Seyfert Galaxies

Seyfert Galaxies are among the biggest and brightest galaxies in our universe. They make up a small minority of galaxies, about 1 in 10.

Similarly to quasars, these galaxies have an active galactic nucleus which makes up a large portion of the galaxies’ overall brightness. In Seyfert galaxies, the nucleus’s brightness is comparable to the brightness in the rest of the galaxy (stars etc), however in a quasar the nucleus overpowers the rest of the galaxy immensely.

The Two Types

The Great Barred Spiral Galaxy. Credit: ESO.

Type I Seyfert galaxies have a lot of broad lines in their spectra, usually from hydrogen I, helium II, and sometimes Oxygen III. A large portion of their output light is in the ultraviolet and x-ray bands.

Type II galaxies are very bright in the infrared, and contain no broad spectral lines, only narrower ones. Some of these are forbidden lines, caused by emission of light when electrons jump unfavourable energy levels. Don’t worry, lots of knowledge on forbidden lines isn’t needed.

Astronomers still to this day do not know why these galaxies split into these two types. However, there are many Seyfert galaxies in an intermediate stage between the two. We’ve seen these galaxies, over the course of a few years, transition to type II.

This may suggest that these galaxies initially are type I, then move onto type II as they burn through more fuel.

What do they look like?

To put it shortly: they look like galaxies. Many of them are spiral galaxies, which may suggest that our regular spiral galaxies used to be Seyferts.

A lot of these Seyfert galaxies are distinctly unique, from having a slightly displaced black hole (NGC 5033) to centres which rotate at relativistic speeds (The Great Barred Spiral Galaxy).

Here’s two of my faves.

NGC 4151 is a Seyfert galaxy in Canes Venatici. It has not one, but TWO supermassive black holes in its centre. And guess what?

Its unofficial name is the Eye of Sauron!

Eye of Sauron in X ray (blue), Optical (gold), and Radio (red). Credit: VLA/NASA/CXC/CfA

Messier 106 is a wild one! Whilst the Active Galactic Nucleus is an interesting feature, M 106 is also a megamaser, and one of the first discovered! M106 is one of the brightest galaxies known to astronomers, and its supermassive black hole is larger than the Milky Way’s by a factor of 10!

Radio Image (purple) stacked onto an IR + optical image of M106. Credit: NASA/CXC/ Uni of Maryland.

The makeup of M106 is (and has been) extremely important to the progress in Astronomy. This Seyfert galaxy is full of Cepheid Variables (stars with very regular luminosity dips), which enabled us to make distance calculations to nearby galaxies with fewer errors.

Unlike other nearby galaxies, M106’s Variable stars were similar in metallicity to the ones in the Milky Way This meant we could make accurate distance calculations using distance ladder.

Also, the presence of a water-vapour megamaser meant distance ladder calibration could be skipped entirely, and lead to the first ever direct measurement of distance to a galaxy! M106’s megamaser is bright purple from the water vapour.

Optical + IR image of M106. Credit: NASA/ESA/HST


That’s all for this week. I’m not sure what will next week’s post will be on, however I’m feeling like we need some more maths!

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