Hide and seek, galaxy style.

There was a big clump of stars (about a billion of them) seen along the line of sight toward the center of our galaxy, but they were too far away to be part of our galaxy, and they were moving in the wrong direction. Instead of orbiting with the rest of the stars in the disk, they were passing through at an angle, still holding together against the overwhelming gravity of the Milky Way. We realized we were seeing a completely separate small galaxy trying to pass through the plane of the Milky Way, and getting ripped apart in the process. This galaxy was dubbed the Sagittarius Dwarf galaxy (as it, and the core of the Milky Way, lie in the constellation Sagittarius), and until recently, it held the record for the nearest galaxy, at about 70,000 light away.

But now, as of November 2003, we think we've just found another, even closer one.

The reason this one eluded us for so long was that it was completely blocked by one of the thick dust clouds in our own galaxy. Even now, we can't detect any of the visible light from the galaxy at all, only infrared light (which we commonly think of as heat) reaches us through the shroud of cold, dusty material that drifts between the stars in the Milky Way. The 2-Micron All-Sky Survey (2 microns, or millionths of a meter, is the wavelength of the infrared light observed) recently completed a map of the entire sky in infrared light. Infrared light is able to pass through obscuring clouds of dust, allowing astronomers to peer into regions of space where visible light is completely blocked. This gives us a huge advantage when looking through our galaxy's disk, and for the first time, we're getting a clear view of how stars are distributed around our entire galaxy, not just the closer regions we can easily see. Infrared light is also a great way to observe cooler, smaller stars, and that's just what the new galaxy seems to be made up of.

So, using infrared light, astronomers have just found a small galaxy of about a billion stars in the constellation Canis Major, that is, astonishingly, only about 25,000 light years away from the Sun. That's closer to us than the center of our own Milky Way! The Canis Major dwarf galaxy is not faring well in its gravitational battle with the Milky Way, and there are streamers of stars being pulled off the smaller galaxy onto the disk of our own.

Some of these cannibalized stars are drifting down to become part of the Milky Way's disk, and others are even heading in the direction of the Sun. It's a pretty weird thought that some of the stars around us may not come from our galaxy at all, but were pulled off the Canis Major dwarf galaxy many millions of years ago. But maybe that's not as strange as it sounds. If the Milky Way is currently engulfing two smaller galaxies, how many has it swallowed in its many billion-year history? Is that, in fact, how large galaxies like the Milky Way come into being, by pulling in and eating any smaller galaxies that get too close?

This idea has certainly gotten the attention of astronomers, who have had some significant problems figuring out how the Milky Way, as a whole, works.

Take the lovely spiral arms, which are actually a great mystery to us. Spiral arms are not rigid structures, but variations in the density of stars inside the galactic disk. The spiral arms move at a different rate than the stars, and our own Sun has existed both in and out of spiral arms during our history. The best analogy I can think of is a traffic jam: it's really just a density wave in a distribution of cars.

Individual cars move in and out of the jam, but the jam persists (especially if you live in Los Angeles, like I do). Spiral arms in a galaxy are just like that. Until now, we couldn't figure out what was causing the traffic jam of stars that form the spiral arms - our gravitational simulations of stars, all orbiting a common center, just couldn't explain it.

But then people started looking at what would happen if a smaller galaxy slams into a larger one, and viola, the equations started working. The combined gravity of a billion stars going the wrong direction is more than enough to induce density waves in a large galaxy, and it looks like the mystery is solved.

Instead of a single system, the Milky Way makes a lot more sense when viewed as a group of interacting galaxies, all merging into an elegant, if somewhat perturbed, whole. There's no reason to think that we've found the last hidden galaxy, either. As we get better at mapping the true location of the stars, who knows what we'll find playing hide and seek with us out there? We've only just opened our eyes.

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