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Atom smasher in deep space

Atoms-for-Peace galaxy

The oddly shaped and oddly named Atoms-for-Peace galaxy is actually a pair of galaxies experience a long, drawn-out merger.

  • Oddly named Atoms-for-Peace galaxy
  • It’s actually two galaxies colliding
  • Named after a speech by former US President Eisenhower

A spectacular new image of the famous Atoms-for-Peace galaxy (NGC 7252) has been released by the European Southern Observatory (ESO).

This galactic pile-up, formed by the collision of two galaxies, provides an excellent opportunity for astronomers to study how galaxy mergers affect the evolution of the Universe.

Atoms-for-Peace is the curious name given to a pair of interacting and merging galaxies that lie around 220 million light-years away. It is also known as NGC 7252 and Arp 226 and is just bright enough to be seen by amateur telescopes as a very faint small fuzzy blob.

This new, deep image was produced by ESO’s Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile.

Galaxy collisions are long drawn-out events that last hundreds of millions of years.

The picture of Atoms-for-Peace represents a snapshot of its collision, with the chaos in full flow. The results of the intricate interplay of gravitational forces can be seen in the tails made from streams of stars, gas and dust.

The image also shows incredible shells that formed as gas and stars were ripped out of the colliding galaxies and wrapped around their merged, single core.

While a lot of material was ejected into deep space, other regions were compressed, sparking bursts of star formation. The result was the formation of hundreds of very young star clusters, which are now around 50 to 500 million years old. They’re also speculated to be the forerunners of what astronomers call “globular star clusters”…vast collections of hundreds of thousands or millions of stars, formed into a spherical grouping.

Close-up of the Atoms-for-Peace galaxy's core

A Hubble close-up of the Atoms-for-Peace galaxy's core. The bluish points swirling around the core are huge clusters of hot, young stars.

Atoms-for-Peace may be a harbinger of our own galaxy’s fate. Astronomers predict that in three or four billion years the Milky Way and the Andromeda Galaxy will collide, much as has happened with Atoms-for-Peace.

But there’s no need to panic—the distance between stars within a galaxy is vast, so it is unlikely that our Sun will end up in a head-on collision with another star during the merger.

See the full-size, high-resolution version here (new window or tab)

And how did the Atoms-for-Peace galaxy pairing get its unusual name?

In December 1953, US President Eisenhower gave a speech that was dubbed Atoms for Peace. The theme was promoting nuclear power for peaceful purposes—a particularly hot topic at the time.

This speech and the associated conference made waves in the scientific community and beyond to such an extent that NGC 7252 was named the Atoms-for-Peace galaxy.

In many ways, this is oddly appropriate—the curious shape that we can see is the result of two galaxies merging to produce something new and grand, a little like what occurs in nuclear fusion. Furthermore, the giant loops resemble a textbook diagram of electrons orbiting an atomic nucleus.

Adapted from information issued by ESO / NASA /  ESA.

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Old galaxies get their second wind

NGC 4150

Images taken with the Hubble Space Telescope reveal fresh star birth in galaxy NGC 4150, 44 million light-years away. The inset shows the core of the galaxy in ultraviolet light, revealing the glow (blue) of young stars.

  • Elliptical galaxies were thought to comprise only very old stars
  • Evidence is now mounting that new stars being born within them
  • Rebirth is due to galactic cannibalism—big galaxies eating little ones

Evidence is mounting that some galaxies that were thought to be in their twilight years, are actually going through a rebirth process by forming lots of new stars.

Galaxies come in many different shapes and sizes, one of the most common being the elliptical type. Elliptical galaxies are generally very large and, unlike our Milky Way galaxy, they don’t have spiral arms.

Ellipticals were once thought to be aging star cities whose star-making heyday was billions of years ago.

But new observations with NASA’s Hubble Space Telescope are helping to show that elliptical galaxies still have some youthful vigour left, thanks to close encounters with smaller galaxies.

Images of the core of the galaxy NGC 4150, taken in near-ultraviolet light with Hubble’s sharp-eyed Wide Field Camera 3 (WFC3), reveal streamers of dust and gas and clumps of young, blue stars that are significantly less than a billion years old.

And evidence shows that the spurt of star birth was sparked when NGC 4150 merged with a dwarf galaxy.

The new study helps bolster the emerging view that most elliptical galaxies have young stars, bringing new life to old galaxies.

“Elliptical galaxies were thought to have made all of their stars billions of years ago,” says astronomer Mark Crockett of the University of Oxford, leader of the Hubble observations. “They had consumed all their gas to make new stars.”

Hubble Space Telescope in orbit

Hubble's Wide Field Camera 3 can pick out the ultraviolet light of young stars in distant galaxies.

“Now we are finding evidence of star birth in many elliptical galaxies, fuelled mostly by cannibalising smaller galaxies.

“These observations support the theory that galaxies built themselves up over billions of years by collisions with dwarf galaxies,” Crockett continues. “NGC 4150 is a dramatic example in our galactic back yard of a common occurrence in the early universe.”

Adding fuel to the cosmic fire

The Hubble images reveal turbulent activity deep inside the galaxy’s core. Clusters of young, blue stars form a ring around the centre that is rotating with the galaxy. The stellar breeding ground is about 1,300 light-years across. Long strands of dust are silhouetted against the yellowish core, which is composed of populations of older stars.

From an analysis of the stars’ colours, Crockett and his team calculated that the star-formation boom started about a billion years ago, a comparatively recent event in cosmological history. The galaxy’s star-making factory has slowed down since then.

