said, let's get started!

Black holes are interesting. But things get more interesting when they're present as supermassive black holes at the centre of galaxies.

(A SMBH can be millions to billions of times the mass of the Sun!)
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About 10% of galaxies are "active" i.e., they emit enormous amount of radiation that originates from nuclei (centres) that are only a few parsecs in size. Sometimes the nucleus can outshine the whole galaxy!
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Some active galaxies are 100 trillion times more luminous than the Sun.

So, Is our Milky Way active?
Of course not. If it were, we wouldn't be here.
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What makes these galaxies active?
The SMBHs in these galaxies are devouring matter in huge amounts - all the gas and dust accreted into a disk, heated up by friction and gravitational force, increasing their temperatures +
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causing it to glow in UV-optical region. These photons from the accretion disk are powered up to X-ray frequencies at the hot material cloud above the disk through a process called 'Inverse Compton scattering' .
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Image credit: Chandra X-ray observatory - Resources page

In addition to the accretion disks, the black holes power jets of ionised matter traveling at speeds close to the speed of light!
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Image credit: http://abyss.uoregon.edu 

The radiation from the jets is due to synchrotron emission (illustrated below). The emission is in radio part of the spectrum and these photons can be inverse Compton scattered to higher frequencies.
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Image credit: http://abyss.uoregon.edu 

Remember the black hole image from EHT? That supermassive black hole is located in elliptical M87 galaxy which is also an active galaxy!

Image Credits: EHT, NASA/ESA
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Active Galactic Nuclei (AGNs) are the largest population of extragalactic gamma ray sources. The image below shows all the AGNs detected by @NASAFermi till 2018. The central bright γ-ray glow is from the Milky Way's plane.
(RIP people with max bright screens)
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Understanding AGNs and the different physical processes occurring inside these galactic engines, help us understand the evolution of galaxies.

Hope you people enjoyed this thread Part 2 will focus on AGN classification and blazars.

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