Active Galactic Nucleus

An active galactic nucleus, or AGN, is an extremely bright central region of a galaxy that is dominated by the light emitted by dust and gas as it falls into a black hole.

An active galactic nucleus (AGN) is a small region at the centre of some galaxies that is far brighter than can be explained by the stellar population alone. The extremely luminous central region is emitting so much radiation that it can outshine the rest of the galaxy altogether. AGNs emit radiation across the entire electromagnetic spectrum, from radio waves to gamma rays. This radiation is produced by the action of a central supermassive black hole that is devouring material that gets too close to it. A galaxy hosting an AGN is called an ‘active galaxy’.

AGNs are the most luminous persistent sources of electromagnetic radiation in the Universe. This means they can be used to discover distant objects. Astronomers have also classified different types of AGN based on their observed characteristics. The most powerful AGNs are known as quasars, which give rise to extremely luminous galactic centres. A blazar is an AGN with a jet of light and energy that is pointed toward the Earth.

Webb's high resolution enables astronomers to study AGNs in great detail. One of Webb's first images was of the galaxy collection known as Stephan's Quintet, which features the active galaxy NGC 7319. This image of the moving gas near NGC 7319's AGN, made with the help of Webb's spectroscopic capabilities, maps out the velocities of the different superheated gases energised by the AGN.