[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.
Dark matter, to re-interpret Obi Wan Kenobi, surrounds us and penetrates us. It binds the galaxy together.
At least that’s what a new scientific study seems to show. Dark matter appears to stretch well beyond the visible limits of galaxies, flowing through and filling even the vast, previously-thought empty space between galaxies. The researchers, led by Shogo Masaki of Nogoya University, used computer simulations to model how dark matter behaves over time as it helps form galaxies, and found that while it’s concentrated in and around galaxies, it doesn’t fade away into nothing with distance. It does get thinner, but still exists to a measurable degree well outside of galaxies. The model structure they found is actually quite lovely:
Remember, this is a model, and not an actual map. It does show concentrations of dark matter along galaxies and clusters of galaxies, but also shows how even "empty" space well outside of galaxies has pervasive dark matter in it.
OK, so what’s the deal then?
Dark matter was discovered a long time ago, when it was found that galaxies that live in clusters were moving way too fast to be held by the cluster gravity. They should just simply shoot away, and clusters would essentially evaporate. This implied that clusters of galaxies were either very young and hadn’t had time to dissolve — which we knew wasn’t true; they’re clearly old — or there must be a lot more gravity holding them together. We can add up all the light from the stars in the galaxies and estimate their total mass, but what you get is only about 5-10% of the mass needed to hold clusters together. So most of the matter making up the clusters must be dark. Otherwise we’d see it.
A lot of things are dark. Cold gas. Dust. Rogue planets. Burned out stars. Black holes. It’s hard to see how there could be more mass in any of these things then all the stars put together, let alone ten times as much! Still, over time, better observations started eliminating all the possibilities. Basically, everything made of normal matter was eliminated as a candidate. The Sherlockian conclusion is that something extraordinary makes up dark matter. The most likely possibility now is an exotic form of matter like axions, subatomic particles that have mass and gravity, but don’t emit light and don’t interact with normal matter. An axion could pass right through you, and like a ghost it would leave no trace.
But that doesn’t mean we can’t detect it. Read More
Before I do anything else, I simply have to present this insanely cool Hubble image of the galaxy cluster MACS J1206, which lies at the mind-numbing distance of 4.5 billion light years from Earth:
[Click to enclusternate, or grab the bigger 2564 x 2328 pixel version.]
Like I said, insanely cool. The cluster has thousands of galaxies in it, and a total mass of something like a quadrillion — that’s 1,000,000,000,000,000 — times the mass of our Sun!
The image was taken as part of a program called CLASH, for Cluster Lensing And Supernova survey with Hubble. A large group of astronomers from ten different countries are observing more than two dozen such distant clusters to look for many interesting things, including exploding stars (which help us gauge the expansion rate of the Universe), very distant galaxies (to help us understand the early Universe), and to look for dark matter.
Dark matter is stuff that doesn’t emit light, but has mass. Careful observations over the years have ruled out pretty much every form of normal matter we can think of, from simple hydrogen clouds to black holes. Whatever this stuff is, it’s weird, not matter as we know it.
But we do know it’s there. Its gravity affects how spiral galaxies rotate, how clusters like MACS 1206 stay together, and can even bend light from more distant galaxies as it passes through. That last bit is the big deal here.
The largest structures in the Universe are superclusters: not just clusters of galaxies, but clusters of clusters. They can stretch for millions of light years and be composed of thousands of galaxies.
Abell 2744, at a distance from Earth of about 3.5 billion light years, is one such megastructure (if you want to sound fancy, astronomers call it "large-scale structure"). Astronomers have been studying Abell 2744 with an arsenal of telescopes, and have discovered that it’s actually the result of the ongoing collision of four galaxies clusters. If you’ve ever wondered what 400 trillion solar masses of material slamming into each other looks like, well, it’s more than a bit of a mess:
[Click to enclusternate.]
Yeah, like I said, it’s a mess.
First off, this picture is a combination of observations from Hubble (in visible light, colored blue, green, and red), the Very Large Telescope (also blue, green, and red), and the Chandra X-Ray Observatory (X-rays, colored pinkish). In visible light you can see literally hundreds of galaxies, probably more, dotting the supercluster. The pink glow is from very hot gas between galaxies; it started its life as gas inside of galaxies that got stripped off and heated to millions of degrees as the galaxies plow through the space around them (I like to think of it as opening a car window to let a noxious smell out — the wind from the car’s motion pushes the air inside the car out the windows).
The blue glow is perhaps the most interesting bit here: it’s a map of the location of dark matter. This type of exotic matter neither emits nor reflects light — hence the name — but it has mass, and that means it has gravity. As I described when this method was used to trace dark matter in the Bullet Cluster, gravity bends space, and light follows that curve. Galaxies farther away get their light distorted by the gravity from dark matter, and that distortion can be measured and used to trace the location of dark matter. The blue glow in the image above maps that.
The thing about dark matter is that it doesn’t interact with normal matter (electron, protons, you, me, lip balm, oranges, whatever). But all that gas between galaxies shown in pink is normal matter, so when one galaxy cluster slams into another at a few thousand kilometers per second that gas gets compressed, mixed-up, and heated. But dark matter just blows right on through. So by comparing the location of the galaxies, the dark matter, and the hot gas, a lot of the cluster’s history can be unraveled.
