Andromeda, we have you surrounded

The Andromeda galaxy

The Andromeda galaxy appears to be surrounded by a circle of dwarf galaxies (not visible in this image). Credit: ESA / Hubble & Digitized Sky Survey 2 / Davide De Martin (ESA/Hubble).

JUST AS BILBO BAGGINS found himself the centre of some unwanted attention from a bunch of dwarfs, the Andromeda galaxy appears to have a bunch of smaller, dwarf galaxies circling it in a single plane, according to new research. The finding, published in the prestigious journal Nature, presents a challenge to ideas of how all galaxies form and evolve.

The surprising research result reveals that around half of Andromeda’s 30-odd known dwarf galaxy satellites are orbiting the larger Andromeda Galaxy – the closest giant cosmic neighbour to our own galaxy, the Milky Way.

The international group of astronomers who discovered the cosmic curiosity include Professor Geraint Lewis from the University of Sydney’s School of Physics, and Anthony Conn, a PhD student at Macquarie University, and Dr Dougal Mackey from the Australian National University.

“Astronomers have been observing Andromeda since Persian astronomers first noted it over a thousand years ago, but it is only in the past decade that we have truly studied it in exquisite detail with the Pan-Andromeda Archaeological Survey,” said Lewis, one of the lead authors on the Nature paper.

Completely unexpected findings

“The Pan-Andromeda Archaeological Survey – cutely called PAndAS – is a large project that ran between 2008 and 2011, using the Canada-France-Hawaii Telescope situated on the Mauna Kea volcano on the Big Island of Hawaii,” explained Lewis. “Now that we’re examining the data it collected, it is providing our first panoramic view of our closest large companion in the cosmos.”

“When we looked at the dwarf galaxies surrounding Andromeda, we expected to find them buzzing around randomly, like angry bees around a hive.

Diagram showing the position of dwarf galaxies orbiting Andromeda

Left: A close up of the Andromeda galaxy. Right: Diagram showing the position of the dwarf galaxies (red dots) detected orbiting Andromeda in a single plane, in the direction of the red arrow. Credit: R. Ibata (PAndAS team).

“Instead, we’ve found that half of Andromeda’s satellites are orbiting together in an immense plane, which is more than a million light years in diameter but only 30,000 light years thick. These dwarf galaxies have formed a ring around Andromeda.”

“This was completely unexpected – the chance of this happening randomly is next to nothing. It really is just weird,” said Professor Lewis.

Not anticipated by computer modelling

Large galaxies, like Andromeda and our own Milky Way, have long been known to be orbited by an entourage of smaller galaxies. These small galaxies, which are individually anywhere from ten to at least hundreds of thousands of times fainter than their bright hosts, were thought to trace independent paths around those galaxies.

For several decades, astronomers have used computer models to predict how dwarf galaxies should orbit large galaxies, and every time they found that dwarfs should be scattered randomly over the sky. Never, in these synthetic universes, did they see dwarfs arranged in a plane like that observed around Andromeda.

“Now that we’ve found that the majority of these dwarf galaxies orbit in a [plane] around the giant galaxy Andromeda, it looks like there must be something about how these galaxies formed or subsequently evolved that has led them to trace out this peculiar coherent structure,” said Professor Lewis.

“Dwarf galaxies are the most numerous galaxy type in the universe, so understanding why and how they form this disc around the giant galaxy is expected to shed new light on the formation of galaxies of all masses.”

PhD student, Anthony Conn, whose research proved key to this study said, “It is very exciting for my work to reveal such a strange structure. It has left us scratching our heads as to what it means.”

There have been similar claims of an extensive plane of dwarf galaxies about our own Milky Way Galaxy, with some claiming that the existence of such strange structures points to a failing in our understanding of the fundamental nature of the Universe.

Adapted from information issued by the University of Sydney.

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