Mystery of Galaxy’s Missing Dark Matter Deepens

When astronomers using NASA’s Hubble Space Telescope uncovered an oddball galaxy that looked like it didn’t have much dark matter, some thought the search was hard to believe and looked for a simpler explanation.

Dark matter, after all, is the invisible glue that makes up most of the matter in the universe. All galaxies seem to be dominated by it; In fact, galaxies are thought to be made up of large haloes of dark matter.

When astronomers using NASA’s Hubble Space Telescope uncovered an oddball galaxy that looks like it doesn’t have much dark matter, some thought the finding was hard to believe and looked for a simpler explanation. Dark matter, after all, is the invisible glue that makes up the bulk of the universe’s contents. All galaxies are dominated by it; in fact, galaxies are thought to form inside immense halos of dark matter. So, finding a galaxy lacking the invisible stuff is an extraordinary claim that challenges conventional wisdom. It would have the potential to upset theories of galaxy formation and evolution.
Credits: NASA’s Goddard Space Flight Center

Thus, finding a galaxy in the absence of invisible things is an extraordinary claim that challenges conventional wisdom. This would have the potential to upset galaxy formation and evolution theories.

Their original find was first reported in 2018 (Missing Dark Matter in the Weedball Galaxy (hubblesite.org)), a team of scientists led by Peter van Dockum of Yale University in New Haven, Connecticut followed up their initial study with a more robust Hubble look at the galaxy, named NGC 1052-DF2. Scientists refer to it simply as “DF2.”

Mystery of Galaxy's Missing Dark Matter Deepens galaxy NGC 1052-DF2, or DF2.
This Hubble Space Telescope image offers a sampling of aging, red stars in the ultra-diffuse galaxy NGC 1052-DF2, or DF2. The galaxy continues to puzzle astronomers because it is lacking dark matter, an invisible form of matter that provides the gravitational glue to hold galaxies together. Precisely establishing the galaxy’s distance form Earth is a step toward solving the mystery. The close-up at right reveals the many aging red giant stars on the outskirts of the galaxy that are used as intergalactic milepost markers. Researchers calculated a more accurate distance to DF2 by using Hubble to observe about 5,400 red giants. These older stars all reach the same peak brightness, so they are reliable yardsticks to measure distances to galaxies. The research team estimates that DF2 is 72 million light-years from Earth. They say the distance measurement solidifies their claim that DF2 lacks dark matter. The galaxy contains at most 1/400th the amount of dark matter that the astronomers had expected, based on theory and observations of many other galaxies. Called an ultra-diffuse galaxy, the galactic oddball is almost as wide as the Milky Way, but it contains only 1/200th the number of stars as our galaxy. The ghostly galaxy doesn’t appear to have a noticeable central region, spiral arms, or a disk. The observations were taken between December 2020 and March 2021 with Hubble’s Advanced Camera for Surveys.
Credits: SCIENCE: NASA, ESA, STScI, Zili Shen (Yale), Pieter van Dokkum (Yale), Shany Danieli (IAS) IMAGE PROCESSING: Alyssa Pagan (STScI)

“We went to an arena in 2018 with our initial Hubble observations of this galaxy,” says Van Dokkum. “I think it was okay for people to question it because it’s such an unusual result. It would be nice if there was a general explanation like a wrong distance. But I think it’s more fun and more interesting if it’s a very strange galaxy.”

Determining the amount of dark matter in a galaxy depends on accurate measurements of how far it is from Earth.

Van Ducum’s team strongly claims that if DF2 is too far from Earth, the galaxy’s dark-matter content could be only a few percent. The team’s conclusion is based on the motions of the stars within the galaxy; Their velocity is affected by gravitational pull. Researchers have found that the number of observed stars is responsible for the total mass of the galaxy, and there’s not much room left for dark matter.

However, according to some astronomers, if DF2 were closer to Earth, it would be intrinsically fainter and less massive. The galaxy, therefore, would need dark matter to account for the observed effects of the total mass.

A better yardstick

Zili Shen, a team member from Yale University, says the new Hubble observations help confirm that DF2 is not only farther from Earth, according to some astronomers, but also slightly farther than the team’s original estimate.

The new distance estimate is that DF2 is 72 million light-years as opposed to 42 million light-years, as reported by other independent teams. This places the galaxy farther away than the original Hubble 2018 estimated 65 light-years away.

The research team based its new result on long exposures with Hubble’s Advanced Camera for Surveys, which provide a deeper view of the galaxy for finding a reliable yardstick to nail down the distance. They have targeted the old red giant star on the outskirts of the galaxy, each reaching the same peak brightness in their evolution. Astronomers can use the stars’ intrinsic brightness to calculate vast intergalactic distances. “Studying bright red giants is a well-established distance indicator for nearby galaxies,” Shen explained.

Team members say, the more accurate Hubble measurements make the researchers’ initial conclusions about the galaxy deficient in dark matter. So the mystery of why DF2 is missing most of its dark matter still persists.

“For almost every galaxy we look at, we say that we can’t see most of the mass because it’s dark matter,” Van Duckcom explained. “What you see is just the tip of the iceberg with Hubble. But in this case, what you see is what you get.It’s It’s not just the tip of the iceberg, it’s the whole iceberg.”

The group’s science paper has been published in The Astrophysical Journal Letters.

A stealthy galaxy

The DF2 is a giant cosmic cotton ball that Van Dokkum calls a “see-through galaxy” where the stars scatter. The galactic orbital is as wide as the Milky Way, but it is only 1/200th the number of stars in our galaxy.

ghostly galaxies do not have a noticeable central region, spiral arms, or a disc. The team estimates that the amount of dark matter is 1/400th higher than DF2 astronomers expect. How the galaxy is formed remains a complete mystery based on the latest measurements of the team.

DF2 is not just a dark matterless galaxy. Shany Danieli of the Princeton Institute for Advanced Study in Princeton, New Jersey, used Hubble in 2020 to get an accurate distance to another haunted galaxy called NGC 1052-DF4 (or DF4 only), which apparently lacks the dark matter. However, in this case, some scientists have suggested that the dark matter has dropped out of the galaxy due to the tidal wave from other galaxies.

The researchers believe that both DF2 and DF4 were members of the galaxy. However, new Hubble observations show that while the two galaxies are 6.5 million light-years away from each other, they were farther away from the first thought. It also appears that DF2 has moved away from grouping and is isolated in space.

Both galaxies were discovered at the New Mexico Sky Observatory with the Dragonfly telephoto array.

“Both of them probably were in the same group and formed at the same time,” Danieli said. “So there was probably something special about the environment they created.”

Researchers are hunting for more of these oddball galaxies.Other teams of astronomers are also searching. In 2020, a team of researchers discovered 19 unusual dwarf galaxies that they say are deficient in dark matter (Off the Baryonic Tully–Fisher Relation: A Population of Baryon-dominated Ultra-diffuse Galaxies – IOPscience).  However, it will take uncovering many more dark matter-less galaxies to resolve the mystery.

Nevertheless, van Dokkum thinks finding a galaxy lacking dark matter tells astronomers something about the invisible substance. “In our 2018 study, we suggested that if you have a dark matterless galaxy and other similar galaxies appear to be present near it, it means that the dark matter is real and exists,” says Van Dokkum. “It’s not a mirage.”

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center operates telescopes in Greenbelt, Maryland. The Space Telescope Science Institute (STSCI) in Baltimore, Maryland conducts Hubble Science activities. STSCI is conducted for NASA by the Association of Astronomy Research in Washington, DC.