The so-called missing satellites problem has been solved. Based on our models of how dark matter clumps together, we know there ought to be more small satellite galaxies around the Milky Way than we have seen. But we haven’t been able to find them.
Now, there is a simple answer: they were there all along, we just missed them.
at Ohio State University in Columbus and her team pored through the data from the (SDSS) and found that extrapolating from the number of galaxies in the survey area – about one third of the sky, thus far – we should be able to detect about 200 satellite dwarf galaxies.
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Comparing that number with what current models of dark matter tell us we should see, they found there was agreement. Some of that is down to more accurate simulations, and some to more precise observations.
“While it doesn’t put the final nail in the coffin, it’s along the lines of what we’ve been thinking for the past decade when it comes to the missing satellite problem: as simulations get better, they predict fewer dwarf galaxies, and as observations get better, they find more dwarf galaxies,” says , also at Ohio State University.
“What used to be a factor of 10 – or worse – mismatch has been reduced to a factor of 2 in the past few years, and this latest work may close the final gap,” he says.
Reality mismatch
The reason the satellite galaxies were thought to be missing in the first place was that we were relying on dark matter models that didn’t match our observations. These models proposed that as the universe aged, dark matter would clump together.
The blobs of dark matter should attract higher concentrations of gas that would then form stars. Simulations showed that there should be hundreds of small galaxies around the Milky Way, but by 2005, we had only found a dozen or so.
Since then, new sky surveys have increased that number to about 50. The SDSS can detect faint dwarf galaxies down to about 340 times as luminous as the sun – notably Segue I, the faintest satellite dwarf galaxy yet detected – and the Dark Energy Survey has picked up some as well, including eight in 2015, after its second year of operation.
Kim and her colleagues note in the paper that models of star formation and galaxy evolution seemed to show that smaller concentrations of matter would actually be less efficient in making stars, so not every small clump, or subhalo, of dark matter would host luminous stars.
Cold versus warm dark matter
“They have essentially put a few things together – correcting surveys for incompleteness plus doing some basic theoretical predictions – and shown that there are no major problems in the number of very faint galaxies we see, as long as below a certain mass of dark matter clump, no galaxy is made,” says at the University of California, Irvine.
The work may also limit models of “warm” dark matter, in which the mysterious matter that fills a majority of our universe is made of very fast-moving particles that don’t interact with light. Most models of dark matter assume it is “cold” – that the particles aren’t very energetic and therefore clump together more readily.
But if dark matter is indeed warm, this work provides a kind of anti-speed limit for those particles. Any warm dark matter particles would need a mass above about 4000 electronvolts in order to clump enough to make dwarf galaxies.
With cold dark matter, the prediction would be for a lot of small halos, the clumps that should form galaxies. In this case, some galaxies might be “missing” because many of these halos were too small to attract enough matter to make stars, says at the Observatories of the Carnegie Institution for Science in Pasadena, California.
Warm dark matter would lead to fewer halos and fewer dwarf galaxies. “But you can’t make it too warm or too strongly interacting, or you would end up with fewer dwarf galaxies than we actually see,” he says.
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Read more: Dark force could keep Milky Way’s neighbours away
