In 2016, astronomers led by Pieter van Dokkum of Yale University published a bombshell paper claiming the discovery of a galaxy so dim, yet so broad and heavy, that it must be almost entirely invisible. They estimated that the galaxy, dubbed Dragonfly 44, is 99.99 percent dark matter.
A heated debate ensued about Dragonfly 44’s properties that remains unresolved. Meanwhile, more than 1,000 similarly big but faint galaxies have turned up.
Dragonfly 44 and its ilk are known as ultra-diffuse galaxies (UDGs). While they can be as large as the largest ordinary galaxies, UDGs are exceptionally dim—so dim that, in telescope surveys of the sky, “it’s a task to filter out the noise without accidentally filtering out these galaxies,” said Paul Bennet, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland. The bright star-forming gas that’s abundant in other galaxies seems to have vanished in UDGs, leaving only a skeleton of elderly stars.
Their existence has caused a stir in galactic evolutionary theory, which failed to predict them. “They didn’t turn up in simulations,” van Dokkum said. “You have to do something special to make a galaxy that big and faint.”
Wild new theories have emerged to explain how Dragonfly 44 and other UDGs came about. And these giant smudges of light may be providing fresh evidence of dark matter’s invisible hand.
Too Much Dark Matter
As gravity brings clumps of gas and stars together, their combined energies and momentums cause the mashup to inflate and rotate. Eventually a galaxy emerges.
There’s just one problem. As galaxies rotate, they should come apart. They don’t appear to have enough mass—and thus gravity—to stick together. The concept of dark matter was invented to provide the missing gravity. In this picture, a galaxy sits inside a larger conglomeration of nonluminous particles. This dark matter “halo” holds the spinning galaxy together.
One way to estimate a galaxy’s rotation speed, and thus its dark matter content, is by counting its spherical clusters of stars. “We don’t know why, from a theory point of view,” Bennet said, but the number of these “globular clusters” correlates closely with those harder-to-measure properties. In the 2016 paper, van Dokkum counted 94 globular clusters inside Dragonfly 44—a number that implied an extraordinarily large dark matter halo, despite how little visible matter the galaxy has.
No one had ever seen anything like it. Van Dokkum and co-authors suggested that Dragonfly 44 could be a “failed Milky Way”: a galaxy with a Milky Way–sized dark matter halo that underwent a mysterious event early on that robbed it of its star-forming gas, leaving it with nothing but aging stars and a giant halo.
Or No Dark Matter
The object attracted the interest of another camp of astronomers who argue that dark matter doesn’t exist at all. These researchers explain galaxies’ missing gravity by tweaking Newton’s law of gravity instead, an approach called modified Newtonian dynamics, or MOND.
According to MOND, the modified gravitational force for each galaxy is calculated from the mass-to-light ratio of its stars—their total mass divided by their luminosity. MOND theorists do not speculate as to why the force would depend on this ratio, but their ad hoc formula matches the observed speeds of most galaxies, without the need to invoke dark matter.
When news broke about Dragonfly 44, MOND advocate Stacy McGaugh, an astronomer at Case Western Reserve University, calculated from its mass-to-light ratio that it should rotate more slowly than van Dokkum’s initial estimate indicated. The MOND calculation didn’t seem to fit the data.