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Mysterious dance of dwarfs may force a cosmic rethink

THE DISCOVERY THAT many small galaxies throughout the universe do not ‘swarm’ around larger ones as bees do but ‘dance’ in orderly orbits is a challenge to our understanding of how the universe formed and evolved.

The finding, by an international team of astronomers, including Professor Geraint Lewis of the University of Sydney, was published in the prestigious science journal Nature today.

“Early in 2013 we announced our startling discovery that half of the dwarf galaxies surrounding the Andromeda Galaxy are orbiting it in an immense plane” said Professor Lewis. “This plane is more than a million light years in diameter, but is very thin, with a width of only 300,000 light years.”

The universe contains billions of galaxies. Some, such as the Milky Way, are immense, containing hundreds of billions of stars. Most galaxies, however, are dwarfs, much smaller and with only a few billion stars.

Many of the larger galaxies have dwarf galaxies circling around them. Astronomers call them satellite galaxies.

Result contradicts standard understandings

For decades astronomers have used computer models to predict how these dwarf galaxies should orbit the large galaxies, and they’d always found that the dwarfs should be scattered randomly.

“Our Andromeda discovery did not agree with expectations, and we felt compelled to explore if it was true of other galaxies throughout the universe,” said Professor Lewis.

Using the Sloan Digital Sky Survey, a remarkable resource of colour images and 3-D maps covering more than a third of the sky, the researchers dissected the properties of thousands of nearby galaxies.

An artist's impression of the orbit of dwarf galaxies about a large galaxy

An artist’s impression of the orbit of dwarf galaxies about a large galaxy. Credit Geraint Lewis. The Hubble Image Archive was used as a source of the galaxies used in this illustration.

They were surprised to find that a large proportion of pairs of satellite galaxies are travelling in opposite directions if they are on opposite sides of larger galaxy hosts, said lead author Neil Ibata of the Lycée International in Strasbourg, France. And each of the dwarfs seemed to orbiting in the same plane, or angle, around the parent galaxy.

“Everywhere we looked we saw this strangely coherent co-ordinated motion of dwarf galaxies,” said Professor Lewis. From this the astronomers have extrapolated that this phenomenon is widespread in the universe, and seen in about 50 percent of galaxies.

“This is a big problem that contradicts our standard cosmological models. It challenges our understanding of how the universe works including the nature of dark matter,” said Professor Lewis.

Keeping an open mind

The researchers think the explanation might lie in some currently unknown physical process that governs how gas flows in the universe, although, as yet, there is no obvious mechanism that can guide dwarf galaxies into narrow planes.

Some experts, however, have made more radical suggestions, including bending and twisting the laws of gravity and motion.

“Throwing out seemingly established laws of physics is unpalatable,” said Professor Lewis, “but if our observations of nature are pointing us in this direction, we have to keep an open mind. That’s what science is all about.”

Adapted from information issued by the University of Sydney.

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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Tiny galaxy with a bright nebula

Dwarf galaxy NGC 2366

The dwarf galaxy NGC 2366 might be small and dim, but it is home to a surprisingly bright, star-forming nebula—the blue patch in the top-right corner—and close enough so that its individual stars can be made out.

THE STARRY SMOG stretching across this image obtained by the Hubble Space Telescope is the central part of the dwarf galaxy known as NGC 2366. The most obvious feature in this galaxy is a large nebula visible in the upper-right part of the image, an object known as NGC 2363.

A nearby yellowish swirl is not in fact part of the nebula. It is a spiral galaxy much further away, whose light is shining right through NGC 2366. This is possible because galaxies are not solid objects. Galaxies are overwhelmingly made up of the empty space between stars.

NGC 2366 and NGC 2363 are located about 10 million light-years away. As a dwarf galaxy, NGC 2366’s size is in the same ballpark as the two main satellite galaxies of our Milky Way, named the Large and Small Magellanic Clouds. Like the Magellanic Clouds, NGC 2366’s lack of well-defined structure leads astronomers to further classify it as an irregular galaxy.

Although NGC 2366 might be small by the standards of galaxies, many of its stars are not, and the galaxy is home to numerous gigantic blue stars. The blue dots scattered throughout the galaxy speak to the burst of star formation that the galaxy has undergone in recent cosmic time. A new generation of these stellar titans has lit up the nebula NGC 2363.

In gas-rich star-forming regions, the ultraviolet radiation from young, big, blue stars excites the hydrogen gas, making it glow. NGC 2363, as well as other, smaller patches seen throughout Hubble’s image, are the latest birth sites for stellar giants.

Imaged through green and infrared filters, these nebulae take on a blue-ish tinge in this image, though their actual colour is a shade of red.

Adapted from information issued by NASA / ESA.

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Galaxy caught blowing bubbles

Hubble Space Telescope image of Holmberg II

The Hubble Space Telescope captured this image of dwarf irregular galaxy Holmberg II. The main part of the galaxy is the spread of stars in the lower left half of the image. Huge bubbles of glowing gas produced by stellar explosions dominate the galaxy; they are now sites of ongoing star formation.

HUBBLE’S FAMOUS IMAGES OF GALAXIES typically show them to be elegant spirals or soft-edged elliptical shapes.

But these neat forms are only representative of large galaxies. Smaller galaxies like the dwarf irregular galaxy Holmberg II come in many shapes and types that are harder to classify.

Holmberg II’s indistinct shape is punctuated by huge glowing bubbles of gas, captured in this image from the Hubble Space Telescope.

The intricate glowing shells of gas were formed by the energetic life cycles of many generations of stars. High-mass stars form in dense regions of gas, and later in life expel strong stellar winds that blow away the surrounding material.

At the very end of their lives, they explode in as a supernova. Shock waves rip through these less dense regions blowing out and heating the gas, forming the delicate shells we see today.

Holmberg II is a patchwork of dense star-forming regionsand extensive barren areas with less material, which can stretch across thousands of light-years.

Keck Observatory view of Holmberg II

A wider view of Holmberg II. Courtesy B. Mendez / Keck Observatory.

As a dwarf galaxy, it has neither the spiral arms typical of galaxies like the Milky Way nor the dense nucleus of an elliptical galaxy.

This makes Holmberg II, gravitationally speaking, a gentle haven where fragile structures such as these bubbles can hold their shape.

A hidden black hole?

While the galaxy is unremarkable in size, Holmberg II does have some intriguing features. As well as its unusual appearance—which earned it a place in Halton Arp’s Atlas of Peculiar Galaxies, a treasure trove of weird and wonderful objects—the galaxy hosts an ultraluminous X-ray source in the middle of the three gas bubbles in the top right of the image.

There are competing ideas as to what causes this powerful radiation—one intriguing possibility is that an intermediate-mass black hole is pulling in material from its surroundings, with the material giving off energy as it nears the black hole.

The colourful image is a composite of visible and near-infrared exposures taken using the Wide Field Channel of Hubble’s Advanced Camera for Surveys. Hubble is a project of international cooperation between the European Space Agency and NASA.

Download the Hubble wallpapers:

Holmberg II (1024×768, 588.2 KB)

Holmberg II (1280×1024, 1.0 MB)

Holmberg II (1600×1200, 1.5 MB)

Holmberg II (1920×1200, 1.8 MB)

Adapted from information issued by HEIC / NASA / ESA.

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