Types of Active Galaxies

There are three main types of active galaxies: Seyfert galaxies, Quasars, and Radio galaxies.
These galaxies are called ‘active’ due to their extraordinarily bright and fluctuating nuclei.

Seyfert galaxies, named after their discoverer Carl Seyfert, are a type of spiral galaxy.
They are characterised by bright, active nuclei producing strong emission lines in their spectrum.
This activity is thought to be caused by intense heat and light from a supermassive black hole at the centre of these galaxies which heats the surrounding gas, creating a brightly glowing accretion disk.

The term Quasar is short for ‘quasi-stellar radio source,’ as these objects appear star-like in optical observations but have strong radio emissions.
Quasars are the most luminous and powerful active galaxies, capable of emitting up to 1000 times the energy of the entire Milky Way galaxy!
They are thought to be fuelled by supermassive black holes that are actively accreting material, much like in Seyfert galaxies, but at a much more extreme rate.

Radio galaxies emit strong radio wave emissions from their nuclei.
These radio emissions are often associated with super-fast jets of material being ejected from the vicinity of the galaxy’s central black hole.
The jets can extend well beyond the visible portion of the galaxy, up to millions of light-years in length!

Studying active galaxies is key to understanding the life cycle of galaxies and the growth and evolution of supermassive black holes.
They also help in the study of cosmology, the study of the universe’s history, due to their extreme luminosity, which allows them to be seen at very large distances. This means we are looking far back in time when observing them.
Active galaxies also provide a way to probe extreme physics conditions not achievable in Earth-based laboratories.