“I spent an hour just staring at this image,” lead researcher, astronomer Pieter van Dokkum of Yale University says as he recalls first seeing the Hubble image of NGC 1052-DF2, a galaxy completely void of dark matter. “This thing is astonishing, a gigantic blob so sparse that you see the galaxies behind it. It is literally a see-through galaxy. It’s so rare, particularly these days after so many years of Hubble, that you get an image of something and you say, ‘I’ve never seen that before.”
No theory that predicts its existence
The universe is not simply a place, as astronomers keep discovering, it’s an unfolding dynamic and developmental process –it’s a story. In 2918, an international team of researchers using the NASA/ESA Hubble Space Telescope and several other observatories for the first time, uncovered a galaxy in our cosmic neighborhood that is missing most — if not all — of its dark matter. A second galaxy missing dark matter was found in the NGC 1052 Group in 2019.
The invisible, mysterious substance known as dark matter is by far the most dominant aspect of any galaxy. Finding a galaxy without any dark matter is completely unexpected; it challenges standard ideas of how galaxies work. There is no theory that predicts its existence— how it was formed is completely unknown. It challenges the standard ideas of how we think galaxies work, and it shows that dark matter is real: it has its own separate existence apart from other components of galaxies. This result also suggests that there may be more than one way to form a galaxy.”
This Hubble Space Telescope image above offers a sampling of aging, red stars in DF2. 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, reports NASA, 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. (SCIENCE: NASA, ESA, STScI, Zili Shen (Yale), Pieter van Dokkum (Yale), Shany Danieli (IAS) IMAGE PROCESSING: Alyssa Pagan, STScI)
But none of the ultra-diffuse galaxies discovered so far have been found to be lacking in dark matter. So even among this unusual class of galaxy, NGC 1052-DF2 is an oddball. “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.”
Challenges currently-accepted theories
This discovery of the two galaxies in the NGC 1052 group challenges currently-accepted theories of galaxy formation and provides new insights into the nature of dark matter.
Astronomers using Hubble and several ground-based observatories accurately confirmed the distance of NGC 1052-DF2 to be 65 million light-years and determined its size and brightness. Based on these data the team discovered that NGC 1052-DF2 is larger than the Milky Way, but contains about 250 times fewer stars, leading it to be classified as an ultra diffuse galaxy.
Further measurements of the dynamical properties of ten globular clusters orbiting the galaxy allowed the team to infer an independent value of the galaxy’s mass. This mass is comparable to the mass of the stars in the galaxy, leading to the conclusion that NGC 1052-DF2 contains at least 400 times less dark matter than astronomers predict for a galaxy of its mass, and possibly none at all.
“Dark matter is conventionally believed to be an integral part of all galaxies — the glue that holds them together and the underlying scaffolding upon which they are built,” explains co-author Allison Merritt from Yale University and currently at the Max Planck Institute for Astronomy.
Its existence –Counterintuitive
Although counterintuitive, the existence of a galaxy without dark matter negates theories that try to explain the Universe without dark matter being a part of it: The discovery of NGC 1052-DF2 demonstrates that dark matter is somehow separable from galaxies. This is only expected if dark matter is bound to ordinary matter through nothing but gravity.
Meanwhile, the researchers already have some ideas about how to explain the missing dark matter in NGC 1052-DF2. Did a cataclysmic event such as the birth of a multitude of massive stars sweep out all the gas and dark matter? Or did the growth of the nearby massive elliptical galaxy NGC 1052 billions of years ago play a role in NGC 1052-DF2’s dark matter deficiency?
These ideas, however, still do not explain how this galaxy formed. To find an explanation, the team is already hunting for more dark-matter deficient galaxies as they analyze Hubble images of 23 ultra-diffuse galaxies — three of which appear to be similar to NGC 1052-DF2.
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.”
New Hubble Data Writes a New, Violent Chapter of Discovery
In 2020, new Hubble data explained the reason behind the missing dark matter in NGC 1052-DF4, which resides 45 million light-years away. Mireia Montes Quiles of the University of New South Wales in Australia led an international team of astronomers to study the galaxy using deep optical imaging. They discovered that the missing dark matter can be explained by the effects of tidal disruption. The gravity forces of the neighboring massive galaxy NGC 1035 are tearing NGC 1052-DF4 apart. During this process, the dark matter is removed, while the stars feel the effects of the interaction with another galaxy at a later stage.
Until now, the removal of dark matter in this way has remained hidden from astronomers as it can only be observed using extremely deep images that can reveal extremely faint features. “We used Hubble in two ways to discover that NGC 1052-DF4 is experiencing an interaction,” explained Montes. “This includes studying the galaxy’s light and the galaxy’s distribution of globular clusters.”
Globular clusters are thought to form in the episodes of intense star formation that shaped galaxies. Their compact sizes and luminosity make them easily observable, and they are therefore good tracers of the properties of their host galaxy. In this way, by studying and characterizing the spatial distribution of the clusters in NGC 1052-DF4, astronomers can develop insight into the present state of the galaxy itself. The alignment of these clusters suggests they are being “stripped” from their host galaxy, and this supports the conclusion that tidal disruption is occurring.
Dark Matter Evaporated
By studying the galaxy’s light, reported The Hubble Site, the astronomers also found evidence of tidal tails, which are formed of material moving away from NGC 1052-DF4. This further supports the conclusion that this is a disruption event. Additional analysis concluded that the central parts of the galaxy remain untouched and only about 7% of the stellar mass of the galaxy is hosted in these tidal tails. This means that dark matter, which is less concentrated than stars, was previously and preferentially stripped from the galaxy, and now the outer stellar component is starting to be stripped as well.
The image at the top of the page presents the region around the galaxy NGC 1052-DF4, taken by the IAC80 telescope at the Teide Observatory in Tenerife, Spain. The figure highlights the main galaxies in the field of view, including NGC 1052-DF4 (center of the image) and its neighbor NGC 1035 (center left).( M. Montes et al).
“This result is a good indicator that, while the dark matter of the galaxy was evaporated from the system, the stars are only now starting to suffer the disruption mechanism,” explained team member Ignacio Trujillo of the Instituto de Astrofísica de Canarias in Spain. “In time, NGC 1052-DF4 will be cannibalized by the large system around NGC 1035, with at least some of their stars floating free in deep space.”