Precise Distance to Galaxy Bolsters Missing Dark Matter Claim
What if oceanographers found the “tip” of an iceberg and nothing else? Mysteriously missing was the iceberg’s immense body, which extends far below the waves.
Astronomers faced this puzzle when they aimed Hubble at the spheroidal galaxy NGC 1052-DF2, or DF2. It looks like a denizen of intergalactic space that is the closest thing there is to nothing, but is still something. It’s physically larger than our Milky Way, but its loose beehive swarm of stars is so thinly scattered that Hubble sees right through it, capturing myriad background galaxies.
The missing “bottom of the iceberg” for DF2 is the lack of dark matter. Galaxies are partly made up of visible matter—stars and gas. But the bulk of a galaxy’s makeup is in dark matter, the invisible glue that keeps a lid on stars, so they don’t escape from the galaxy.
Because this innocuous galaxy challenges conventional theories of how galaxies are put together, astronomers were naturally skeptical when it was first announced that the universe harbored such a rule breaker. After all, the entire cosmos is built on the invisible scaffolding of dark matter.
To double-check their conclusion, the researchers used a lot more Hubble exposures to better nail down the distance to the stealthy galaxy. If DF2 were closer than they thought, the dark matter mystery goes away.
They actually found that the galaxy is a little bit farther away than first measured. The researchers say the new milepost helps them confirm that dark matter is really missing in the galactic oddball. They say it’s now up to theorists to figure out why.
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. Credit: NASA’s Goddard Space Flight Center
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 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 matter. All galaxies appear to be 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.
To bolster their original finding, first reported in 2018 (Dark Matter Goes Missing in Oddball Galaxy), a team of scientists led by Pieter van Dokkum 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.”
“We went out on a limb with our initial Hubble observations of this galaxy in 2018,” van Dokkum said. “I think people were right to question it because it’s such an unusual result. It would be nice if there were a simple explanation, like a wrong distance. But I think it’s more fun and more interesting if it actually is a weird galaxy.”
Determining the amount of the galaxy’s dark matter hinges on accurate measurements of how far away it is from Earth.
If DF2 is as far from Earth as van Dokkum’s team asserts, the galaxy’s dark-matter content may only be a few percent. The team’s conclusion is based on the motions of the stars within the galaxy; their velocities are influenced by the pull of gravity. The researchers found that the observed number of stars accounts for the galaxy’s total mass, and there’s not much room left for dark matter.
However, if DF2 were closer to Earth, as some astronomers claim, 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
Team member Zili Shen, from Yale University, says that the new Hubble observations help them confirm that DF2 is not only farther from Earth than some astronomers suggest, but also slightly more distant than the team’s original estimates.
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 than the original Hubble 2018 estimate of 65 light-years distance.
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 targeted aging red giant stars on the outskirts of the galaxy that all reach the same peak brightness in their evolution. Astronomers can use the stars’ intrinsic brightness to calculate vast intergalactic distances. “Studying the brightest red giants is a well-established distance indicator for nearby galaxies,” Shen explained.
The more accurate Hubble measurements solidify the researchers’ initial conclusion of a galaxy deficient in dark matter, team members say. 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 Dokkum explained. “What you see is only the tip of the iceberg with Hubble. But in this case, what you see is what you get. Hubble really shows the entire thing. That’s it. It’s not just the tip of the iceberg, it’s the whole iceberg.”
The team’s science paper has been published in The Astrophysical Journal Letters.
A Stealthy Galaxy
DF2 is a giant cosmic cotton ball that van Dokkum calls a “see-through galaxy,” where the stars are spread out. 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 team estimates that DF2 contains at most 1/400th the amount of dark matter than astronomers had expected. How the galaxy formed remains a complete mystery based on the team’s latest measurements.
DF2 isn’t the only galaxy devoid of dark matter. Shany Danieli of the Institute for Advanced Study in Princeton, New Jersey, used Hubble in 2020 to obtain an accurate distance to another ghostly galaxy, called NGC 1052-DF4 (or simply DF4), which apparently lacks dark matter, too. In this case, however, some scientists suggest the dark matter may have been stripped out of the galaxy due to tidal forces from another galaxy.
The researchers think both DF2 and DF4 were members of a collection of galaxies. However, the new Hubble observations show that the two galaxies are 6.5 million light-years away from each other, farther apart than they first thought. It also appears that DF2 has drifted away from the grouping and is isolated in space.
Both galaxies were discovered with the Dragonfly Telephoto Array at the New Mexico Skies observatory.
“Both of them probably were in the same group and formed at the same time,” Danieli said. “So maybe there was something special in the environment where they were formed.”
The researchers are hunting for more of these oddball galaxies. Other teams of astronomers are searching, too. In 2020, a group of researchers uncovered 19 unusual dwarf galaxies 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 paper, we suggested that if you have a galaxy without dark matter, and other similar galaxies seem to have it, that means that dark matter is actually real and it exists,” van Dokkum said. “It’s not a mirage.”
Reference: “A Tip of the Red Giant Branch Distance of 22.1 ± 1.2 Mpc to the Dark Matter Deficient Galaxy NGC 1052–DF2 from 40 Orbits of Hubble Space Telescope Imaging” by Zili Shen, Shany Danieli, Pieter van Dokkum, Roberto Abraham, Jean P. Brodie, Charlie Conroy, Andrew E. Dolphin, Aaron J. Romanowsky, J. M. Diederik Kruijssen and Dhruba Dutta Chowdhury, 9 June 2021, The Astrophysical Journal Letters.
The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.