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Paris - Astronomers said on Tuesday they have examined the remotest parts yet of an elliptical galaxy, Centaurus A, and discovered its starry halo was bigger, more lopsided and richer in heavy elements than expected.
The Hubble Space Telescope, a joint project of Nasa and the European Space Agency, scanned the outskirts of the giant galaxy from its position in low Earth orbit, using ultra-sensitive onboard cameras.
At a distance of over 12 million light years, Centaurus A is nevertheless one of the closer galaxies to Earth.
Hubble mapped a region about 450 000 light years of the galaxy's length, and about 295 000 light years of its width - and it still hasn't reached the halo's outer edge.
"These are large distances if you consider that the main visible component of the Milky Way is around 120 000 light years in diameter," said an ESA press statement.
Galaxies have a bright, glowing centre surrounded by swirling, spiral arms or a disk-shaped fuzz made up of dust, gas, stars and dark matter, which are in turn surrounded by dim halos of stars.
Little is known about these halos because of their faint and spread-out nature.
Now Hubble has revealed that Centaurus A's halo extends much deeper into space from the galaxy's centre than had been believed, and that it has a weird shape.
"We found more stars scattered in one direction than the other, giving the halo a lopsided shape - which we hadn't expected," said study co-author Marina Rejkuba of the European Southern Observatory in Garching, Germany.
"Tracing this much of a galaxy's halo gives us surprising insights into galaxy formation, evolution and composition," she added.
Using the Hubble data, the research team also found elements heavier than hydrogen and helium in the gas that made up the stars within the halo.
The stars in halos of the Milky Way and other nearby spiral galaxies are generally low in heavy elements.
The study was published online in Astrophysical Journal.
