Massive black hole spotted without galaxy

By Ker Than

updated 3:58 p.m. ET Sept. 14, 2005

In a strange reversal, astronomers have detected a massive black hole but can find no traces of the surrounding galaxy that should be feeding it.

At the center of most large galaxies, our own Milky Way included, are extremely dense black holes that have masses hundreds of millions times that of the Sun.

Called quasars, these massive black holes are the most radiant objects in the universe, outshining even the brightest galaxies. While the black holes themselves are undetectable, friction and heat from the swirling matter they ingest emit huge amounts of radiation that can be detected by radio telescopes.

To maintain their fierce brightness, however, quasars must feed off the very galaxies they live within. That is why the discovery of a galaxy-less quasar is so surprising.

Using images from the Hubble Space Telescope and spectroscopy from the Very Large Telescope (VLT) in northern Chile, an international team of astronomers selected 20 quasars located at moderate distances from Earth to study the properties of their host galaxies.

In 19 of them, the astronomers found encircling galaxies as predicted. But when they looked at HE0450-2958, a quasar located some 5 billion light-years away, they didn’t find any sign of a galaxy.

“We must therefore conclude that, contrary to our expectations, this bright quasar is not surrounded by a massive galaxy,” said Pierre Magain, an astronomer from the University of Liege in Belgium and lead author of a new study documenting the finding.

Quasars are relatively small compared to the galaxies they outshine. They are only about the size of our solar system, but they can emit up to 100 times as much radiation as an entire galaxy.

While the intense brilliance of quasars make them visible from clear across the universe, it also makes detection of their host galaxies difficult because light and radiation from the galaxies get lost in the glare of the quasars.

In the late 1990s, mathematical “deconvolution” algorithms were developed that could be applied to images after they were transmitted to Earth and which were capable of separating light from quasar’s from that of their host galaxies. Since then, astronomers have shown that nearly all quasars are encircled by a host galaxy.

Instead of a galaxy, the researchers detected a cloud of ionized gas about 2,500 light years in size near HE0450-2958. Dubbed “the blob,” the researchers believe this gas cloud is what’s feeding the black hole, allowing it to become a quasar. The researchers estimate that the quasar is siphoning off about one sun’s worth of mass each year from blob to satisfy its ravenous appetite.

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