Galactic rendezvous likely to trigger reaction in Milky Way

The Milky Way appears to have some galactic fireworks in its future. Our galaxy, some 100 billion stars strong, is headed for a close encounter with the Andromeda galaxy in less than 4 billion years. This could awaken a slumbering black hole thought to lie at the core of the Milky Way, turning the hole into an energy powerhouse that would strip the galaxy of gas for thousands of light-years from its center. This gas is crucial to the formation of stars.

``You cannot avoid the Milky Way encountering'' the Andromeda galaxy, says Marshall McCall, a University of Toronto astronomer who is studying the evolution of the local group of galaxies that includes the Milky Way. ``And it's going to get closest in less than 4 billion years.'' He says, ``Close encounters have been theorized as being responsible for activity in galactic nuclei, so one could expect a significant increase in activity in our own galaxy.'' What Dr. McCall terms an increase in activity means the release of staggering amounts of energy.

The most extreme example of active galactic cores are quasars, says Mitchell Begelman, an astronomer from the University of Colorado at Boulder's Joint Institute for Laboratory Astrophysics. He and McCall are among the hundreds of astronomers attending the joint meeting of the American and Canadian Astronomical Societies here this week.

``We are pretty confident that quasars are the nuclei of galaxies which happen to be extremely hyperactive,'' Dr. Begelman says of the objects, which can unleash more than 10 trillion times the energy output of the sun. In the process, they far outshine all the other energy sources in their galaxies and constitute the youngest, most distant objects we can see.

A large body of what Begelman terms ``largely circumstantial'' evidence points to black holes as the central engines of quasars. Black holes are thought to be objects with gravity so strong that not even light can escape their pull. The black holes in quasars are thought to be from 100 million to 1 billion times as big as the sun.

Observations made over the last couple of years suggest that many normal galaxies have a high probability of hosting massive black holes in their centers - including the Milky Way.

If there is a black hole in Earth's galaxy, why isn't it more active? Because there isn't enough matter being drawn into it. Begelman tested this notion by simulating what would happen when more matter was ``fed'' into the black hole. He pumped enough matter into it to turn it into a quasar. He found that the intense output of X-rays from the core would clear the galaxy of the gas and dust that are crucial to the formation of stars and solar systems. From the standpoint of Earth's solar system, if such an event took place, there would be no noticeable effect; by the time the X-rays traveled the 25,000 light-years from the core to the sun, they would be too diffuse to do any damage. But were it to have happened 5 billion years ago, when the solar system was forming, there'd have been no solar system today.

Fortunately, he says, observations have strengthened the notion that any black hole that may anchor Earth's galaxy isn't massive enough to turn into a full-blown quasar. But it does have enough mass to ensure that, if activated, its X-ray emissions will clear the gas out to a few thousand light-years from the core.

Presumably, the close encounter with the Andromeda galaxy could activate the Milky Way's core by adding its gravitational pull to that of the black hole - thus exerting an extra tug on material on the far side of the galactic nucleus. But because of the time scales involved, no one has actually witnessed the start-up of a quasar or other active galaxies, Begelman says.

An encounter with the Andromeda galaxy could lead to other effects, Begelman and McCall say. If the Milky Way passes by Andromeda, it could lose some stars and other material to the more-massive galaxy or to deep space. If the two galaxies collide, the chances of individual stars colliding are exceedingly small. But the gas in the galaxies would heat up, form shock waves, and perhaps form into clumps that could lead to new stars.
