Galaxies swarm and light bends under dark matter's sway

Bad Astronomy
The entire universe in blog form
Oct. 14 2011 7:08 AM

Galaxies swarm and light bends under dark matter's sway

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:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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[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.

Take a look at the cluster again. See those bright, weirdly drawn-out smears, as if someone grabbed a galaxy at both ends and stretched? Those are galaxies on the other side of the cluster from us, farther away. As the light from those galaxies passes through the cluster on the way to Earth, it gets distorted by the gravity of the galaxies in the cluster, and so we see the shape distorted. This is called gravitational lensing, and by carefully mapping that distortion, the distribution and amount of dark matter in MACS 1206 can be determined!

And that's the whole point.

Now take another look at the picture. See how so many of the galaxies appear to lie perpendicular to the galaxy center? That's another effect of the dark matter distortion; the smearing is tangential to the direction to the cluster center. So a lot of those distorted galaxies aren't just smeared out, they look like little arcs, centered on the cluster center. Again, by measuring this, we can figure out a lot about the dark matter in the cluster.

I also got a kick out of this wiggly galaxy off to the right of the cluster center. How weird is that thing? It appears to snake around those two galaxies I marked, and I have to wonder if they have enough gravity to actually distort the light from that more distant galaxy that much, or if it's just a coincidence. I'm having a hard time thinking it's anything but the gravitational influence of those two galaxies doing that, but I've never seen anything like it! The astronomers published a paper about the program, but didn't mention that squirmy guy in particular.

Also, if you look at the embiggened picture of the cluster, you'll see it dotted with tiny red galaxies (I've cropped out a section here and marked three of them; many more can be found). I'm really wondering about those. Are they smaller cluster members, galaxies with so much dust that the starlight from them is reddened (the same way sunlight is redder at sunset), or are they big galaxies so far away they're diminished in size through sheer distance, their light red shifted to that ruddy glow?

I don't know. But the astronomers running CLASH will find out. Part of the program is to use the Very Large Telescope to get spectra of some of the galaxies in the survey, and I would bet a lot of money some of those little red dudes will be targeted. If they really are very far away, then they could prove very useful in understanding a lot about the early Universe.

I'm really impressed with this work so far, and in the ambitious nature of the survey. I literally gasped when I read that CLASH will get 524 orbits of Hubble to do this survey! That's at least 300 hours of solid observing time, which is incredible. When I worked on Hubble, I was on some projects that got two orbits, and we were happy to get them! So 524 is an astonishing amount, and indicates pretty clearly that the folks in charge of giving astronomers time on Hubble think very highly of this project.

Given how much data they'll be getting, I expect we'll learn a lot about the mid- to very distant Universe in this survey, including how galaxies behave when they're young, how the invisible fingers of dark matter shape normal matter, and even how the Universe itself is expanding on large scales. All of that, and much more, by taking a telescope, putting it up above our atmosphere, and letting it stare at one target after another for a long, long time.

That, plus the human ability to make sense of nature -- which is what science is. That goes a long way, too.

Image credit: Credit: NASA, ESA, M. Postman (STScI) and the CLASH Survey Team



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