A Viking galaxy found by Hipparcos shows how the Milky Way grew
05 November 1999A dozen ancient stars, scattered all over the sky, are survivors from a small galaxy that invaded the Milky Way like a shipload of Vikings. The European Space Agency's Hipparcos satellite, which measured the motions of many thousands of stars, enabled astronomers from Leiden in the Netherlands and Garching in Germany to make this astonishing discovery. It provides clear evidence in favour of the theory that great assemblies of stars, like the Milky Way Galaxy where we live, grew by the amalgamation of smaller galaxies.
Like archaeologists reconstructing a ship from stray wreckage, the Leiden-Garching team visualize a galactic intruder containing about 30 million stars, similar to dwarf galaxies that still exist in the Milky Way's vicinity. The invader entered our Galaxy about 10 billion years ago, long before the birth of the Sun and the Earth 4.6 billion years ago.
"This was not an accidental discovery," says Amina Helmi of Leiden Observatory. "We calculated that a dwarf galaxy colliding with the Milky Way would break up, over a very long period, into streams of stars retaining a common memory of their origin. We looked for them using the Hipparcos data, and there they were!" The stars in question are among the oldest in the Sun's vicinity, identified by their chemical simplicity. They are called halo stars because they spend most of their time in a sparsely populated halo around the Galaxy. The Leiden-Garching team looked for groups of halo stars travelling on nearly identical tracks through space. Whilst the stars' speeds towards or away from the Sun were already well known from ground observations, measurements of their sideways movements (proper motions) by Hipparcos were essential for picturing their paths in 3-D.
Two coherent streams appeared in the analysis, one represented by nine stars and the other by three. The streams are travelling in opposite directions at 250 kilometres per second. Their tracks slant at about 60 degrees across the band of the Milky Way itself, which marks the crowded disk of the Galaxy inhabited by many younger stars, including the Sun.
Cosmologists used to imagine that large galaxies were born large, but support has switched to the idea that they grew from smaller galaxies by collisions. This "hierarchical" picture fits better with current theories that try to explain how a complex and beautiful Universe grew from the primeval furnace of the Big Bang. ESA's Planck mission (2007) will use radio microwaves to measure small irregularities in the Big Bang that apparently grew into galaxies. The proposed NASA-ESA Next Generation Space Telescope (2008) will look by infrared light for the first stars in the precursors of galaxies. Meanwhile, Hipparcos has opened the way to a different method of investigating galactic origins.
"To find out how galaxies were born, 10-12 billion years ago, we now have two choices," says Simon White of the Max-Planck-Institut f|r Astrophysik in Garching. "We can use large telescopes to push to the limits of visibility, peering 10 billion light-years out into space and back in time. Or we can search for 10 billion-year-old fossils in our own backyard. The old galaxy that we have rediscovered is just the first of many fossils waiting to be found." Amazing opportunities for galactic antiquarians would come from a proposed successor to Hipparcos now under consideration by ESA, for a possible launch around 2010. Called Gaia, it could chart a billion stars and find accurate distances and motions across a large volume of space, including most of the Milky Way Galaxy. A member of the Leiden-Garching team, Tim de Zeeuw of Leiden, is busy also with the science studies for Gaia.
"Human life can be seen as a by-product of the history of the Milky Way," de Zeeuw comments. "So we want to understand exactly how our Galaxy formed, evolved and survived, as a possible home for life. Hipparcos has given us a glimpse of the possibilities. With Gaia we could transform the apparently random scatter of ancient and middle-aged stars into identified families with shared origins. For example, we might find long-lost sisters of the Sun."
The discovery of the old star-streams is announced in the journal Nature on 4 November. The reference is: Amina Helmi, Simon D.M. White, P. Tim de Zeeuw and HongSheng Zhao, "Debris streams in the solar neighbourhood as relicts from the formation of the Milky Way," Nature, Vol. 402, pp. 53-55, 1999.
Contacts for press enquiries:
Amina Helmi, lead author, Leiden
Simon White, co-author, Garching
Tim de Zeeuw, co-author, Leiden
Michael Perryman, Hipparcos project scientist, Noordwijk