Groningen astronomer Amina Helmi joint discoverer of new stellar stream in the Milky Way

An international team of astronomers, including Groningen professor Amina Helmi, have discovered a new stellar stream in the Milky Way. The ‘Aquarius stream’ has been named after the Aquarius constellation in which it lies.

The stellar stream is the remnant of a dwarf galaxy in our immediate galactic environment, which was pulled apart about 700 million years ago by the gravitational force of the Milky Way. The discovery was made within the RAVE project, which is determining the characteristics and speeds of 250,000 individual stars.

The Aquarius stream, unlike previously discovered streams such as the Helmi stream, is within the galactic plain and was thus more difficult to find. Within the galactic plain, virtually all stars travel in roughly circular orbits around the centre of the Milky Way. At first sight, the stream cannot be differentiated from other stars.

With the help of RAVE data, the astronomers determined the radial velocity (the speed at which a star travels towards us or away from us) of 12,000 stars in this plain and discovered that 15 stars followed a different pattern to the others. They moved with a relative speed of up to 15,000 kilometres per hour through the rotating disc of the Milky Way.

A comparison of the star parameters with simulations revealed that these stars formed part of a larger stream from a dwarf galaxy that was attracted and swallowed up by the Milky Way about 700 million years ago. This means that the Aquarius stream is an extremely young colony in an extraordinary place – other known streams can be as much as several billion years old and are usually in the outer regions of our galaxy.

The researchers hope to discover more of this type of stream with the RAVE data. Helmi: ‘By 2012 we want to have charted the characteristics of 1 million stars in our Milky Way. RAVE provides us with information about how our Milky Way came into being. We want to know how often dwarf galaxies were swallowed up in the past, and how often this will happen in the future.’

The next major collision with a galaxy will happen in about 3 billion years, with the neighbouring Andromeda galaxy.

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Prof. Amina Helmi

RAVE is a multinational project incorporating scientists from Australia, Germany, France, the United Kingdom, Italy, Canada, the Netherlands, Switzerland, Slovenia and the United States.