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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