The Search for Dark Satellites

“This is the first time in my work that I’ve really gone out on a limb and made a very specific prediction – I didn’t give myself any elbow room… If we’re right, then it’s a huge success and you can find very dim or effectively dark galaxies simply by analysing disturbances in the gas disk.” –Sukanya Chakrabarti

There’s a long-standing problem in the field of dark matter research, which is so distressing that it has led a few people to abandon dark matter altogether.

What am I talking about?

Image credit: The Millenium Simulation.

This — roughly — is what the matter in our Universe looks like on the largest scales. Great filaments of matter connect in a great cosmic web, and the intersections of the largest, strongest filaments correspond to the densest, richest collections of matter in our Universe today, such as clusters and superclusters of galaxies.

Our simulations and our observations, on these large scales, match up as accurately as we’ve been able to measure. But what happens if we come down to smaller scales?

On the scale of just a single large galaxy — like us, perhaps — our simulations tell us that each galaxy should get a large, diffuse halo of dark matter with a thin disk of atoms at the center. Which is perfectly consistent with what we do observe, as well.

But they also predict a large number of very low mass satellites, and we don’t observe those.

“Hang on a minute,” you might say. “What about the Magellanic Clouds?”

Of course the Milky Way does have many small satellite galaxies. Dozens of them, in fact, and that number gets up into the hundreds if you include globular clusters. But our simulations predict many, many more than we observe, and this is a genuine problem for our models of structure formation. We’re pretty confident that we’ve nailed it on large scales, but the small scales have issues. What are the possible resolutions?

  1. Our models for structure formation are all wrong. There’s no dark matter, and our laws of gravity are wrong, and the fact that large-scale structure works as well as it does is nothing more than a lucky coincidence.
  2. There’s a problem with our simulations on small scales. Perhaps dark matter has low-energy interactions that we don’t understand well, perhaps the way galaxies merge and grow works different than our simulations tell us, or perhaps we keep making an unidentified mistake over and over again.
  3. The simulations and our models are both correct, and the flaw lies with our observations. Perhaps these satellite galaxies are there, but they’re much dimmer than we anticipated, and we haven’t found them simply because we don’t know where to look.

On most days, I think option 2 is the most likely. But a BBC story alerted me
to this paper (full text here) by Sukanya Chakrabarti and her collaborators, entitled Finding Dark Galaxies From Their Tidal Imprints.

If option 3 is correct (or even partially correct), this is a group of astronomers really sticking their necks out there to find them. Here’s what they’re doing.

Image credit: Tony and Daphne Hallas.

This is M51, the Whirlpool Galaxy, a gorgeous face-on spiral galaxy. The more astute among you will notice that it’s also an interacting galaxy, as the large spiral is in the process of merging with a smaller galaxy about one-third its size (and slightly behind it, from our point of view).

This and all subsequent images from S. Chakrabarti et al., except where noted.

But if we only focus on the hot hydrogen gas from this object, we see that it sweeps out in a great tail-like shape, with some disruptions in it! It turns out, as this paper discusses, that this exact phenomena can be modeled by simulations, which the authors do.

And just from their model, they can tell you precisely where (and of what mass) the secondary, smaller galaxy is. But that’s a well-known one, where the small galaxy is only a third the mass of the large one. What about a more extreme case, like we’re likely to find near the Milky Way?

Image credit: Hubble Space Telescope.

Say hello to NGC 1512, as shown in both visible and ultra-violet light. With dusty lanes and hot gas, it’s another ideal candidate for this analysis. But unlike M51, the companion galaxy to NGC 1512 is only a few percent of its mass. And when we look at the hot Hydrogen gas, we find the same disturbances in the tail of the gas.

Again, the same analysis — through simulations — allows us to predict where the companion galaxy ought to be and what mass it ought to have to produce these patterns. And what do we find?

It nails it again. Chakrabarti says that this method should be useful for finding the locations of satellite galaxies as small as 0.1% the mass of the large spiral, and will be getting telescope time to search for a satellite to the Milky Way where she predicts it ought to be.

These dwarf galaxies could really be there, and they could just be much dimmer than we’d be able to detect without a dedicated search for them. I will, of course, be skeptical until the galaxy is actually found in the location it was predicted to be. But this is my favorite kind of science: using only the laws we know and understand to make concrete predictions that, if verified, will be a huge step towards solving this puzzle.