Pancakes and Sausages

Monday, July 01, 1996
Even the strongest telescopes can’t discern the full three- dimensional shapes of galaxies. So astronomers create computer models to tell them what galaxies look like. But for at least one important class of galaxies, it seems the computer models have been wrong, says David Merritt, an astronomer at Rutgers University.

The problem, says Merritt, arises with models of elliptical galaxies, which make up about half of all galaxies. Astronomers over the years have come up with three possible shapes for these galaxies: oblate (sort of like a pancake), prolate (like a sausage), and triaxial (a slightly squashed sausage). In the past decade or so, the triaxial model has emerged as the favorite--in part because astronomers have been able to reproduce it successfully with computer simulations. So they thought, ‘Well, if we can do it on the computer, then nature probably can as well,’ Merritt says.

But nature, Merritt suspects, had other ideas. Astronomers using the Hubble Space Telescope have recently discovered that all elliptical galaxies seem to have cusps--dense knots of stars--or, in some cases, black holes at the galactic center. Those two types of high-density regions, Merritt and his colleagues have found through detailed mathematical calculations, can have a huge effect on the shape of their parent galaxies. We found that the gravitational forces produced by these two things can deflect the orbit of any stars that pass through the galaxy’s center, says Merritt. They are essentially kicked in some random direction, and then the next time they come around they are kicked and change direction again.

This wouldn’t happen much in pancake- or sausage-shaped galaxies, says Merritt, because the stars in those galaxies rarely pass near the galactic center: they orbit it the way the planets orbit the sun, without hitting it. (The prolate shape--or cylindrical sausage--is created by countless elliptical orbits, all going around the center but tilted at different angles.) Thus oblate and prolate galaxies, according to Merritt’s calculations, are stable. But in a triaxial galaxy--the squashed sausage-- it turns out that the stars do go near the center, so they do get kicked. As a result their orbits will eventually become chaotic. That chaos, because it destroys the orbits, makes it very hard to maintain triaxiality as a shape, says Merritt.

That is not to say, however, that elliptical galaxies are never triaxial. In fact, Merritt suspects that they all may start out that way. But over the eons, driven by the gravitational kicks of cusps and black holes, they evolve into a more durable oblate or prolate shape.
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