The Enigma of Dragonfly 44, the Galaxy That’s Nearly Invisible

In 2016, astronomers led by Pieter van Dokkum of Yale College revealed a bombshell paper claiming the invention of a galaxy so dim, but so broad and heavy, that it have to be virtually solely invisible. They estimated that the galaxy, dubbed Dragonfly 44, is 99.99 % darkish matter.

A heated debate ensued about Dragonfly 44’s properties that is still unresolved. In the meantime, greater than 1,000 equally huge however faint galaxies have turned up.

Dragonfly 44 and its ilk are generally known as ultra-diffuse galaxies (UDGs). Whereas they are often as massive as the biggest odd galaxies, UDGs are exceptionally dim—so dim that, in telescope surveys of the sky, “it’s a process to filter out the noise with out by chance filtering out these galaxies,” mentioned Paul Bennet, an astronomer on the Area Telescope Science Institute in Baltimore, Maryland. The brilliant star-forming gasoline that’s plentiful in different galaxies appears to have vanished in UDGs, leaving solely a skeleton of aged stars.

Their existence has prompted a stir in galactic evolutionary idea, which didn’t predict them. “They didn’t flip up in simulations,” van Dokkum mentioned. “You need to do one thing particular to make a galaxy that huge and faint.”

Wild new theories have emerged to clarify how Dragonfly 44 and different UDGs happened. And these big smudges of sunshine could also be offering recent proof of darkish matter’s invisible hand.

Too A lot Darkish Matter

As gravity brings clumps of gasoline and stars collectively, their mixed energies and momentums trigger the mashup to inflate and rotate. Finally a galaxy emerges.

There’s only one downside. As galaxies rotate, they need to come aside. They don’t seem to have sufficient mass—and thus gravity—to stay collectively. The idea of darkish matter was invented to supply the lacking gravity. On this image, a galaxy sits inside a bigger conglomeration of nonluminous particles. This darkish matter “halo” holds the spinning galaxy collectively.

One technique to estimate a galaxy’s rotation pace, and thus its darkish matter content material, is by counting its spherical clusters of stars. “We don’t know why, from a idea viewpoint,” Bennet mentioned, however the variety of these “globular clusters” correlates intently with these harder-to-measure properties. Within the 2016 paper, van Dokkum counted 94 globular clusters inside Dragonfly 44—a quantity that implied a very massive darkish matter halo, regardless of how little seen matter the galaxy has.

Nobody had ever seen something prefer it. Van Dokkum and co-authors instructed that Dragonfly 44 could possibly be a “failed Milky Manner”: a galaxy with a Milky Manner–sized darkish matter halo that underwent a mysterious occasion early on that robbed it of its star-forming gasoline, leaving it with nothing however growing older stars and a large halo.

Or No Darkish Matter

The thing attracted the curiosity of one other camp of astronomers who argue that darkish matter doesn’t exist in any respect. These researchers clarify galaxies’ lacking gravity by tweaking Newton’s regulation of gravity as an alternative, an strategy known as modified Newtonian dynamics, or MOND.

In line with MOND, the modified gravitational drive for every galaxy is calculated from the mass-to-light ratio of its stars—their complete mass divided by their luminosity. MOND theorists don’t speculate as to why the drive would rely on this ratio, however their advert hoc method matches the noticed speeds of most galaxies, with out the necessity to invoke darkish matter.

When information broke about Dragonfly 44, MOND advocate Stacy McGaugh, an astronomer at Case Western Reserve College, calculated from its mass-to-light ratio that it ought to rotate extra slowly than van Dokkum’s preliminary estimate indicated. The MOND calculation didn’t appear to suit the info.