New Scientist

20 MAY 1998

Mistaken Identity: A Family Of Galaxies Masquered As Stars In Our Own Back Yard

Galaxies so compact that astronomers mistook them for single stars in our own Milky Way have been found nearby, astronomers say. "It's a bit embarrassing having them so close and not knowing they were there," Michael Drinkwater, an astronomer from the University of New South Wales told this month's ScienceNOW!, an Australian National Science Forum in Melbourne.

There may be hundreds of thousands of the new galaxies lurking nearby, Drinkwater says. If so, they could be the long-lost relatives of a population of blue galaxies known to lie in the distant Universe.

Drinkwater, along with a team of colleagues in Britain and the US, made the discovery with the help of an instrument called the 2dF (Two degree Field) facility on the 3.9 metre Anglo-Australian Telescope at Siding Spring Observatory in northern New South Wales. Normally astronomers would take about two hours to collect the light of an object such as a faint galaxy, but the 2dF can do 400 objects at once, or about 2000 during a night's viewing.

The team looked at 1000 objects in an area of the sky about the size of 16 full moons. The objects were thought to lie within our own Galaxy, which is about 100 000 light years across.

"Based on photographic surveys of the sky, people have assumed that these light sources were stars," Drinkwater says. "Until now, no one has had the time to measure them properly."

By analysing the spectra of the objects, the astronomers could work out their distances. It turned out that seven of the objects lie about 0.5 to 2 billion light years away, well beyond our Galaxy.

Drinkwater says their complex light emission is the telltale sign of so-called blue galaxies, composed of hot, young stars that emit blue-white light. "They have very active star formation too, so maybe more stars have been formed in the recent history of the Universe than we thought," he says.

The astronomers estimate that some of the galaxies are far smaller than the Milky Way. While most seem about 10 000 light years wide, some are as small as 5000 light years across. "They are very compact galaxies," says Drinkwater.

The new discoveries could help resolve a long-standing puzzle about the distribution of galaxies in the Universe, Drinkwater adds. Astronomers had found blue galaxies like these before, but only in very distant parts of the Universe as far away as five billion light years. Their relatives in the nearby Universe had seemed puzzlingly elusive ("Mystery of the missing blue blobs", Science, New Scientist, 15 July 1995, p 17).

Drinkwater and his colleagues estimate that an extensive survey covering the whole sky could well eventually find another 150 000 to 200 000 similar galaxies lying up to 2 billion light years away, yet masquerading as stars in our Galactic back yard. He suspects that these could make up most of the shortfall.

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