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Milky Way is a galactic cannibal

Galaxy NGC 1300

The barred-spiral galaxy NGC1300 has a "bar" structure—the elongated section through the middle of the galaxy from which the spiral arms extend. The Milky Way is thought to have a bar like this too.

LATEST RESEARCH HAS GIVEN ASTRONOMERS new insight into how our Milky Way galaxy may have formed, including its history of devouring smaller neighbouring galaxies that get too close.

One such incident, the focus of this recent work, could be responsible for the shape of our galaxy.

Astronomer Dr Kenji Bekki of the International Centre for Radio Astronomy Research (ICRAR) in Perth worked with international collaborators to simulate a merger between a smaller galaxy and the infant Milky Way some nine billion years ago.

“Our computer model shows a distinct bar-shape in a portion of our galaxy called the thick disc. If observed, this bar would be clear evidence for a merger taking place in the early history of the Milky Way,” says Dr Bekki, who is based at The University of Western Australia node of ICRAR.

Side-on view of the simulated Milky Way

A side-on view of the simulated Milky Way, showing its different parts—the thin disc in blue and the thick disc in red. The green dot shows the location of the Solar System within the thin disc.

Bar—or elongated—central sections are seen in many galaxies.

The Milky Way is shaped like two fried eggs placed back to back, where the yolks are a puffy collection of older stars called the bulge. The whites are a bright collection of younger stars known as the thin disc. The thick disc is a puffed up version of the thin disc, but is ten times lighter.

Current ideas predict that the thick disc used to be shaped like the thin disc, but was ‘puffed up’ during a merger with a smaller galaxy. The thin disc we observe today was then slowly formed from other material in our galaxy.

The idea that our galaxy was shaped in this way by galactic merging has been around for about 30 years, but until now this hasn’t been directly testable. The new research provides the best avenue yet to determine whether or not the merger actually occurred.

“If our predicted bar-shape is not detected within the thick disc, then we know it can’t have formed as early as we think. We would then need some new ideas for how our galaxy came to look the way it does today,” says Dr Bekki.

“Detecting the shape of the thick disc involves working out the movement of individual stars, a lengthy painstaking process. From our vantage point within the thin disc of the Galaxy, it’s difficult for us to know exactly what shape our Galaxy is,” he adds.

ICRAR is a joint venture between Curtin University and The University of Western Australia providing research excellence in the field of radio astronomy.

The research will be published in the Astrophysical Journal on July 10, 2011.

Adapted from information issued by ICRAR. Images courtesy Dr Kenji Bekki (ICRAR) / Credit: NASA, ESA, and The Hubble Heritage Team STScI/AURA).

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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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