In 2016, astronomers led by Pieter van Dokkum of Yale University published a paper showing that there is a galaxy so dark, but so big and so rich, that it must be invisible. They estimated that the galaxy, called Dragonfly 44, is 99.99 percent dark matter.
Fierce negotiations ensued over the Dragonfly 44’s cargo which was not resolved. So far, about 1,000 massive but faint galaxies have been discovered.
Dragonfly 44 and its counterparts are known as Ultra-diffuse galaxies (UDGs). Although they may be as large as other large galaxies, UDGs are exceptionally dark—so much so that, in telescopes, “it’s a job to filter out the noise without accidentally filtering out these galaxies,” says Paul Bennet. an astronomer at the Space Telescope Science Institute in Baltimore, Maryland. The luminous star-forming gas that is abundant in other galaxies appears to have disappeared in UDGs, leaving only the shell of old stars.
Their existence has caused chaos in the theory of the evolution of galaxies, which failed to predict them. “They didn’t make any assumptions,” van Dokkum said. “You have to do something special to make a galaxy big and faint.”
Wild new theories have emerged to explain how Dragonfly 44 and other UDGs originated. And this great light may provide new evidence of the invisible hand of dark matter.
A Very Dark Story
As gravity brings clusters of gas and stars together, their combined forces and energies cause the mashup to spin and spin. At the end a galaxy appeared.
There is only one problem. As galaxies rotate, they must split apart. It appears that they do not have enough mass—and thus gravity—to hold together. The concept of dark matter was developed to provide the missing gravity. In this picture, the galaxy is inside a giant collision of small particles that make up the light. This dark matter “halo” holds the surrounding galaxy together.
One way to determine a galaxy’s velocity, and thus whether it contains dark matter, is to count its orbiting star clusters. “We don’t know why, in terms of theory,” Bennet said, but the number of “groups of all kinds” is closely related to things that are hard to measure. In a 2016 paper, van Dokkum counted 94 clusters of each type within Dragonfly 44 – a number that means the largest black halo, despite how small the galaxy is.
No one had ever seen anything like it. Van Dokkum and his co-authors suggested that Dragonfly 44 may be a “failed Milky Way”: a galaxy with Milky Way-type galaxies that underwent a mysterious past that robbed it of its star-forming gas, leaving it with nothing but old stars and light. great.
Or No Darkness
The object attracted the attention of a camp of astronomers who say that dark matter does not exist at all. The researchers explain the missing galaxies’ gravity by modifying Newton’s law of gravity instead, a method called modified Newtonian dynamics, or MOND.
According to MOND, the modified gravity of each galaxy is calculated from the ratio of the magnitude and luminosity of its stars—its total mass divided by its luminosity. MOND theorists do not understand why the force would depend on this number, but their method of detection is similar to the speed of many galaxies, without the need to mention dark matter.
When news of Dragonfly 44 was heard, MOND representative Stacy McGaugh, an astronomer at Case Western Reserve University, calculated from the light ratio that it would rotate more slowly than van Dokkum had originally estimated. The MOND calculations did not match the data.
