The mystery of Dragonfly 44, the galaxy that is almost invisible

2016, Astronomers led by Yale University’s Pieter van Dokkum, released a bombshell paper claiming the discovery of a galaxy so faint, yet so wide and massive that it must be almost entirely invisible. They estimated that the galaxy, named Dragonfly 44, is 99.99 percent dark matter.

A heated debate about the characteristics of Dragonfly 44 ensued, which remains unresolved. More than 1,000 similarly sized but faint galaxies have now surfaced.

Dragonfly 44 and its ilk are known as ultra-diffuse galaxies (UDGs). Though they can be as large as the largest ordinary galaxies, UDGs are exceptionally faint — so faint that in telescopic 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 Space Telescope Science Institute in Baltimore, Maryland. The bright star-forming gas, abundant in other galaxies, appears to have disappeared in UDGs, leaving only a skeleton of older stars.

Her existence has caused a stir in galactic evolutionary theory that she could not have predicted. “They didn’t show up in simulations,” said van Dokkum. “You have to do something special to make a galaxy this big and faint.”

Wild new theories have emerged to explain how Dragonfly 44 and other UDGs came about. And these giant blobs of light could provide new evidence of dark matter’s invisible hand.

Too much dark matter

As gravity brings blobs of gas and stars together, their combined energies and momentum cause the mashup to inflate and spin. Eventually a galaxy is formed.

There’s only one problem. As galaxies spin, they should fall apart. They don’t seem to have enough mass – and therefore gravity – to hold them together. The concept of dark matter was invented to provide the missing gravity. In this image, a galaxy sits within a larger conglomerate of non-luminous particles. This “halo” of dark matter holds the spinning galaxy together.

One way to estimate a galaxy’s rotational speed, and therefore its dark matter content, is to count its globular star clusters. “We don’t know why from a theoretical perspective,” Bennet said, but the number of these “globe clusters” correlates closely with these more difficult-to-measure properties. In the 2016 publication, van Dokkum counted 94 globular clusters within Dragonfly 44 — a number that suggests an exceptionally large dark matter halo, even though the galaxy has little visible matter.

Nobody 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 early on underwent a mysterious event that robbed it of its star-forming gas, leaving nothing but aging stars in its wake and a huge halo.

Or no dark matter

The object caught the interest of a different camp of astronomers, who argue that dark matter doesn’t exist at all. These researchers explain the lack of gravity in galaxies by instead fitting Newton’s law of gravitation, 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. MOON theorists do not speculate as to why the force would depend on this ratio, but their ad hoc formula fits the observed velocities of most galaxies without the need for dark matter.

When news broke about Dragonfly 44, MOON proponent Stacy McGaugh, an astronomer at Case Western Reserve University, calculated from its mass-to-light ratio that it should spin slower than van Dokkum’s original estimate. The MOND calculation did not seem to match the data.

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