This Huge Galaxy Has Tiny Galaxies Orbiting It, Like Planets Around a Star

A galaxy called Centaurus A has been harbouring a strange secret. Researchers have found a plane of dwarf galaxies orbiting it on a plane - much like planets do around stars, and even stars around galactic cores.
This is not only really cool - it also challenges a long-held theory that dwarf galaxies hang around larger galaxies in all directions, like bees in a hive.

Centaurus A, a galaxy around between 10 and 16 light-years from the Solar System in the southern constellation of Centaurus, is one of the most studied galaxies in the sky.

It's very close, yet outside the Local Group of galaxies; it has an active galactic nucleus, which means it's very bright and shooting huge relativistic jets into the surrounding space; and it has an unusual shape, either elliptical or lenticular.

It's also now the first galaxy outside the Local Group observed to have dwarf galaxies on an orbital plane.

Both the Milky Way and Andromeda have dwarf galaxies orbiting on a plane, but cosmological simulations based on the standard Lambda cold dark matter model predicted that smaller galaxies would be randomly scattered around larger ones, and move in all directions, making the Milky Way and Andromeda outliers.

The new observations of Centaurus A suggests that they're not.

"Cold dark matter theory made astronomers believe that the best studied galaxies in the Universe - the Milky Way and Andromeda - are the odd ones out," said researcher Helmut Jerjen from the Australian National University Research School of Astronomy and Astrophysics.
"It seems that our Milky Way and Andromeda are normal galaxies after all, and spinning pancake-like systems of satellite galaxies are more common than scientists expected."

These satellite galaxy planes do differ from planetary ones, which normally sit at the equator of the central host star. The dwarf galaxies orbiting in a plane around Centaurus A - and the Milky Way and Andromeda, for that matter - are swinging around perpendicularly to the host galaxy, around the poles of the supermassive black hole at the core.
For the Milky Way and Andromeda, the galaxy planes are pretty easy to find, the researchers said.

On Earth, you just take pictures of the sky every few years and look at where the stars are. Andromeda is around 2.5 million light-years away, and edge-on to Earth, so the perpendicular plane of the orbiting dwarf galaxies is also easy to see.

With Centaurus A, it's a little more tricky - but it being one of the most studied objects in the sky means there are certain perks, such as years' worth of archived data. And this is where the research team found information on 16 of the dwarf galaxies orbiting Centaurus A.
The Doppler shift, or light wavelength, revealed the direction the galaxies were travelling.

"We see that half of them are red-shifted, meaning they are receding from us, and the other half are blue-shifted, which tells us they are approaching," said astronomer Marcel Pawlowski from the University of California, Irvine.

This revealed that 14 of the 16 dwarf galaxies are following the same pattern of movement. Of 11 found orbiting the Milky Way, 8 are on a perpendicular plane; and of 27 found orbiting Andromeda, 15 are on a perpendicular plane.

Because the phenomenon has been observed thrice, the researchers said, randomness can be ruled out. This also suggests that it's more common than previously thought, since the standard models suggest it should only be found in 0.5 percent of nearby galaxies.

"This means we are missing something," Pawlowski said. "Either the simulations lack some important ingredient, or the underlying model is wrong. This research may be seen as support for looking into alternative models."

Lead author Oliver Müller from the University of Basel in Switzerland said that the results strengthen the idea that collisions between two large galaxies - which are more likely to have collided at some point - produce smaller dwarf galaxies from debris ejected by tidal forces.

The team's paper has been published in the journal Science.!