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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.

Galaxy record broken again

A cluster of galaxies nearly 10 billion years old

Though this cluster of galaxies (the red dots) is nearly 10 billion years old, it has a surprisingly modern appearance.

Using NASA’s Spitzer Space Telescope, a Texas A&M University-led team of astronomers has uncovered what may very well be the earliest, most distant cluster of galaxies ever detected.

The group of roughly 60 galaxies, called CLG J02182-05102, is nearly 10 billion years old — born just 4 billion years after the Big Bang.

This follows close on the heals of another distant galaxy cluster discovery.

However, it’s not the size nor the age of the cluster that amazes the team of researchers led by Dr Casey Papovich, an assistant professor in the Texas A&M Department of Physics and Astronomy. Rather, it’s the surprisingly modern appearance of CLG J02182-05102 that has them baffled — a huge, red collection of galaxies typical of only present-day galaxies.

“It’s like we dug an archaeological site in Rome and found pieces of modern Rome amongst the ruins,” explains Papovich.

While its neighbouring galaxies appear vastly smaller and far fainter, Papovich says CLG J02182-05102 stands out as a densely-populated bundle of ancient galaxies. Enormous, red galaxies at the centre contain almost 10 times as many stars as our Milky Way, he notes, combining for a total size which rivals that of the most monstrous galaxies of our nearby Universe.

Before now, Papovich says, such a finding would be considered by many astronomers to be highly unlikely, considering the time frame in which they were found.

“The predictions are that these things should be very rare when the Universe was 4 billion years old, and yet, we found them,” Papovich explains. “Not only did we find them, it looks for all intents and purposes like they had already formed completely and evolved into the large concentrations of galaxies that we see in clusters today.”

Exactly why these particular galaxies are fully formed that early is what Papovich and his collaborators — which include astronomers from NASA’s Jet Propulsion Laboratory at the California Institute of Technology (Caltech) as well as Carnegie Observatories — hope to one day uncover. But for now, studying CLG J02182-05102 could help them and other researchers better understand how galaxies form and cluster in general.

Galaxies with a rock ‘n’ roll lifestyle

Artist's impression of the Spitzer Space Telescope

Artist's impression of the Spitzer Space Telescope

The find resulted from a project initiated two years ago when Papovich and his team observed an area of the sky that could encompasses 250 full Moons, the largest extragalactic survey of space ever made — the Spitzer Wide-area InfraRed Extragalactic (SWIRE) survey.

The team focused on a cosmic region of the survey that previously had been observed by other instruments including Japan’s Subaru telescope on Mauna Kea, Hawaii, and the European Space Agency’s orbiting XMM-Newton telescope.

This, combined with infrared data from the United Kingdom Infrared Telescope — also in Hawaii — and Spitzer’s Public Ultra Deep Sky survey instantly revealed a number of distant galaxies.

It wasn’t until Papovich’s group studied faint light from CLG J02182-05102’s least-dim galaxies that they were able to determine they had found a cluster that contained about 60 galaxies full of old, red stars, at a time when the Universe was only 4 billion years old — about 30 percent of the Universe’s current age of 13.7 billion years.

At that point in time, most other galaxies would still have been forming their very first stars and certainly would not have congregated with other galaxies yet.

In essence, Papovich said the galaxies in CLG J02182-05102 must have subscribed to a rock ‘n’ roll lifestyle: They lived fast and died young. It’s another mystery Papovich hopes to solve through deeper observations, including spectroscopy, with the Hubble Telescope later this year.

“That’s one of the reasons this is so interesting,” he adds. “It seems that they somehow had a premonition they would end up in these big clusters, so that’s another thing we want to find out.”

Adapted from information issued by Texas A&M University / NASA / JPL-Caltech / C. Papovich (Texas A&M University) / Adam Hadhazy.

Galaxy distance record smashed

A cluster of galaxies 9.6 billion light-years away

Astronomers have spotted galaxies 9.6 billion light-years away (circled). The arrows indicate galaxies that are likely located at the same distance, clustered around the centre of the image. The contours indicate X-ray emission coming from the cluster. This false colour image covers an area of the sky about 1/10th the size of the Moon.

A team of astronomers from Germany and Japan has discovered the most distant cluster of galaxies known so far — 9.6 billion light-years away.

The X-ray and infrared observations showed that the cluster hosts predominantly old, massive galaxies, demonstrating that the galaxies must have formed earlier than 9.6 billion years ago, ie. when the universe was still very young.

These and similar observations therefore provide new information not only about early galaxy evolution but also about history of the universe as a whole.

Clusters of galaxies are the largest “building blocks” in the universe. Our galaxy, the Milky Way, is part of the Virgo cluster, comprising some 1,000 to 2,000 galaxies.

By observing galaxies and clusters that are very distant from Earth, astronomers can look back in time, as the galaxies’ light was emitted a long time ago and took millions or billions of light-years to reach the astronomers’ telescopes.

Invisible to the naked eye

Astronomers had to use infrared wavelengths, invisible to the naked eye, because the expansion of the universe — which forces distant galaxies to have large velocities — shifts their light from visible to infrared wavelengths.

The Multi-Object Infrared Camera and Spectrometer (MOIRCS) at the Subaru Telescope detects near-infrared wavelengths, at which the galaxies are most luminous.

“The MOIRCS instrument has an extremely powerful capability of measuring distances to galaxies. This is what made our challenging observation possible,” says Masayuki Tanaka from the University of Tokyo.

The Subaru Observatory

The Japanese Subaru Observatory, located in Hawaii.

“Although we confirmed only several massive galaxies at that distance, there is convincing evidence that the cluster is a real, gravitationally bound cluster.”

That the individual galaxies are indeed held together by gravity is confirmed by observations in a very different wavelength band. The gas between the galaxies in clusters is heated to extreme temperatures and emits light at much shorter wavelengths. The team therefore used the XMM-Newton space observatory to look for this radiation in X-rays.

“Despite the difficulties in collecting X-ray photons … we detected a clear signature of hot gas in the cluster,” explains Alexis Finoguenov from the Max Planck Institute for Extraterrestrial Physics.

Record smashed by 400 million light-years

The combination of these different observations led to the pioneering discovery of the galaxy cluster at a distance of 9.6 billion light-years — some 400 million light-years further into the past than the previously most distant cluster known.

An analysis of the data collected about the individual galaxies shows that the cluster contains already an abundance of evolved, massive galaxies that formed some two billion years earlier.

As the processes for galaxy aging are slow, the presence of these galaxies means the cluster must have come about through the merger of massive galaxy groups, each nourishing its dominant galaxy.

The team is continuing the search for more distant clusters.

Adapted from information issued by MPE /NAOJ / Subaru.