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THERE鈥橲 a giant chicken-and-egg problem lurking in every galaxy. Astronomers
had been leaning towards the notion that black holes formed first, but now an
Italian astronomer thinks it might be the galaxies.

Every galaxy contains a supermassive black hole at its centre. In certain
young galaxies called quasars, the black hole pulls in matter so strongly that
the superheated material spews out powerful X-ray emissions. But some young
galaxies don鈥檛 put on such a show. Paolo Salucci at the International School for
Advanced Studies in Trieste, Italy, decided to find out why.

He looked at the distribution of matter in more than 900 spiral galaxies and
calculated that the black holes at their centres were all smaller than 10
million solar masses. He says this is too small for the black holes ever to have
powered quasar X-ray emissions. In elliptical galaxies, by contrast, the black
holes are about 100 times larger.

Doug Richstone of the University of Michigan at Ann Arbor thinks the results
sound right but unsurprising. 鈥淲e鈥檝e already shown that the amount of bulge in
an elliptical galaxy is related to the size of the black hole, and spiral
galaxies tend to have a very small bulge.鈥� But Salucci says the black holes in
spiral galaxies are even smaller than one would expect from the size of the
bulges.

Salucci says spiral galaxies have much more angular momentum than
ellipticals, so in the early Universe matter was kept out of the black hole as
the galaxy was forming. This means that the black holes in spiral galaxies have
remained virtually unchanged ever since they formed.

These different views lead to different answers to the chicken-and-egg
question. Richstone likes to emphasise the role of the black hole in determining
galaxy formation, but Salucci says it鈥檚 the galactic environment in which the
black hole forms that determines its size.