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Pumping gas to fuel a galaxy's active core.

Pumping gas to fuel a galaxy's active core

Many galaxies have central regions that appear extremely active and spew out tremendous amounts of radiation. In some cases, this activity takes the form of a strong burst of star formation at a galaxy's core. Other galaxies seem to have black holes at their centers, which apparently pull in vast quantities of matter. What fuels this activity has long mystified astrophysicists.

One possibility is that the disturbing effects of a modest cluster of stars merging with a large galaxy can drive much of a galaxy's interstellar, molecular gas toward its center. New computer simulations support this scenario.

"Gas distributed throughout a galaxy responds strongly to the tidal field of a companion during a merger," says Lars Hernquist of the Institute for Advanced Study in Princeton, N.J. Hernquist's computer simulations, reported in the Aug. 31 NATURE, track the behavior of interstellar gas during a merger between a parent galaxy similar to the Milky Way -- having stars and gas in a spiral-armed disk -- and a smaller, satellite galaxy containing only stars.

Initially orbiting the disk, the satellite galaxy gradually spirals in and loses energy, in effect "rubbing" against the parent galaxy's stars, which gain energy. At the same time, the satellite's perturbing gravitational field causes gas streams to intersect, building up regions of high-density gas. Some of this gas collects into a large, gravitationally bound cloud that sinks to the galaxy's center.

The concentrated gas at a galaxy's core could initiate star formation, possibly spurring the eventual formation of a black hole while supplying a reservoir of fuel for future activity. "Continued accretion of gas by the black hole may provide sufficient power to explain quasars and nuclear activity in otherwise normal galaxies," Hernquist says.

"Hernquist's calculations are particularly interesting because he simulates the dynamics of both the gas and the stars simultaneously," says astrophysicist Mitchell C. Begelman of the University of Colorado in Boulder. "The processes are so complicated it's now difficult to make theoretical progress in this area without these kinds of computer simulations." Other groups, including researchers at Tokyo University and the Paris Observatory, have done similar calculations and continue to explore various scenarios.

Hernquist's model predicts that the origin and evolution of activity in at least some galaxies could be controlled by events on a galactic scale. However, his simulations do not directly tackle the question of what happens to the gas after it reaches a galaxy's core.

"It is certainly plausible that the large gas density built up at the galactic center should result in vigorous star formation . . . but there is no reliable quantitative theory for this," writes Cedric Lacey of the University of California, Berkeley, in a commentary accompanying the research report. Hernquist's model also neglects the effect of subsequent supernova explosions of massive, dying stars, which could heat the gas and inhibit processes that tend to concentrate gas at the galaxy's center, Lacey adds.

Moreover, mechanisms other than galaxy mergers may also cause the concentration of gas, which in turn could lead to activity in galactic nuclei. "Some people have gone a little too far in the direction of believing that mergers or close encounters between galaxies are the only way to trigger this kind of activity," Begelman says. "A merger is certainly one possible process that could lead to a lot of gas plunging into the nucleus of a galaxy, but I don't think it's the only or even necessarily the main effect. It's not clear how common these kinds of mergers are."

Observational evidence gathered so far remains contradictory. Some, but not all, quasars appear buried at the center of disturbed galaxies. At the same time, studies of nearby Seyfert galaxies, which also have active cores, show that weakly interacting pairs of galaxies have an excess of active nuclei, whereas strongly disturbed galaxies have fewer active nuclei than expected.

"It's a confused situation," says William C. Keel of the University of Alabama in Tuscaloosa. "Statistically, you can make a case that whatever a galaxy does to turn on nuclear activity, it's a little easier if there's a companion nearby. But there's no evidence that anything as simple and straightforward as [galaxy mergers causing active nuclei] is going on." That leaves lots of room for debate.
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Author:Peterson, I.
Publication:Science News
Date:Sep 2, 1989
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