A smattering of small galaxies appear to be missing a whole lot of dark matter.
Most of a typical galaxy is invisible. This elusive mass, known as dark matter, seems to be an indispensable ingredient for creating a galaxy — it’s the scaffolding that attracts normal matter — yet reveals itself only as an extra gravitational tug on gas and stars.
But now, researchers have found 19 dwarf galaxies — all much smaller than the Milky Way — that defy this common wisdom. These newly identified outliers have much less dark matter than expected. The finding, published November 25 in Nature Astronomy, more than quintuples the known population of dark-matter renegades, adding fuel to an already simmering mystery.
“We are not sure why and how these galaxies form,” says Qi Guo, an astrophysicist at the Chinese Academy of Sciences in Beijing. Typical dwarf galaxies concentrate dark matter far more than their larger cousins, she notes. Their smaller size leads to weaker gravity, which has trouble holding on to tenuous clouds of gas. That usually shifts the balance of mass in dwarf galaxies away from normal matter and toward dark matter.
“This new class of galaxy is straining our ability to explain all galaxies in one cohesive framework,” says Kyle Oman, an astrophysicist at Durham University in England who was not involved in this research.
In 2016, Oman and his colleagues identified two galaxies that appeared to be missing dark matter. In short order, two more oddballs turned up (SN: 3/28/18).
Guo and her colleagues wondered if these galaxies had more company. So using existing data from the Arecibo radio telescope in Puerto Rico, the team weighed dwarf galaxies by looking at how fast hydrogen whipped around each one. Higher speed means more total mass. The researchers then combined the mass of the hydrogen and of all the stars, inferred from starlight, to estimate how much of each galaxy’s mass is made up of normal matter.
For every galaxy, total mass added up to more than the mass of the gas and stars — not surprising, as that extra mass is the dark matter. But in about 6 percent of cases, there wasn’t as much extra mass as expected.
One oddball, designated AGC 213086, weighs in at around 14 billion suns. If it were typical, about 2 percent of its mass — nearly 280 million solar masses — would be gas and stars. Instead, its actual inventory of normal matter is about 3.8 billion solar masses, or about 27 percent of its total mass.
Of 324 dwarf galaxies analyzed, 19 appear to be missing similarly large stores of dark matter. Those 19 are all within about 500 million light-years of Earth, and five are in or near other groups of galaxies. In those cases, the researchers note, perhaps their galactic neighbors have somehow siphoned off their dark matter. But the remaining 14 are far from other galaxies. Either these oddballs were born different, or some internal machinations such as exploding stars have upset their balance of dark matter and everyday matter, or baryons.
It may not be a case of missing dark matter, says James Bullock, an astrophysicist at the University of California, Irvine. Instead, maybe these dwarf galaxies have clung to their normal matter — or even stolen some — and so “have too many baryons.” Either way, he says, “this is telling us something about the diversity of galaxy formation…. Exactly what that’s telling us, that’s the trick.”