“We are seeing this galaxy after the major starburst has occurred,” explains team member Joseph Silk of the University of Oxford. “The most massive stars are already gone. The youngest stars are between 50 million and 300 to 400 million years old. By comparison, most of the [rest of the] stars in the galaxy are around 10 billion years old.”

“We believe that a merger with a small, gas-rich galaxy around one billion years ago supplied NGC 4150 with the ‘fuel’ necessary to form new stars,” says team member Sugata Kaviraj of the Imperial College London and the University of Oxford.

The abundance of elements heavier than hydrogen and helium in the young stars is very low, suggesting the galaxy that merged with NGC 4150 was also poor in heavy elements.

This points towards a small, dwarf galaxy, around one-twentieth the mass of NGC 4150.

Magnified view of NGC 4150's core

A magnified view of NGC 4150's core. The blue areas indicate a flurry of 'recent' (less than a billion years) star birth. Most of the rest of the stars in the galaxy are about 10 billion years old.

Hubble’s vision reveals new stars

Minor mergers such as this one are more ubiquitous than interactions between hefty galaxies, the astronomers say. For every major encounter, there are probably up to 10 times more frequent clashes between a large and a small galaxy.

Major collisions are easier to see because they create incredible fireworks—distorted galaxies, long streamers of gas, and dozens of young star clusters. Smaller interactions are harder to detect because they leave relatively little trace.

Over the past five years, however, ground- and space-based telescopes have offered hints of fresh star formation in elliptical galaxies.

Ground-based observatories captured the blue glow of stars in elliptical galaxies, and satellites such as the Galaxy Evolution Explorer (GALEX), which looks in far- and near-ultraviolet light, confirmed that the blue glow came from fledgling stars much less than a billion years old. Ultraviolet light picks out the glow of hot, young stars.

Crockett and his team selected NGC 4150 for their Hubble study because a ground-based telescope analysis gave tantalising hints that the galaxy’s core was not a quiet place. The survey, called the Spectrographic Areal Unit for Research on Optical Nebulae (SAURON), revealed the presence of young stars and dynamic activity that was out of sync with the galaxy.

“In visible light, elliptical galaxies such as NGC 4150 look like normal elliptical galaxies,” Silk says. “But the picture changes when we look in ultraviolet light. At least a third of all elliptical galaxies glow with the blue light of young stars.”

Adds Crockett: “Ellipticals are the perfect laboratory for studying minor mergers in ultraviolet light because they are dominated by old red stars, allowing astronomers to see the faint blue glow of young stars.”

The astronomers hope to study other elliptical galaxies in the SAURON survey to look for the signposts of new star birth. The team’s results have been accepted for publication in The Astrophysical Journal.

Adapted from information issued by STScI / NASA / ESA / R.M. Crockett (University of Oxford, U.K.), S. Kaviraj (Imperial College London and University of Oxford, U.K.), J. Silk (University of Oxford), M. Mutchler (Space Telescope Science Institute, Baltimore), R. O’Connell (University of Virginia, Charlottesville), and the WFC3 Scientific Oversight Committee.

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Say hello to the halo

A partial view of the large spiral galaxy M81.

A partial view of the large spiral galaxy M81. Japan's Subaru telescope has studied its outskirts, looking for "fossil" remains of smaller galaxies that were devoured by M81.

  • Galaxy M81, 11.7 million light-years away
  • It’s outskirts are different to the Milky Way’s
  • Halo forms through merger of galaxies

Astronomers have used Japan’s giant Subaru telescope to study the outskirts of large spiral galaxy, in an effort to understand more about galaxy growth.

Astronomers think that large galaxies such as our Milky Way, become bigger over time by gobbling up smaller galaxies.

In the case of our Galaxy, there’s plenty of evidence for this process – several actual small galaxies have been spotted crashing into the Milky Way, attracted by its huge gravity. And swarms of stars all moving together within the Milky Way are thought to be the remnants of past episodes of galactic cannibalism.

But the “gobbling up” process would not be absolute. Many stars from the smaller galaxies would get left behind, loitering on the outskirts of the Milky Way in a region astronomers call the “halo“.

By studying galaxy halos, astronomers can learn more about these “fossil” remains of past galaxies, and thereby learn more about the process of large galaxy growth.

There are still lots of things we don’t know about the Milky Way’s halo, or galaxy halos in general for that matter. For a start, it’s hard to study our Galaxy’s halo from the “inside”, and most other galaxies are too far away to detailed observations to be made.

Japan's giant Subaru Telescope in Hawaii.

Japan's giant Subaru Telescope in Hawaii.

Outer limits

Enter Japan’s huge 8.2-metre Subaru telescope, situated in Hawaii. Astronomers have used it to make observations of the outskirts of a spiral galaxy called M81, the largest of a group of 30-plus galaxies over 11 million light-years from Earth.

They managed to identify a faint outer region to the galaxy, beyond its bright main section. They also gathered information on enough individual stars in this region to analyse its chemical properties.

They found that what they saw does not quite fit in with the conventional notion of a galaxy halo. It has basically the same type of spread of stars as the Milky Way’s halo, but overall it could be several times brighter and contain nearly twice as much matter in the form of heavy elements.

Questions raised include: does the definition of a halo need to be widened? Do M81’s outskirts have a different structure to the Milky Way’s? Is this because M81 gobbled more or different galaxies in its past, compared to the Milky Way?

Either way, it seems to be becoming clearer that the outer limits of galaxies are more complex than previously expected.

Written by Jonathan Nally, Images courtesy NAOJ.