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Milky Way’s Tiny Satellite Galaxies May Help Solve Dark Matter Mystery

Hubble observations of the smallest galaxies known help explain why so few of them exist
Hubble photo of ultra-faint dwarf galaxy Leo IV



NASA, ESA, and T. Brown (STScI)

As galaxies go, these are a bunch of wimps. The so-called ultrafaint dwarfs are like ghost galaxies—tiny wisps against the sky that were first discovered less than a decade ago. Now about a dozen have been found that orbit our Milky Way galaxy, and they’re helping to solve an astronomical riddle called the missing satellites problem.

Dark matter is an intangible, invisible substance thought to make up 23 percent of the universe, and models suggest that it should be at the heart of thousands of mini galaxies orbiting the Milky Way and other large spirals. Yet only a few dozen have ever been observed, counting both the ultrafaint dwarfs and the not-quite-as-small dwarf galaxies. The mismatch between hypothesis and observation has caused some physicists to question dark matter models’ prediction of the existence of so many small galaxies. Now, new Hubble Space Telescope observations of some of the tiniest Milky Way satellite galaxies are offering clues about where the rest of their siblings may be hiding.

The first ultrafaint dwarf galaxies—some of which are so dim that a single bright giant star can outshine them—were discovered in 2005 in data collected by the Sloan Digital Sky Survey. That systematic mapping survey, which covers a quarter of the sky, eventually revealed the presence of more dwarf galaxies, but not nearly as many as hypotheses predicted. To take a closer look, Tom Brown of the Space Telescope Science Institute in Baltimore and his colleagues chose to observe a representative sample of six ultrafaint dwarf galaxies with two of Hubble’s cameras. The scientists recently shared the full data gathered on all six galaxies at a meeting on Milky Way cosmology in Finland, and the results will be published in an upcoming issue of the Journal of the Italian Astronomical Society.

The Hubble data created portraits of these six galaxies that allowed the researchers to measure the age of the galaxies’ stars with unprecedented precision. The tiny objects are incredibly aged, almost as old as the universe itself, and they likely stopped birthing stars long, long ago. “These are the only galaxies known so far that are so ancient,” Brown says. “It looks like the vast majority of these stars is older than 12 billion years old.” (The universe itself, by comparison, is roughly 13.8 billion years old). The finding supports one idea put forward to explain the missing satellites problem, which suggests that most of the dark matter clumps predicted by models are too small to attract gas and form stars at all—thus, they would be invisible. If this idea is correct, there should be some transition objects between the starless dark matter blobs and the fully formed dwarf galaxies, where star formation started but was halted prematurely.

But what could cause such a break? Just after the big bang the universe was too hot to form atoms, so protons and electrons floated freely as ions. Eventually the universe cooled to the point that the protons and electrons could join forces, and most of the universe’s matter, in the form of hydrogen, became neutrally charged. When the first stars formed, their radiation excited the hydrogen, causing it to ionize again during an epoch known as reionization. The process might have halted star formation in the smallest galaxies, which wouldn’t have had enough mass to hold on to hot, excited gas. “This gas is the fuel for star formation, so when it goes away because of reionization, all star formation shuts down along with it,” says James Bullock of the University of California, Irvine, one of the authors of this reionization hypothesis.

The new observations provide a welcome opportunity to test this hypothesis, says Beth Willman, an astronomer at Haverford College who wasn’t involved in the research. “The data really represent an important step forward in our understanding of the history of these dwarfs,” Willman added. Still, the missing satellites problem is not a closed case, because factors aside from reionization might also have shut down star formation in the dwarfs, such as the violent gravitational forces that might have resulted if the satellite galaxies had formed elsewhere and were captured later by the Milky Way.

Furthermore, to really solve the missing satellites quandary astronomers must first make a complete census of the dwarf galaxies—a task that is far from complete, researchers say. “It’s funny this is called the missing satellites problem when we haven’t actually finished looking for them yet,” Willman says. “We need to understand which things actually are missing and which things are not.”

The progress being made in understanding dwarfs is at least calming fears that the basic ideas about dark matter that predicted a plentitude of satellite galaxies are not unsound. “When the problem was first pointed out, there was some concern that it meant that our understanding of dark matter or cosmology itself might be flawed,” Bullock says. “Today we have come to understand that there are a lot of reasons why dwarf galaxy formation might be inefficient. Observations like this one are fascinating, as they draw a possible connection between the very local universe and the events that were happening in the universe's infancy.”

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