Cannibal galaxy leaves halo to mark mealtime

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Astronomers from Birmingham and Southampton universities have discovered traces of an act of intergalactic cannibalism - where one large galaxy swallows several smaller neighbours, leaving only a halo of hot gas like the smile on the Cheshire cat.

The hot gas represents the last relict of the smaller galaxies which have been absorbed by the larger one. But, according to Dr Laurence Jones of Southampton University, this 'fossil' halo is too large to be held in place by the gravitational pull of the stars alone.

The previously undiscovered galaxy is given the formal scientific name of RX J1340.6 + 4018 in today's issue of the journal Nature. 'But we call it Trevor's fossil,' Dr Jones said, in honour of Dr Trevor Ponman, from Birmingham University, the leader of the team that found the galaxy.

Their discovery, made with researchers from Finland and Hawaii, adds to the growing body of evidence for the existence of 'dark matter' - which differs from the atoms and particles which make up the ordinary world here on Earth but which may account for more than nine-tenths of the composition of the cosmos.

The astronomers believe that only dark matter - which betrays its presence by its gravitational pull - can be responsible for keeping the halo of gas in place. The action of gravity is also responsible, they believe, for making the gas so hot that it emits X-rays rather than ordinary visible light.

The galaxy and its surrounding halo are more than a billion light years away from Earth. They were detected using two optical telescopes - one owned by the University of Hawaii on Mauna Kea in the Pacific and the Nordic telescopes on La Palma in the Canary Islands - and the X-ray satellite Rosat. The Earth's atmosphere absorbs X-rays and so astronomers can only conduct observations at such wavelengths by sending satellites out into space.