18 July 2011

The Milky Way is a galactic cannibal

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A merger between the infant Milky Way and a smaller galaxy has been detected with the help of a new theoretical model, providing evidence that our galaxy is a barred-spiral galaxy.
galaxy (NGC1300)

An image of a barred-spiral galaxy (NGC1300) where you can clearly see the bar structure in the thin disk of the galaxy (the line through the middle of the galaxy which the spiral arms start from). The Milky Way is thought to have a bar like this in its thin disk, as well as a similar shape in the thick disk as predicted by Dr Bekki’s simulation.
Credit: NASA, ESA, and The Hubble Heritage Team STScI/AURA

GOSFORD: A merger between the infant Milky Way and a smaller galaxy has been detected with the help of a new theoretical model, providing evidence that our galaxy is a barred-spiral galaxy.

The new model simulates a merger between a smaller galaxy and the Milky Way some nine billion years ago and shed light on how the Milky Way was formed, reveals its history of devouring smaller galaxies and may strongly support a new model of the galaxy formation.

“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,” said co-author Kenji Bekki from the University of Western Australia’s node of ICRAR (International Centre for Radio Astronomy Research) of the research to be published in the Astrophysical Journal.

How were the discs formed?

The Milky Way’s shape is often referred as being like two fried eggs placed back to back, where the yolks are older stars in the centre bulge. The whites are the younger stars referred to as the thin disk, while the thick disk is a ‘puffed up’ version of the thin disk.

The physical properties of the two disks are quite different in masses, thickness, kinematics, ages and stellar metallicity. The thick disk has a smaller mass (10% of the thin disk), larger scale-height, smaller amplitude of rotation, and older and metal-poor stars.

But it was unclear as to when and how these two distinct stellar disks were formed in the history of our Milky Way galaxy.

The violent event

A new theoretical model based on numerical simulations has proposed a self-consistent scenario, whereby the thick disk was first formed by merging between the early galactic disk and a smaller galaxy about nine billion years ago. Afterwards, the thin disk slowly formed by gas accretion from the galactic halo surrounding our galaxy.

The new model explains the fundamental properties of the thin and thick disks and their differences and also predicts the presence of a giant bar about 19,500 light years across in the thick disk.

“After the last major merger event, the galaxy has not experienced such a violent event. If it had of experienced even just one such event, there would be no thin stellar disk, where we are living,” said Bekki.

A new way of testing theories

The formation and evolution of disks in galaxies is best understood by the combined powers of ‘n-body simulations’, or the numerical simulation of a dynamic system, chemical evolution models and observations.

Glen Mackie, an astronomer at Swinburne University of Technology in Melbourne, agreed with the research, saying, “It is an area rich in promise and one in which the interplay of theory and observation should allow us to improve our knowledge of galaxy formation scenarios.”

Astronomers have believed that our galaxy was shaped in this way by galactic merging for the last 30 years, but until now this hasn’t been directly testable. This new study provides a valid means of determining whether or not any merger actually occurred.

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