WHERE quasars come from has been a mystery since they were first discovered
40 years ago鈥攂ut maybe not for much longer. The AAS heard evidence that
puts them at the heart of 鈥渟tarburst鈥� galaxies, which have high concentrations
of young massive stars and supernovae.
Quasars emit so much light that we can see them from the farthest corners of
the Universe. Astronomers have long suspected that they form the cores of
distant galaxies, and that they are probably powered by supermassive black
holes. But they are so dazzling that they drown out light from galaxies around
them, making it virtually impossible to tell anything about these galaxies.
Now a team led by Nick Scoville of the California Institute of Technology in
Pasadena has found evidence that quasars are surrounded by gas-rich spiral
galaxies, or by galaxies that result from the merging of other gas-rich
galaxies. Since gas-rich galaxies also tend to form lots of stars, 鈥渢his links
quasar activity to star-forming activity鈥�, says Scoville.
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Scoville鈥檚 team studied a number of bright and relatively nearby quasars,
using the Owens Valley Radio Observatory array in California to look for the
signature of carbon monoxide in their spectra. From the strength of the signal,
they were able to calculate the total amount of molecular gas in the host
galaxies. Two-thirds of the observed quasars turn out to be in galaxies that
contain between 2 and 10 billion solar masses of gas each, enough to make them
starburst galaxies.
Backing this idea, Kimberly Weaver of NASA鈥檚 Goddard Space Flight Center in
Greenbelt, Maryland, told the AAS how a quasar might form at the heart of a
starburst galaxy. Dense starburst regions in the galaxy form extremely massive
stars, she suggests. At the end of their lives, the stars collapse into huge
black holes that can later merge into what astronomers call middleweight black
holes, weighing tens to hundreds of times as much as the Sun. Later, says
Weaver, these middleweights, which have been discovered in many nearby starburst
galaxies, could spiral into the centre and further merge into a supermassive
black hole.
However, she says, this happens too slowly to explain the really distant
quasars, which formed in the early days of the Universe. 鈥淲e just don鈥檛 know yet
how supermassive black holes form,鈥� says Weaver.
Another complication is that not every galaxy that hosts a quasar shows signs
of vigorous star formation. Scoville suggests that this could be because the
original starburst activity may have subsided, while the central black hole
continues to be fed and fuelled by the interstellar gas. This could happen,
Scoville says, if quasars evolve from what are called ultra-luminous infrared
galaxies, or ULIRGs. These galaxies are believed to result from the collision
and merging of separate gas-rich galaxies.
According to Scoville鈥檚 theory, when such galaxies begin to merge they will
first undergo huge bursts of star formation and show up as an ULIRG. In a later
stage, the cores of the colliding galaxies merge into a single supermassive
central black hole, which gobbles up much of the remaining gas and shines as a
brilliant quasar.
鈥淪tarburst activity is a wonderful way to produce what you need to create
[quasars] in the centre of a galaxy,鈥� says Weaver. 鈥淣ick Scoville鈥檚 model is one
way to make them.鈥�
