The Sciences

Voyaging deep into the Universe

Bad AstronomyBy Phil PlaitNov 7, 2008 3:46 PM


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The European Southern Observatory just released a picture that I can only call astonishing: the deepest ground-based look into the universe ever undertaken. It's part of the Chandra (X-Ray Observatory) Deep Field South, an effort to map out distant regions of the Universe across the entire spectrumwith incredible resolution and depth. The image is mostly in the near ultraviolet, or UV. It's a total of 55 hours of observations. Despite that, like many things, at first glance it doesn't look like much:

<img src=""Click any of the images in this post to embiggen.

Sure, you see lots of stars in it, right? But the image I displayed here has been compressed; if you download the complete full-res image (warning, 32MB file!) you get a much clearer view of this field. Those dots you saw above? Those aren't stars, they're galaxies. Thousands of them. Tens of thousands. I don't know how many, exactly. A lot. Scanning the full-res image is incredible. There's so much to see! Each dot, each smudge, is a full-blown galaxy, a collection of billions of stars. They're very, very far away; some of these galaxies are estimated to be 10 billion light years distant; you're seeing them as they were just a couple of billion years after the Universe itself began, and the faintest are one-billionth as bright as objects you can see with your own eye. They come in all shapes and sizes (though the apparent size depends mostly on distance; the bigger ones are almost certainly much closer than the smaller ones). Some appear to be loners, while others group together. Here's an interesting cluster that, from the sizes of the galaxies, is at an intermediate distance from us:

Our Milky Way is a member of a small cluster called the Local Group, and I imagine from a couple of billion light years away it looks a lot like this one. When I look at this little cluster, I can't help but wonder if anyone is looking back. When the Universe was younger, it was smaller, and clusters were smaller. That means collisions between galaxies were more common back then. So you expect to see a few galaxies twisted up, distorted from the impact. Guess what?

This looks like a spiral galaxy that had a smaller galaxy plow through it. Gravity warps the gas and stars into a ring surrounding the nucleus, and the gas forms new stars. These stars shine brightly, so the ring is a very obvious structure. The apparently interlocking ring to the lower left is a bit puzzling, though. It might be the interloping galaxy, torn apart by tidal stresses from the bigger galaxy, or it might be a tendril of material torn off the bigger galaxy. The perspective makes it hard to tell. But see that faint glowing fan of material to the right? That points right to the center of the ring galaxy, and that is a pretty clear indicator it's a jet, a focused beam of material being spewed out by the galaxy. Every big galaxy has a monster black hole in its core (ours does, too). When galaxies collide, huge amounts of gas fall to the galactic center. The physics of the situation pretty much demand that the material forms a flat disk which swirls around the black hole. Near the Final Plunge, the disk gets incredibly hot (amazingly, through friction). This hot material tries to escape, and magnetic and other forces focus it into twin beams or jets of matter and energy that scream out in opposite directions at nearly the speed of light. Galaxies like this are said to be active, an oddly understated word for the mind-numbing amount of sustained violence going on -- the energy contained in those beams can be millions of times the Sun's total output. We're talking cosmic blowtorches here. Seeing a jet coming from a distorted galaxy, especially one shaped into a ring, is dead giveaway. That dude had a head on crash, and not terribly long ago. Maybe a couple of hundred million years at most. Another fan of material is coming from a closer galaxy in the image, too:

This time the jet is fainter, but the galaxy itself looks more normal. Maybe it had a recent collision too, but the offending collider was much smaller, so the bigger galaxy doesn't appear overtly distorted. But again, the jet points right back to the core of that nearly edge-on spiral, so it's a pretty clear indicator of an active galaxy. I love galaxies, but I have to say that my favorite spot on this whole ginormous picture shows not some distant collection of billions of stars, but instead a single star, one that is far, far closer:

No, not the bright one. See the little weird thing to the left of it? The things that's green on top and reddish-pink below? That's a nearby star, one so close that its motion across the sky caused it to blur during the few years it took to get the observations of this image! Normally, you don't notice the stars' motions in the sky; the constellations look pretty much the same as they did thousands of years ago. But stars do move across the sky as they follow their separate orbits around the Milky Way center. In high-magnification images, some fast-moving stars can be seen to change position. The changing colors for this star are because some exposures were made with one filter, and then later exposures with a different filter, with the star moving in the intervening time. Unlike the galaxies billions of light years away, this star is inside our own Galaxy, most likely only a few dozen light years away at most. If it were much farther its motion would be dimmed by distance, and it would appear as rock steady as the other objects in this picture. Incidentally, I found another object blurred in a similar multi-hued way, but it looks like a galaxy. Since galaxies don't move that quickly (they are too far away to see that motion) I think it might be a reflection inside the telescope from a bright nearby star. Such ghost images are common in deep exposures like this, and play havoc with analysis. These are a handful of close-ups I found in a few minutes of perusing this devastating image. What will astronomers find over years of study? No doubt they'll look at statistics of the galaxies; how close together they appear to be, how clustered they are as opposed to spread out. Once distances are found to some of these objects we can begin to learn more about how the early Universe behaved itself. We can figure out how galaxies formed at such an early time, and what the structure of the Universe was like back then. And remember as you look at this picture: you're seeing one teeny tiny fraction of the area of the entire sky. No matter where we look in the sky, we see galaxies splayed out like this. The cosmos is a grand place, full of wonder and delight. Exploring it is just about the noblest thing we can do. All images credit: ESO/ Mario Nonino, Piero Rosati and the ESO GOODS Team.

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