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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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Star that changed the universe

Andromeda Galaxy with insets of star V1

Observations of a star in the Andromeda Galaxy that changes its brightness in a regular pattern, convinced astronomers that our cosmos was huge. Edwin Hubble's further studies of such stars showed that the universe is expanding.

THOUGH THE UNIVERSE IS FILLED with billions upon billions of stars, observations of a single star in 1923 altered the course of modern astronomy. And, at least one famous astronomer of the time lamented that the discovery had shattered his worldview.

The star goes by the inauspicious name of Hubble variable number one, or V1, and resides two million light-years away in the outer regions of the Andromeda Galaxy. V1 belongs to a special class of pulsating star called Cepheid variables, which can be used to make reliable measurements of large cosmic distances.

The star helped Edwin Hubble show that Andromeda lies beyond our galaxy. Prior to the discovery of V1 many astronomers, including Harlow Shapley, thought ‘spiral nebulae’, such as Andromeda, were part of our Milky Way Galaxy.

Others weren’t so sure. In fact, Shapley and Heber Curtis held a public debate in 1920 over the nature of these nebulae. But it took Edwin Hubble’s discovery just a few years later to settle the debate.

Hubble sent a letter, along with a light curve of V1, to Shapley telling him of his discovery. After reading the note, Shapley reportedly told a colleague, “Here is the letter that destroyed my universe.”

The universe became a much bigger place after Edwin Hubble’s discovery.

Andromeda Galaxy with an overlay of a Cepheid star light curve

Cepheid variable stars like V1 change their brightness with a regular pattern. This characteristic enables astronomers to use them to measure distances in the cosmos, by comparing their apparent brightness with their calculated theoretical brightness. Courtesy NASA, ESA, and Z. Levay (STScI), HHT (STScI/AURA), AAVSO. Acknowledgment: T. Rector (University of Alaska, Anchorage).

Cosmic distance ladder

In commemoration of this landmark observation, astronomers with the Space Telescope Science Institute’s Hubble Heritage Project partnered with the American Association of Variable Star Observers (AAVSO) to study the star.

AAVSO observers followed V1 for six months, producing a plot, or light curve, of the rhythmic rise and fall of the star’s light. Based on this data, the Hubble Heritage team scheduled Hubble Space Telescope time to capture Wide Field Camera 3 images of the star at its dimmest and brightest light levels.

“This observation is a reminder that Cepheid variables are still relevant today,” explains Max Mutchler of the Heritage team. “Astronomers are using them to measure distances to galaxies much farther away than Andromeda. They are the first rung on what astronomers call the cosmic distance ladder.”

Edwin Hubble's original photo of Andromeda

Edwin Hubble's original photo of Andromeda, showing three stars of interest marked 'N'. The one at the top became even more interesting when it was recognised as being variable (hence 'VAR'). This is Hubble's V1 star.

Copies of the photograph Edwin Hubble made in 1923 flew onboard space shuttle Discovery in 1990 on the mission that deployed Hubble. Two of the remaining five copies were part of space shuttle Atlantis’s cargo in 2009 for NASA’s fifth servicing mission to Hubble.

The most important star

Edwin Hubble’s observations of V1 became the critical first step in uncovering a larger, grander universe. He went on to measure the distances to many galaxies beyond the Milky Way by finding Cepheid variables within them. The velocities of those galaxies, in turn, allowed him to determine that the universe is expanding.

“V1 is the most important star in the history of cosmology,” says astronomer Dave Soderblom of the Space Telescope Science Institute, who proposed the V1 observations.

The space telescope that bears his name continues using Cepheids to refine the expansion rate of the universe and probe galaxies that were far beyond Edwin Hubble’s reach.

Adapted from information issued by STScI. Images courtesy NASA, ESA, and the Hubble Heritage Team (STScI/AURA). Acknowledgment: R. Gendler.

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The Andromeda apparition

THE ANDROMEDA GALAXY is a huge spiral galaxy about 2.5 million light-years away from Earth, making it the nearest big galaxy to our Milky Way.

Both Andromeda and our Milky Way are moving through space toward each other, and are expected to crash head-on in about 4.5 billion years from now.

The European Space Agency’s fleet of space telescopes has captured views of Andromeda, also known as M31, in different wavelengths. Most of these wavelengths are invisible to the eye and each shows a different aspect of the galaxy’s nature.

Visible light, as seen by optical ground-based telescopes and our eyes, reveals the various stars that shine in the Andromeda Galaxy, yet it is just one small part of the full spectrum of electromagnetic radiation. There are many different wavelengths that are invisible to us but which are revealed by ESA’s orbiting telescopes.

Starting at the long wavelength end, the Planck spacecraft collects microwaves. These show up particles of incredibly cold dust, at just a few tens of degrees above absolute zero. Slightly higher temperature dust is revealed by the shorter, infrared wavelengths observed by the Herschel space telescope. This dust traces locations in the spiral arms of the Andromeda Galaxy where new stars are being born today.

The XMM-Newton telescope detects wavelengths shorter than visible light, collecting ultraviolet and X-rays. These show older stars, many nearing the end of their lives and others that have already exploded, sending shockwaves rolling through space. By monitoring the core of Andromeda since 2002, XMM-Newton has revealed many variable stars, some of which have undergone large stellar detonations known as novae.

Ultraviolet wavelengths also display the light from extremely massive stars. These are young stars that will not live long. They exhaust their nuclear fuel and explode as supernovae typically within a few tens of millions of years after they are born. The ultraviolet light is usually absorbed by dust and re-emitted as infrared, so the areas where ultraviolet light is seen directly correspond to relatively clear, dust-free parts of Andromeda.

By putting all of these observations together, and seeing Andromeda in its many different colours, astronomers are able to follow the life cycle of the stars.

Adapted from information issued by ESA.

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Birth and death in Andromeda

M31 Andromeda galaxy

The Andromeda Galaxy, seen at several wavelengths to reveal different stages of the stellar life cycle. Infrared shows reservoirs of gas in which stars are forming. Optical shows adult stars. X-rays show the violent endpoints of stellar evolution, in which individual stars explode or pairs of stars pull each other to pieces.

  • Andromeda Galaxy is the nearest large spiral galaxy
  • Contains a strange dust ring 75,000 light-years wide
  • Infrared and X-ray views show stars forming and dying

TWO SPACE TELESCOPES have combined forces to show the Andromeda Galaxy in a new light.

Using data from the European Space Agency’s (ESA) Herschel and XMM-Newton telescopes, the image shows the light of newborn stars and X-ray emission from dying stars.

Andromeda, also known as M31, is the nearest large spiral galaxy and is similar to our own Milky Way. Both contain several hundred billion stars.

Herschel was used to produce the most detailed far-infrared image of Andromeda ever taken, showing clearly that more stars are being added to the galaxy.

Sensitive to far-infrared light, Herschel sees the clouds of cool dust and gas where stars can form. Inside these clouds are many dusty cocoons containing still-forming stars, each one pulling itself together in a slow gravitational process that can last for hundreds of millions of years.

Once a star reaches a high enough density, it will begin to shine at optical wavelengths, whereupon it will become visible to normal telescopes.

Andromeda is interesting because it shows a large ring of dust about 75,000 light-years wide encircling the centre of the galaxy. Some astronomers speculate that this ring might be a “scar” that formed after a recent collision with another galaxy.

Herschel space telescope

Artist's impression of the Herschel space telescope

The new Herschel image reveals yet more intricate details, with at least five concentric rings of star-forming dust apparent.

X-rays of stellar corpses

Superimposed on the infrared image is an X-ray view taken almost simultaneously by XMM-Newton. Whereas infrared shows the beginnings of star formation, X-rays usually show the endpoints of stellar evolution.

