[The Desktop Project is my way of clearing all the pretty pictures off my computer's desktop, by posting one per day until they're gone. I think this week is it - I'm almost out!]
Dark matter is funny stuff. We’ve known about its existence for many decades, and the more we look the better our evidence gets. We know it has mass, and therefore gravity, but we don’t know what it is! We do, however, know what it isn’t: normal matter of any kind, like cold gas, rogue planets, black holes, dead stars, or anything else made of protons, neutrons and the other types of particles we deal with in everyday life.
Since careful observations have shown clearly it can’t be any kind of normal matter, it therefore must be some sort of exotic flavor of matter, some kind of particle we haven’t yet seen.
One thing we’re pretty sure about it, though, is that it doesn’t interact with normal matter except through gravity. Dark matter can pass right through you and you’d never know it. But put enough of it in one spot, and its gravity will reveal its presence.
Which is why the galaxy cluster Abell 520 is such a mystery. Here’s the beauty shot:
Pretty, isn’t it? Abell 520 is a galaxy cluster about 2.4 billion light years away, and a mass of several trillion times our Sun’s — it’s made of galaxies, each with billions of stars in them. And a galaxy cluster is a collection of hundreds or even thousands of galaxies bound together by their gravity. In fact, Abell 520 is more than one cluster: it’s actually a collision between two or more clusters! As they move through space, clusters can collide, and actually quite a few of these cosmic train wrecks are known.
When clusters collide, a lot of things happen. The gas clouds in between galaxies in the two cluster slams into each other, heating up to millions of degrees and glowing in X-rays. In the picture above, that gas has been colored green so you can see it (invisible to the eye, the X-rays were detected by the Chandra Observatory). The orange glow is from stars in galaxies (as seen by the Canada-France-Hawaii and Subaru telescopes). The blue is actually a map of the dark matter made using Hubble observations. The gravity of dark matter distorts the light passing through from more distant galaxies, making it possible to map out the location of the otherwise invisible stuff (you can read about how that’s done here and here).
Since dark matter doesn’t interact with normal matter, we expect it to simply pass through the collision point, sailing on as if nothing had happened. That’s been seen in a half dozen other galaxy cluster collisions, including the Bullet Cluster — hailed as definitive proof of the existence of dark matter — as well as Abell 2744 aka Pandora’s cluster (seen here on the right), and the newly found Musketball cluster.
But Abell 520 isn’t like those others. The problem is, there’s a clear peak in the dark matter right in the middle of the cluster, not off to the sides as you might expect. It looks as if the dark matter slammed to halt in the middle of the collision instead of sailing on.
Here’s the thing: this does not mean dark matter doesn’t exist, or we’re wrong about it. The other clusters I mentioned above make it clear we do have a pretty good grip — so to speak — on the behavior of dark matter.
Every now and again something weird and wonderful happens in the sky, and for a few minutes I’m totally perplexed about what it is.
And then there’s something that makes me literally gasp and say "WHAT THE FRAK WAS THAT?"
Yeah. Check out this amazing video:
Holy Haleakala! What was that?
The footage is from a webcam mounted outside the CFHT astronomical observatory in Hawaii (another view of it from a different webcam can be found here; sadly, both webcams are on Mauna Kea, not Haleakala). You see some stars and the horizon, then suddenly an ethereal pale arc pops into view. It rapidly expands into a thin circular shell, then fades away as it fills the view. The whole thing takes a few minutes to expand; you can see the stars moving during the event (some of the pixels on the webcam are very sensitive and make stationary "hot spots" in the field of view).
So what is it? Is it a trans-dimensional portal into the future, some wormhole from the Pegasus galaxy, or two alien spaceships battling it out?
In point of fact, we are seeing something related to space war…
I first saw this video on Starship Asterisk, the discussion forum for the wildly popular Astronomy Picture of the Day website. The conversation there about this event is going pretty well, and I think this whole thing has been nailed down to a reasonable series of events. First, let’s look at a still frame from the video:
What does a half million galaxies look like? Something like this:
Whoa. That’s a part of a huge image just released by the Canada-France-Hawaii Telescope Legacy Survey Deep Field #1, a ginormous mosaic of the night sky… and by ginormous, I mean GINORMOUS. It covers a solid square degree of sky — 5 times the area of the full Moon — and tips the scale at a whopping 370 megapixels! It took 5 years and several hundred hours of observing time with the 3.6 meter telescope on top of Mauna Kea to get this massive mosaic.
The image itself may look cool and all, but the true power comes when
you give in to the dark side you use the interactive zoom feature. You can surf the entire mammoth 370 million pixel image, zooming in on galaxies galore. And you won’t run out of objects to investigate any time soon: there are an estimated 500,000 galaxies in the image. Like the Hubble image I posted about yesterday, almost everything you see in the image above is a galaxy, not a star.
The images were taken to look for very distant supernovae. It was the investigation of these far-flung stellar explosions that led astronomers to determine the Universal expansion is accelerating, and to postulate the mysterious dark energy that powers this phenomenon. The CFHT is being used to map the same area of the sky over and over again, looking for the tell-tale blobs of light that mark the spots of a distant, dying suns. The more of these we see, the better we can nail down the physical characteristics of the cosmic expansion, and of the dark energy about which we know so little.
Of course, astronomers will squeeze a lot of science from this and other images… but it’s also OK to simply scan and pan through them at home, too, marveling that the Universe is so deep and so deeply beautiful.
For more deep and gorgeous images like this, see Hubble Digs Deep to See Baby Galaxies, The Milky Way Bulges with Cannibalized Corpses, Hubble Pokes at a Galactic Bulge, or just search in the Pretty Pictures category of this blog.