Seyfert Galaxies & Double-Lobed Radio Galaxies

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  • 0:01 Active Galaxies
  • 0:46 Seyfert Galaxies
  • 1:48 The Nuclei of Seyfert Galaxies
  • 3:48 Double-Loved Radio Galaxies
  • 5:13 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will cover two particular kinds of active galaxies called Seyfert galaxies and double-lobed radio galaxies and their peculiarities, such as hot spots and synchrotron radiation.

Active Galaxies

In general, there are several kinds of different galaxies in our universe. There are the elliptical ones, lenticular, and spiral. But even within these, there are different kinds of galaxies. I mean, we can also say that in general there are several different kinds of cars. There are the sedans, SUVs, and trucks. But of course, each kind has different makes and models. So, this lesson will focus on two peculiar examples of spiral galaxies and elliptical galaxies, the Seyfert and double-lobed radio galaxies, respectively, which are both a kind of active galaxy, a galaxy whose center emits a large amount of energy.

Seyfert Galaxies

In 1943, astronomer Carl Seyfert found some barred and spiral galaxies with inconspicuous spiral arms that have a very small and very bright, point-like nucleus that fluctuates in brightness that are now termed Seyfert galaxies. Around 2% of spiral galaxies are Seyfert galaxies, and these galaxies come in two types: Seyfert 1 and Seyfert 2 galaxies.

The spectra of Seyfert galaxies, namely Seyfert 1 galaxies, contain broad emission lines. Emission lines are made by hot or excited gases that are of a low density. And because they are broad emission lines, their width indicates that the velocities at the centers of Seyfert galaxies are much higher than those found in normal galaxies, about 30 times higher! Such high velocities imply that there is a lot of crazy stuff happening at the centers of these galaxies.

The Nuclei of Seyfert Galaxies

The nuclei of Seyfert galaxies fluctuate quite a bit, particularly at X-ray wavelengths. Because a Seyfert galaxy's nucleus can change its brightness at X-ray wavelengths in a very small space of time, this means that its center must be quite small in diameter.

Again, since a Seyfert galaxy's nucleus changes brightness very quickly, it must be very small. This is because an object's brightness can change only as fast as it takes light to cross its diameter. A larger object has a larger diameter and light has a finite speed. That means it will take light longer to cross a larger diameter in order to change its brightness. Let's figure out why this is so with something closer to home.

Let's pretend that you are standing at one end of a very large room and travel at a set speed (just like light). When you reach the other end of the room, you flip a switch to change the brightness in the room up or down, your choice. As you can envision, if the room is large (it has a large diameter), it will take you much longer to cross the room. That means it will take you longer to change the room's brightness compared to a much smaller room you can cross much more quickly at any given speed.

Despite the very small size of a Seyfert galaxy's nucleus, about the size of Earth's orbit, such galaxies can nevertheless emit a hundred times more energy than our very own galaxy.

Because a ton of energy is produced in a very small space at high velocities and hot temperatures, astronomers think that the centers of Seyfert galaxies contain supermassive black holes where matter swirls around in very hot accretion disks. An accretion disk is a disk of matter spinning around a black hole.

Double-Lobed Radio Galaxies

While Seyfert galaxies are spiral galaxies, almost without exception, double-lobed radio galaxies are elliptical galaxies that emit radio radiation from two lobes that are on opposite sides of the galaxy.

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