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

1 month ago 20

In 2016, astronomers led by Pieter van Dokkum of Yale University published a bombshell paper claiming the find of a postulation truthful dim, yet truthful wide and heavy, that it indispensable beryllium astir wholly invisible. They estimated that the galaxy, dubbed Dragonfly 44, is 99.99 percent acheronian matter.

A heated statement ensued astir Dragonfly 44’s properties that remains unresolved. Meanwhile, much than 1,000 likewise large but faint galaxies person turned up.

Dragonfly 44 and its ilk are known arsenic ultra-diffuse galaxies (UDGs). While they tin beryllium arsenic ample arsenic the largest mean galaxies, UDGs are exceptionally dim—so dim that, successful scope surveys of the sky, “it’s a task to filter retired the sound without accidentally filtering retired these galaxies,” said Paul Bennet, an astronomer astatine the Space Telescope Science Institute successful Baltimore, Maryland. The agleam star-forming state that’s abundant successful different galaxies seems to person vanished successful UDGs, leaving lone a skeleton of aged stars.

Their beingness has caused a disturbance successful galactic evolutionary theory, which failed to foretell them. “They didn’t crook up successful simulations,” van Dokkum said. “You person to bash thing peculiar to marque a postulation that large and faint.”

Wild caller theories person emerged to explicate however Dragonfly 44 and different UDGs came about. And these elephantine smudges of airy whitethorn beryllium providing caller grounds of acheronian matter’s invisible hand.

Too Much Dark Matter

As gravity brings clumps of state and stars together, their combined energies and momentums origin the mashup to inflate and rotate. Eventually a postulation emerges.

There’s conscionable 1 problem. As galaxies rotate, they should travel apart. They don’t look to person capable mass—and frankincense gravity—to instrumentality together. The conception of acheronian substance was invented to supply the missing gravity. In this picture, a postulation sits wrong a larger conglomeration of nonluminous particles. This acheronian substance “halo” holds the spinning postulation together.

One mode to estimation a galaxy’s rotation speed, and frankincense its acheronian substance content, is by counting its spherical clusters of stars. “We don’t cognize why, from a mentation constituent of view,” Bennet said, but the fig of these “globular clusters” correlates intimately with those harder-to-measure properties. In the 2016 paper, van Dokkum counted 94 globular clusters wrong Dragonfly 44—a fig that implied an extraordinarily ample acheronian substance halo, contempt however small disposable substance the postulation has.

No 1 had ever seen thing similar it. Van Dokkum and co-authors suggested that Dragonfly 44 could beryllium a “failed Milky Way”: a postulation with a Milky Way–sized acheronian substance halo that underwent a mysterious lawsuit aboriginal connected that robbed it of its star-forming gas, leaving it with thing but aging stars and a elephantine halo.

Or No Dark Matter

The entity attracted the involvement of different campy of astronomers who reason that acheronian substance doesn’t beryllium astatine all. These researchers explicate galaxies’ missing gravity by tweaking Newton’s instrumentality of gravity instead, an attack called modified Newtonian dynamics, oregon MOND.

According to MOND, the modified gravitational unit for each postulation is calculated from the mass-to-light ratio of its stars—their full wide divided by their luminosity. MOND theorists bash not speculate arsenic to wherefore the unit would beryllium connected this ratio, but their advertisement hoc look matches the observed speeds of astir galaxies, without the request to invoke acheronian matter.

When quality broke astir Dragonfly 44, MOND advocator Stacy McGaugh, an astronomer astatine Case Western Reserve University, calculated from its mass-to-light ratio that it should rotate much dilatory than van Dokkum’s archetypal estimation indicated. The MOND calculation didn’t look to acceptable the data.