XMM-Newton highlights hundreds of X-ray sources within Andromeda, many of them clustered around the centre, where stars are more crowded together.

Some of the X-ray sources reveal shockwaves rolling through space from exploded stars. Others indicate pairs of stars locked in a gravitational fight to the death.

In the latter case, one star has already died and is pulling gas from its still-living companion. As the gas falls through space, it heats up and gives off X-rays.

The living star will eventually be greatly depleted, having had much of its mass torn from it by the stronger gravity of its denser partner. As the stellar corpse wraps itself in this stolen gas, it could explode.

Both the infrared and X-ray images show information that is impossible to collect from the ground because these wavelengths are absorbed by Earth’s atmosphere.

Adapted from information issued by ESA. Image credits: Infrared, ESA / Herschel / PACS / SPIRE /J. Fritz, U. Gent; X-rays, ESA / XMM-Newton / EPIC / W. Pietsch, MPE; optical, R. Gendler.

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Intergalactic pile-up, but no witnesses

The Andromeda Galaxy

The Andromeda Galaxy is the largest member of our Local Group of about 40 galaxies. Simulations suggest it formed during a merger of two galaxies.

  • Local Group of galaxies has around 40 members
  • Milky Way and Andromeda are biggest are the biggest of them
  • Andromeda and two smaller galaxies could have come from a cosmic collision

Did a major collision between two massive galaxies occur in the ‘Local Group’ of galaxies six billion years ago? Computer simulations suggest it could have.

The study—by a team of six researchers from Paris Observatory, the French National Centre for Scientific Research (CNRS), and the National Astronomical Observatories of Chinese Academy of Science (NAOC)—found that our biggest neighbour, the Andromeda Galaxy, as well as the smaller Magellanic Cloud galaxies, may well have been formed during a gigantic collision between galaxies.

The Magellanic Clouds are small, ‘irregular’ galaxies close to our Milky Way. They can be seen with the unaided eye under dark skies.

The Local Group includes nearly 40 galaxies and is dominated by two giant spiral galaxies—Andromeda (Messier 31) and our own galaxy, the Milky Way.

Many astronomers think Andromeda might have been formed through the merger of two galaxies of smaller mass. When could such a major event have occurred, and how would it have affected our neighbourhood?

The team, led by Francois Hammer from Paris Observatory, used computer simulations to model for the first time the detailed structural evolution of the Andromeda Galaxy.

They concluded that Andromeda might well have been the result of a collision between two galaxies, one slightly more massive than the Milky Way, and the other about one third as massive.

The first stage of the collision would have been about 9 billion years ago, with the final fusion slightly less than 5.5 billion years ago.

The following video shows how it might have happened.

Origin of the Magellanic Clouds

The simulations also predict that an amount of mass equivalent to one third of that of the Milky Way could have been expelled during the interaction, through the formation of gigantic tidal ‘tails’.

The Large Magellanic Cloud

The Large Magellanic Cloud, a small, irregular galaxy that orbits the Milky Way. Did it form from the wreckage of the Andromeda galaxy's birth?

Most of this matter would have been gas, and a large part of this matter would have been ejected in a particular direction…towards the Milky Way.

The researchers propose that the Magellanic Cloud galaxies formed within one of the tidal tails. They would have been ejected towards the Milky Way, at a very large velocity that has been recently re-evaluated to be one million kilometres per hour (350 km/s)!

This scenario could explain why the Magellanic Clouds are the only gas-rich and irregular galaxy companions of the Milky Way.

The researchers used the measured velocities of these galaxies to trace their positions back several billion years, and they found many solutions for which they could have originated from the Andromeda Galaxy.

If confirmed, these results may support both the hypothesis that most spiral galaxies have been formed by galactic mergers, and the prediction that many dwarf galaxies may originate from tidal tails during such events.

Adapted from information issued by the Observatoire de Paris / ESA / Hubble / NASA / R. Gendler.

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