The good folks at the
Space Telescope Science Institute European Space Agency just released this gorgeous Hubble picture of the globular cluster NGC 1806:
Wow! I actually cropped it a bit and shrank it to get it to fit correctly on the blog, so click it to see it in all its 3741 x 2303 pixel glory.
Globular clusters are spherical collections of hundreds of thousands and even sometimes millions of stars, held together by their mutual gravity. The stars orbit every which-way, and I like to think of them as stellar beehives. The clusters as a whole orbit galaxies on long paths that sometimes take them well away from their parent galaxy, so we see them scattered across the sky.
NGC 1806 is actually part of another galaxy: the Large Magellanic Cloud (or LMC to those in the know), an irregular smear of a billion or so stars that orbits the Milky Way itself as a satellite galaxy. Given that this means the globular cluster is something like 170,000 light years away — 1.7 quintillion km, or a quintillion miles — it’s a pretty clear picture!
This is fascinating news: 90% of the distant Universe was thought to be missing, but it was recently found. And what’s weird is, it was found to be in the red. Quite literally.
[Note: before you ask, this has nothing to do with dark matter. See below!]
First, a bit of background. Galaxies are filled with hydrogen gas, and that gas is a major component of the clouds that collapse to form stars. When that happens, the hot stars ionize the gas: the flood of ultraviolet light strips the electron away from the proton, freeing both. If the electron gets near the proton again, they can recombine. Because of quantum mechanics, the electron can only exists in certain energy states, which are a bit like steps in a staircase. You can jump from the third step down to the second, but there is no second-and-a-halfth step.
So it is with electrons. It used to be taught that this levels were like orbits, but that’s not a great analogy; the staircase is better. So if the electron is on the second level and drops to the first, it gives off energy in the form of light (just like when you step down you lose a bit of energy too, and it takes energy to go up a step). For the 2 to 1 step in hydrogen, the photon emitted is in the ultraviolet, and has a special name: Lyman alpha.
Ionized hydrogen gas clouds tend to blast out lots of Lyman alpha. This makes it a good way to search for distant star forming regions; just look for that wonderful wavelength of light associated with the 2 – 1 transition of hydrogen.
As it happens, we know that when the Universe was young, about a quarter the age it is now, star formation was going on at a much higher rate on average than it does now. So astronomers figured, hey, why not do searches for distant galaxies using Lyman alpha? They should pump it out, and make them easy to see.
So they looked. And to their surprise, they only found about 10% of the galaxies they predicted they should!
This has been a problem for some time. But it’s not anymore: a recent experiment by astronomers shows that the galaxies are there, but they’re hidden!
What they did is look in one part of the sky, using the GOODS South field (part of which is pictured above), trying to find Lyman alpha emitting galaxies. Then they looked at the same region, but looked instead for H alpha, the line emitted when an electron jumps down from the third energy level to the second. And guess what they found: tons of galaxies!
The problem, they surmised, is that the galaxies are actually there and emitting Lyman alpha. But before that ultraviolet light can get out of one of those galaxies, it gets reabsorbed by gas inside the galaxy itself. We never see it.
But H alpha can more easily escape the galaxies once it’s produced. For one thing, it’s red light, and that can penetrate the gas and dust better than the ultraviolet Lyman alpha light can. There are other more complicated reasons as well, but the point is, the galaxies were simply hidden from us before, but not anymore. By extrapolating their results, it looks like they found 90% of the distant Universe!
I’ll note: this has nothing to do with dark matter. As it happens, 90% of the matter in the Universe is in a form that emits no light, but affects other matter through gravity. We know it exists, and you can find out why here. We know it exists locally, in nearby galaxies and clusters of galaxies, too. This new result doesn’t affect that, since the now un-hidden galaxies are very far away, like many billions of light years away. They can’t possibly affect nearby galaxies, so they don’t account for dark matter.
I love this study. It’s a great application of simple logic, though it wasn’t so simple to do: they had to use a lot of time on a monster 8 meter telescope to do it! But they were able to answer a question that has been around for some time, and it really does look like they’ve solved it.
And, as always, it makes me wonder what else is lurking out there in space, hidden but for a leap of logic and technology that will allow us to unveil it. Science is all about thinking around problems, and peeking into dusty corners. Sometimes the most interesting things are found there… including, in this case, the vast majority of the Universe!
One of the secondary goals of the Fermi gamma ray satellite is to look for the signature of dark matter. One idea for dark matter is that it’s composed of weird (and as yet undetected) particles called WIMPs (weakly interacting massive particles). A very odd property about them is that they are self-annihilating: when two of them touch, they turn into energy (and other, more easily detectable particles). When I first read about this several years ago I was pretty excited, because this is finally a testable hypothesis about dark matter.
My fellow Hive Overmind blogger and astronomer Sean Carroll writes that it’s possible Fermi has done just this. The data are not conclusive, but very provocative nonetheless. He has the details.
But I can’t resist adding that on The Big Bang Theory a few weeks ago, Raj and Sheldon were investigating building a detector to look for this very type of dark matter. I wrote David Saltzberg, the science advisor (whom I met on the set last month when I was visiting LA; more on him and that at a later date) and told him this, and he noted that I was right. Well, how about that! It had to happen sometime. Now, to publish…
The picture above shows a cosmic bulls-eye of epic alignment. But before I can tell you about it, I have to tell you about how the dart got thrown.
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.