Active Galactic Nuclei

-Preity Sukla Sahani and Nabendu Sekhar Mishra

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Our planet is situated in the Milky Way, one of the million galaxies present in the Universe. Out of these, there are some galaxies, which have a very active central region, giving radiations in electromagnetic spectrum ranging from Radio to X-Rays. These have a very compact nucleus at their centres and are called Active Galactic Nuclei or AGNs. The main driving force in an AGN is a Super Massive Black Hole at its nucleus.

The AGNs can be classified into three broad classes:

1. Seyfert galaxies:

These are the nearby Spiral galaxies having unusually bright central part or nuclei. The spectra of their nuclei are studied in detail and give us details about their nature. There are plenty of emission lines which are broadened. The broadening is due to the high rate of accretion of the super massive black hole present at the centre.

Image of M77, a Seyfert galaxy

2. Radio Galaxies:

These are generally giant elliptical galaxies. They do not generate significant radiations in optical band but giant lobes of relativistic plasma can be seen in radio bands. The radio galaxies are broadly classified as:

FR-I or Fanaroff Riley-I galaxies:

It is also called edge-darkened or centre-brightened galaxy. These show plumes.

Image of FR-I galaxy 3C31

FR-II or Fanaroff Riley-II galaxies:

It is also called edge-brightened or centre-darkened galaxy. These show lobes. The hotspots arise due to the fact that the relativistic electrons emitted from the plasma hit the interstellar medium and lose energy in the form of light.

Image of FR-II galaxy 3C98

3. Quasi-Stellar Radio Objects or Quasars:

Image of  3C273

Quasars are highly redshifted distant galaxies with central nuclei so bright, that they appear as stars. The host galaxy is comparatively very less bright and so cannot be seen easily. Their luminosities are very high as compared to Seyfert galaxies which are less violent versions of Quasars. Like Seyfert galaxies, the accretion rate is high in case of Quasars too. Some of them also show linear features like ‘jets’.

Quasar 3C273 with its jet

AGNs can be treated as one of the important fields to study in both radio astronomy and multi-wavelength astronomy.  Advancements in the technologies involving ground-based telescopes and space-based telescopes can help us know more about them.

References:

1. Arnab Rai Choudhuri-Astrophysics for Physicists
2. https://www.wikiwand.com/en/Messier_77
3. https://www.researchgate.net/figure/Quasar-3C-273-The-panels-show-radio-optical-and-X-ray-images-of-3C-273-and-its-jet_fig1_33429481
4. https://esahubble.org/images/potw1346a/
5. https://en.wikipedia.org/wiki/Radio_galaxy#/media/File:Radio_galaxy_3C31.png
6. https://en.wikipedia.org/wiki/Radio_galaxy#/media/File:Radio_galaxy_3C98.png

Written By-

Preity Sukla Sahani and Nabendu Sekhar Mishra