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European astronomers have discovered a true space oddity: a quasar without a detectable home galaxy. The discovery has led to speculation that the host galaxy could be made up entirely of dark matter, or that a normal galaxy collided with another object that happened to contain a quasar.

The research team studied 20 relatively close quasars (a mere five billion light years away) drawing on data from both the Hubble Space Telescope and the ESO Very Large Telescope (VLT). In 19 of the 20 cases, they found that the quasar was, as expected, sited at the centre of a massive galaxy.

Normal quasar (on the right) compared with the no-host quasar

The researchers note that observing the host galaxy of a quasar is often challenging work because the quasar completely outshines the host, masking the galaxy’s underlying structure.

By combining observation data from Hubble and the ESO VLT, and making observations of a reference star at the same time, the astronomers were able to disentangle the quasar light from any possible light from an underlying galaxy.

But in the case of quasar HE0450-2958, no host galaxy could be found. This means the galaxy must be six times fainter than normal, or as much as 170 times smaller than normal. A quasar's host galaxy's radius is typically between 6,000 and 50,000 light-years, but this one can be no bigger than 300 light-years from end to end.

Nearby, astronomers observed a star-forming galaxy showing signs of a recent collision, and a strange blob - a cloud of gas about 2,500 light-years wide - just next to the quasar.

"The absence of a massive host galaxy, combined with the existence of the blob and the star-forming galaxy, lead us to believe that we have uncovered a really exotic quasar", says Frédéric Courbin from the Ecole Polytechnique Federale de Lausanne, Switzerland.

"There is little doubt that an increase in the formation of stars in the companion galaxy and the quasar itself have been ignited by a collision that must have taken place about 100 million years ago. What happened to the putative quasar host remains unknown," he said.

A quasar is a very distant source of vast quantities of radiation. Originally, astronomers thought they were nearby stars, but it turned out that they were much more distant, and so much brighter, than anyone had thought. Hence the name: quasi-stellar objects.

Quasars are thought to form when a galaxy's central super massive black hole gobbles up matter at a particularly fast rate. The galactic dust and gas fall into the black hole's accretion disk producing some very hot regions where huge quantities of energy are released.

The researchers say that the case of HE0450-2958, published in the 15 September issue of Nature, will need more study before the mystery can be unravelled. ®

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