Missing galaxies may not be missing
The lack of small satellites around the Milky Way has long been a problem – but all may not be as it seems
In the past couple of decades, we’ve converged on a new standard model of cosmology. This theory has its problems: it depends on the existence of mysterious dark matter and even more enigmatic dark energy, but it keeps passing observational tests.
It’s not surprising, then, that attention focuses on any possible discrepancy between the predicted and the real Universe. The most famous case is the ‘missing satellites’ problem. Large simulations of the Universe that use the standard model of cosmology predict that large galaxies such as the Milky Way should be surrounded by many more small satellite galaxies than we see.
The Milky Way does have satellites, as anyone who has been to the southern hemisphere and seen the beautiful Large and Small Magellanic Clouds will tell you. There just aren’t enough detected to match the predictions, but a new paper argues that we shouldn’t give up just yet. The missing satellites are small, and thus hard to detect. Just because we don’t see them, it doesn’t mean they don’t exist: even deep surveys of the sky can’t claim to have carried out a complete census. The Sloan Digital Sky Survey, which mapped the northern sky and which is the go-to dataset for such studies, is only reliable out to a distance one-fifth of the way to that of the most distant predicted systems.
Making matters worse is the likely composition of galaxies. Once you’re smaller than about a hundredth of the mass of the Milky Way, a limit of about a billion solar masses or so, it’s hard to reach the density of gas that is required to kick-start star formation. That means most of these smaller satellites, precisely those which might be hiding, will be even dimmer than expected and harder to detect.
“Large simulations predict that galaxies such as the Milky Way should be surrounded by more small satellite galaxies than we see”
One way to deal with this situation is to build deeper surveys, capable of spotting even the puniest satellite galaxies. I hope we’ll do that one day. In the meantime, Stacy Kim from Ohio State University and colleagues Annika Peter and Jonathan Hargis have decided to get on with it, building a complicated statistical model which can predict, from the data we already have in hand, the number of such galaxies that are out there.
Essentially, they make sensible guesses for how the number of satellites we see relates to the number of (mostly smaller) systems that are out there still to be discovered. This is one of those tasks that sounds simple, but is actually devilishly complicated, with the relationship between the mass of a galaxy and the number of brightly shining stars it contains very difficult to constrain. Yet the results are stunning.
For sensible guesses as to how the Universe behaves, the authors find that the number of satellites around the Milky Way matches the predictions of the standard cosmological model. There is, in fact, absolutely no missing satellite problem, and while we should still check by looking, it seems our best guess as to what the Universe is really like has passed another test.