AAS #8: The cosmic mid-life crisis

Contributed by
Jan 9, 2008
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Look out in the sky today, and you'll see only a few kinds of galaxies. Most are elliptical, fuzzy round blobs containing billions of stars. Others are spirals, of course, flattened disks with magnificent spiral arms. Others are irregular in shape, and the fourth kind are peculiar; they have a definite shape (rings, for example) which are due to collisions with other galaxies.

When two galaxies merge -- physically collide like two trucks on the highway -- they might start out as nice pretty spirals, then distort into peculiars midway through the merger, then finally, after billions of years, they settle into elliptical shapes as gravity smooths out the distribution of stars. Detailed calculations of how galaxies interact gravitational have supported this idea, as well as observations of countless galaxies.

But we don't know enough about how all this works. We need more data, and more detailed data. Happily, when astronomers desire, Hubble and Spitzer provides. The Galaxy Evolution and Morphology (GEMS, which is easier to remember) survey was undertaken a few years ago using the Hubble Advanced Camera for Surveys and the Spitzer (infrared) Space Telescope. GEMS was an international effort to look at the galactic merger history over the last 7 billion years.

They looked at 5000 galaxies that appear to be merging systems, examining the overall shapes of the galaxies, as well as the infrared light that points out where new stars are forming -- colliding galaxies slam their gas and dust clouds together, which can cause a burst of star formation. Bright young stars heat up the dust around them, causing these regions to shine in infrared.

The survey found that when the Universe was young -- a couple of billion years old -- 40% of the massive galaxies were actively merging. However, in more recent times -- in the last 7 billion years -- that drops to only 10%. That's not too surprising, since the Universe has been expanding, and the chances of collision drop as the space between them increases. Also, as more galaxies collide, there are fewer left to collide, so the numbers of interactions drops.

The big surprise, however, is that the star formation in these interacting galaxies isn't as vigorous as thought. In other words, in recent times, collisions don't seem to overly enhance bursts of star formation. There is an enhancement, it's just not as strong as it should be -- at least, as strong as we thought it should be. Only 20% of cosmic star formation is coming from interacting galaxies -- the rest is coming from normal galaxies like the Milky Way.

There have been other surprising results too. Most spirals have a central bulge, a big round component in the middle of the galaxy -- a downtown region if you will. Disk galaxies with no bulges should be rare according to prediction. However, 20% of modern galaxies have no bulge, and the same is true even a long time ago in the Universe. This means we don't understand everything that goes into how galaxies form, how they interact, how the stars and gas in galaxies "talk" to each as the galaxies merge. According to theory, bulges should be a natural result of how spirals form, depending on the amounts of gas and dust are ion the galaxies before they merge, and how this all comes together. But since bulges don't actually crop up as much as we predicted, it means we need to observe more and figure out why the Universe isn't behaving the way we expect.

These surveys will continue to investigate the Universe around us, and tell us more about the neighborhood in which we live. Sometimes this may seem pretty far removed from our everyday lives, but don't forget: it's because of all this that you have an everyday life to begin with.