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Group Politics

Week of August 9, 1999

Last week, we saw how some things can be missed even when they are right in front of our faces. Dwingeloo 1 is a galaxy that's very close, yet no one saw it until pretty recently. But there's more to this story. As the saying goes, no man is an island, and the same can be said about galaxies (even though galaxies used to be called ``island universes'').

Dwingeloo 1 is not alone in the cosmos; it is part of a close group of several galaxies. The group is pretty close to us, as the distance to Dwingeloo 1 indicates: roughly 10 million light years away. The group appears to be headed away from us, which means that in the past it was closer. Actually, they were a lot closer.

The galaxy group containing Dwingeloo 1 (actually, the group is named after the brightest member, IC 342) probably formed at the same time as the Milky Way, from a truly enormous cloud of hydrogen. This proto-galaxy collapsed and broke up a bit; one part formed us, the Andromeda Galaxy, and a few other smaller nearby galaxies, including our own satellite galaxies called the Magellanic Clouds. Another part of the hydrogen cloud collapsed to form the IC 342 group.

Life can be rough for the smaller galaxies in a group. The bigger galaxies, having more mass, can bully the little ones. The dance of gravity is subtle and strong; it can let a small galaxy orbit a bigger one for billions of years, then suddenly and violently eject it. Such an occurrence is actually routine when many galaxies are involved. Two large ones, orbiting each other, can pass a bit of that orbital energy to a smaller galaxy. What seems like a small amount of energy to a giant like the Milky Way is huge for a tiny galaxy, or even a group of tiny galaxies. That energy gets translated into velocity, and the little galaxy or galaxies suddenly pick up speed. If they get too much energy, they can actually launch themselves away from the parent galaxy.

Apparently, this is just what happened to the IC 342 group. They were once close to the Andromeda Galaxy. At some point, perhaps 5 to 10 billion years ago, Andromeda swallowed up a dwarf galaxy (galaxies have terrible manners, and sometimes cannibalize smaller ones). This played havoc with the energy balance of the system, and the IC 342 group was ejected. They are now headed away from our neck of the woods, and it's unlikely they'll be back.

Unfortunately for us humans, billions of years after they were ejected, the group wound up almost exactly along the plane of the Milky Way, making them very difficult to detect. We know of 5 galaxies in the group (Dwingeloo 1 and 2, Maffei 1 and 2, and of course IC 342), but there may be more. Perhaps there are dim, dwarf galaxies tagging along for the extragalactic excursion. We may never know, since they are so faint that detection may be impossibly difficult. They might be hard enough to see even if we weren't looking through our dusty gassy Galaxy!

This story of cannibalism, energy transfer and ostracism may sound fanciful, but it has hard numbers backing it up. Though the exact script for the scene is not known, the general layout is. For a more detailed description of the events (including some really nifty animations!), try Hartmut Frommert and Christine Kronberg's webpage about the dynamics of our galaxy group. They have links to scientific results from people that have studied the events. The animations are fascinating!

©2008 Phil Plait. All Rights Reserved.

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