Galactic Cannibals

In the sky this week…

  • Mercury lies very low in the northwest after sunset.
  • Saturn is in the Southwest, still close to the star Porrima.
  • Jupiter rises around midnight and Mars about 3 a.m.
  • The Moon will be New on July 30.

Ken Tapping, July 27, 2011

In the 1840s, the Earl of Rosse in Parsonstown, Northern Ireland, made himself a telescope. It had a 72-inch (1.8 m) mirror, which was mounted in a tube 54 feet (16.5 m) long, and became known as the Parsonstown Leviathan. It was the biggest telescope in the world at the time and retained the title until 1917, when the 100-inch (2.5 m) Hooker Telescope was built in the United States.

One of the observations he made was a beautiful sketch of Messier 51, also known as the Whirlpool Galaxy. His drawing showed a lovely, face-on, spiral galaxy with a link of stars, gas and dust to a nearby, much smaller galaxy. Now we have much better images, and know that there is nothing peaceful about this picture at all.  It shows one galaxy in the process of eating another. We also now know this is how little galaxies become big ones.

This is not the impression we get with modern images of galaxies - beautiful pinwheels of stars, with pinkish blobs where new stars are being born. Instead we get the impression of a vast object moving majestically through space for billions of years, as stars, planets and probably living things come and go inside them. The reality is different; it is “dog-eat-dog” on the galactic scale too.

The large Whirlpool Galaxy (Messier 51) is known for its sharply defined spiral arms. Their prominence could be the result of the Whirlpool's gravitational tug-of-war with its smaller companion galaxy. Credit: NASA, ESA, S. Beckwith (STScI), and The Hubble Heritage Team (STScI/AURA)

The large Whirlpool Galaxy (Messier 51) is known for its sharply defined spiral arms. Their prominence could be the result of the Whirlpool's gravitational tug-of-war with its smaller companion galaxy. Credit: NASA, ESA, S. Beckwith (STScI), and The Hubble Heritage Team (STScI/AURA)

Until we realized this, our scheme of things for galaxies comprised three styles: flat spirals, like the Milky Way and the Great Galaxy in Andromeda, elliptical galaxies, like Andromeda’s companion, Messier 32, and irregular galaxies, like the Magellanic Clouds which are only visible only from the Southern Hemisphere. Then we started to see some galaxies that are oddly shaped. Some look like rings, another looks like two mice with long tails dancing around each other. These are the results of galactic collisions.

We used to think that galaxies were mostly empty space, lots of stars separated by huge distances, and that a collision between two galaxies would be an anticlimax; they would simply pass through each other with maybe a bit of distortion. Now we know about the huge amounts of gas and dust in galaxies, and then there is the dark matter and huge black holes in the middle to take into account.

Now that computer power is available, we can create simulated galaxies and cause them to collide to observe what happens. These simulations make fascinating but scary movies; there are some very good ones accessible on the World Wide Web. One shows two identical spiral galaxies approaching each other for a heavy but glancing collision. When their dust and gas clouds collide, they become very hot. Parts of the galaxies are spun off as long tails. The remnants move away from each other, but now, thanks to the colliding gas and dust clouds, they are moving more slowly, and fall back together. They proceed to smash each other to pieces, with shreds flying in all directions. In the simulation, the two central black holes collide like a bomb going off in the middle of the devastation. Finally the shreds fall back together and form into a new galaxy. More locally, our galaxy is eating the Magellanic Clouds, and the Milky Way is heading for a collision with the Andromeda Galaxy. Fortunately we have billions of years to prepare.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355
E-mail: ken.tapping@nrc-cnrc.gc.ca

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