ALMOST all quasars are found in blob-like elliptical galaxies, say
astronomers in Britain. This contradicts the widely held theory that the host
galaxies of most quasars are spirals like our own Milky Way.
Quasars, some of the brightest objects in the Universe, are thought to be
giant black holes feeding on the stars and gas of a surrounding galaxy. James
Dunlop of the University of Edinburgh and his colleagues looked at 22 quasars
using the Hubble Space Telescope, and in all cases they detected the surrounding
鈥渇uzz鈥� of a large elliptical galaxy.
Astronomers thought most quasars reside in spirals because of observations of
other 鈥渁ctive galaxies鈥� called Seyfert galaxies, thought to be relatives of
quasars. 鈥淪eyfert galaxies, which are radio-quiet, are spirals,鈥� says Dunlop.
鈥淭his led people to think that the majority of quasars which are radio-quiet
might also be in spirals.鈥�
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The new findings, presented at last week鈥檚 National Astronomy Meeting in St
Andrews, also confound claims made a few years ago that some quasars have no
host galaxy. Such 鈥渘aked鈥� quasars challenged accepted ideas, because there would
be no fuel to feed them.
Quite why quasars appear only in elliptical galaxies is not yet clear.
Naively, you would expect them to exist in spiral galaxies because these have
plenty of gas to fuel them. However, the gas in spirals would also impede the
jets of ultra-high-energy particles, which are characteristic of the most
powerful quasars.