How a Galaxy Can Get Booted From Its Home

Cosmic collisions may slingshot tiny galaxies into intergalactic space.

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The round fuzzy patch shown near the core of the Great Andromeda Galaxy is the compact elliptical galaxy M32. M32 may have had a violent past, similar to that of its wandering brethren.

The cosmic world is full of runaways, including billions of planets and stars. Now astronomers can add entire galaxies to the ranks of the homeless.

A team of Russian astronomers has found evidence that compact galaxies can be gravitationally whipped out of their host galaxy cluster. The study appears today in Science.

Extremely compact elliptical galaxies are generally tight, shaped like a ball, and much smaller than most other galaxies that populate the universe, containing at most a few billion stars. Our own run-of-the-mill galaxy, the Milky Way, in contrast, is home to about 100 billion suns.

These compact elliptical galaxies are fairly rare. Until 2013, only 30 or so had been found, and they were all huddling next to giant galaxies near the center of large clusters of galaxies.

Then, two years ago, a compact elliptical was found wandering by itself, nowhere near any large galaxy. Were there more of these galactic vagabonds out in space? Or was this just a cosmic oddity?

In their new study, Igor Chilingarian, of the the Sternberg Astronomical Institute in Russia and the Harvard-Smithsonian Center for Astrophysics, and Ivan Zolotukhin, of the Sternberg Astronomical Institute and L'Institut de Recherche en Astrophysique et Planétologie in France, document eleven more of these wandering compact ellipticals, each many millions of light-years from any galaxy cluster.

"These small galaxies face a lonely future, exiled from galaxy clusters they were formed and used to live in,” Chilingarian said in a statement.

Astronomers believe that compact elliptical galaxies result from ancient collisions between galaxies. Hundreds of millions of years ago, the gravity of a massive galaxy pulled in a smaller galaxy and tore away its outer layers, leaving behind a naked and compact core.

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In this illustration of how a compact elliptical galaxy may end up wandering, a spiral galaxy (left) approaches the path of a compact elliptical (cE). The smaller galaxy receives a gravitational kick from the intruder that modifies its orbit (middle). It then leaves the system and the intruder is devoured by a giant galaxy in the cluster center (right).

A wanderer is a compact elliptical that got tossed into intergalactic space after interacting with the gravity of two other huge galaxies. In such a three-body encounter, the lightest object gets slung into space. Similar processes are thought to take place on a smaller scale near the center of the Milky Way, where a supermassive black hole can fling away one of two stars in a binary system and devour its companion.

"This is the same phenomenon, but working on a different scale,” Zolotukhin said.

This discovery, the researchers say, may aid our understanding of dark matter, which is thought to help maintain stability in most galaxies but appears to be absent in compact ellipticals.

See For Yourself

Messier 32, a prime example of one of these compact elliptical galaxies, is fairly close to Earth and bright enough to hunt down with modest equipment. The galaxy can be seen at this time of year in pre-dawn skies in the constellation Andromeda, the princess.

Visible with binoculars and small telescopes, M32 is a small companion galaxy next to the much larger and brighter spiral M31, also known as the Great Andromeda Galaxy. At 2.6 million light-years away, M31 represents the closest large galaxy to our Milky Way and is just visible to the naked eye under dark skies and a very easy target with binoculars, even from cities. Astronomers think that M32 may have been involved in a hit-and-run with M31 at around the time dinosaurs were roaming the Earth.

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During pre-dawn hours of late April, use the Great Square of Pegasus and the Andromeda constellation to track down the Great Andromeda Galaxy and its companion M32 in the low northeastern sky.

To find M32, first locate the Great Andromeda Galaxy. Start with the Great Square of Pegasus, the main stars of which form a big square low in the northeast pre-dawn sky. Locate the left corner star, then count upwards along three bright stars (within the Andromeda constellation). When you reach the third, move to the right a little, and on a good night, you will see a patch of light with your naked eye. Turn your scope or binoculars to it, and there is M31.

Look carefully below the large oval disk, and the tiny fuzzy spot tucked underneath is the elliptical companion M32. Another fuzzy patch should be visible a bit farther away, just above the the large spiral. That is another companion dwarf galaxy called M110.

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