Friday, January 11, 2008

Louis J Sheehan Esquire 30029

Space is a dangerous place. Stars explode, black holes gobble up matter… but some violent events are so huge they affect entire galaxies, mayhem on a scale so vast it numbs the mind.
Galaxies are island universes, cities of billions or even hundreds of billions of stars. Some galaxies, like our Milky Way, live pretty much on their own, but others live in vast complexes called clusters. These galaxy clusters may have hundreds or thousands of denizens, all orbiting each other due to their mutual gravity, looking something like bees buzzing around hive.
But there is more there than just the matter we see. Dark matter is there as well; invisible stuff that adds to the gravity of the cluster due to its mass, but gives off no light. However, it betrays its presence in two ways: its gravity changes the motion of the galaxies in the cluster, and it distorts the light from more distant galaxies due to gravitational lensing.
A team of astronomers has used the Hubble Space Telescope to examine the galaxy cluster Abell 901/902 (they call their project STAGES: Space Telescope A901/902 Galaxy Evolution Survey). They wanted to very carefully map out many aspects of the cluster: how many galaxies it contains, what kinds of galaxies they are (spirals, ellipticals, etc.), and, using lensing, determine where the dark matter is. By making a map of all of these characteristics, they hoped to be able to understand the history of the cluster, since the present configuration of the cluster can provide clues to its past.
For the first time, these cosmic archaeologists were able to map out the dark matter of this cluster, and found four very large concentrations of it scattered throughout Abell 901/902. These clumps of invisible stuff are enormous: they total a stunning 100 trillion times the Sun’s mass, or 500 times the mass of our entire galaxy.
Needless to say, that much mass exerts a powerful gravitational pull. Galaxies round the clumps are falling in toward them, inexorably drawn in by the clumps’ gravity. And as they fall in from the suburbs to the downtown regions, they change. They slam into the thin gas between galaxies, which can blow out the gas inside the galaxies (like leaving you car window open on a highway can air out the inside of the car), for one. But as the galaxies fall in, the inevitably interact with one another, colliding and merging as the make the downhill slide. This distorts the galaxies’ shapes, and that in turn allows the astronomers to determine the past history of the objects.
What’s interesting is that they found that galaxies tend to be more distorted on their way in to the centers of the clusters than they are when they are actually at the center. It appears that as they fall, they have time to interact and merge, changing their shape, but once they aproach the center they are falling so quickly they simply don’t have time to distort much as they pass each. Also, it takes time to settle in at the center, so the galaxies at the center appear to be very old, and have finished their transformation from being unsettled and twisted into more sedate, round, elliptical galaxies. The astronomers also determined that the galaxies at the edge of the cluster still produce stars, but by the time they reach the center that has mostly turned off. Their gas — needed to make stars — gets blown out of the galaxies on the way in, and the mergers trigger vast bursts of star formation, which also uses up the gas.
These discoveries were possible only through the use of Hubble, Spitzer, and other telescopes, each of which unpeeled another layer of the puzzle. I’ll note that for Hubble’s part, this represents the largest area of sky ever observed by the grand dame of space ’scopes; it took 80 separate pointings of Hubble to complete the survey of the cluster, and they mapped the locations and shape of 60,000 galaxies in all, a truly staggering amount.
One last thought: the Milky Way is more or less alone in space, being part of a loose collection of other galaxies. But we are headed toward the Andromeda galaxy, and in a couple of billion years we’ll collide and merge with it. I hope that in this far flung future, some distant astronomers can use our own violent fate to learn a little more about the Universe, too. It only seems fair.

One of the more amazing aspects of looking into deep, deep space is that the path there is tortured and twisted. Space itself can be distorted by mass; it gets bent, like a road curves as it goes around a hill. And like a truck that must follow that road and steer around the hill, a photon must follow the curve of space.
Imagine a distant galaxy, billions of light years away. It emits light in all directions. One particular photon happens to be emitted almost — but not quite — in our direction. Left on its own, we’d never see it because it would miss the Earth by thousands or millions of light years.
But on its travels, it passes by another massive galaxy. This galaxy warps space, and the photon does what it must do: it follows that curve in pace, and changes direction… and it just so happens that the curve is just right to send it our way.
The intervening galaxy is essentially acting like a lens, bending the light. If the more distant galaxy is exactly behind the lensing galaxy, we see the light from that more distant galaxy distorted into a perfect ring, a circle of light surrounding the lens. We call this an Einstein Ring. If the farther galaxy is off to the side a bit, we see an arc instead of a complete ring. Gravitationally lensed arcs and rings are seen all over the sky, and they can be used to determine the mass of the intervening galaxy! The more mass, the more distorted the light from the farther galaxy. So the Universe has given us a nice method to let us weigh it.
In a surprising twist, astronomers have found a new type of lensed galaxy: a double ring! In a rare alignment, there are two distant galaxies aligned behind an intervening lensing galaxy. They’re like beads on a wire, lined up just right such that both more distant galaxies are lensed by the nearer one. In this case, the lens is about 3 billion light years away, and the other two are 6 and 11 billion light years away, an incredible distance.
This image is amazing, but it is also a powerful scientific tool. It allows us to measure not just the mass of the lensing galaxy, but also the amount of mysterious dark matter nearby. We cannot see the dark matter, but it too bends light, and contributes to the lensings. By observing lenses like this, we can take a sample of dark matter in the Universe, and that’s a crucial first step in understanding it. Even better, these double rings allows us to measure the amount of total mass not just in the nearest galaxy, as is usual, but also in the middle galaxy as well, since it distorts the light from the galaxy behind it (turns out it’s a rather lightweight one billion solar masses; our own Galaxy has more than 100 times that mass, so the middle galaxy is considered a dwarf).
This is a beautiful happenstance; it gives us a measure of the Universe at two points, with one being for free. In fact, Tommaso Treu, the astronomer at U.C. Santa Barbara who investigated this lens, points out that if we can find as few as 50 of these double rings, we can get a much better idea of the distribution of not just dark matter, but also the even more mysterious dark energy in the Universe. That’s one of the biggest goals of modern astronomy… and we may get a handle on it due to a coincidental ring toss.

No comments: