This large, blurry-looking galaxy is so diffuse that astronomers refer to it as a « see-through » galaxy . . .  because they can clearly see distant galaxies behind them. The ghost object, cataloged as NGC 1052-DF2, is one of the few candidate galaxies, along with nearby NGC 1052-DF4, that may not have dark matter at all.
In recent years astronomy has had an enormous puzzle to solve. If you look at all the large-scale structures in the universe – large galaxies, galaxy groups and clusters, the huge cosmic web, and even the all-sky radiation left over from the Big Bang – the result is the same universal picture. In addition to all normal matter from Standard Model Particles in all their forms, an additional source of invisible mass is required: dark matter. Everywhere we look at all of these large scales, the same 5: 1 ratio of dark to normal matter explains each of our observations adequately.
But on a small scale, the story should be very different. All of the different forces and effects should create two populations of small galaxies: those with enormous amounts of dark matter relative to their normal matter that should persist for long periods of time, and those with very little relative dark matter that should be destroyed in short cosmic terms Time scales. One galaxy, NGC 1052-DF4 (called DF4 for short), made things hugely complicated as it appears to have no dark matter but has not formed new stars in about 7 billion years. In a brilliant new study led by Mireia Montes, that mystery has finally been solved as an otherwise diffuse galaxy is in the final stages of being torn apart. Here’s the science how we found out.
According to models and simulations, all galaxies should be embedded in halos of dark matter, their . . .  Densities peak at the galactic centers. In sufficiently long periods of perhaps a billion years, a single particle of dark matter will complete an orbit at the edge of the halo. The effects of gas, feedback, star formation, supernovae and radiation make this environment difficult and make it extremely difficult to extract universal dark matter predictions. Such complications are absent on larger cosmic scales and in earlier times.
The theory. Theoretically, both dark matter and normal matter permeate the universe, but react differently from one another. If you have a gravitational field, e.g.. B.. a region in which the matter density is greater than the surrounding regions, both normal and dark matter experience the same forces of attraction. But normal matter becomes:
On the largest scales, gravity is the only force that matters, so these differences don’t matter much. However, at small scales, and especially in small, low-mass galaxies, these differences become immediately apparent. This difference is most common in low mass galaxies (i.e.. H. . e. Galaxies with small escape speeds suddenly form large numbers of stars. When these stars start to shine and generate a lot of ultraviolet radiation, the gaseous normal matter can be pushed out and expelled completely while the dark matter is left untouched.
The cigar galaxy M82 and its supergalactic winds (in red) showing the fast new star . . .  Education occur in him. This is the closest massive galaxy to undergo such rapid star formation, and its winds are so strong that almost all of the heavy elements created by the death of these stars are permanently ejected with no dark matter to keep them gravitationally bound.
This creates a population of low mass galaxies with a much larger ratio of dark matter to normal matter than the typical 5-to-1 ratio we see at larger scales in the universe. When we form new stars in the universe, they come in a myriad of masses and colors, with the most massive generating the greatest amount of winds and high-energy radiation, which allows normal matter (but not dark matter) to accelerate to high speeds. When a galaxy has too little mass, that normal matter is ejected, bringing the ratio of dark matter to normal matter in the hundreds to 1 or even thousands to 1 range.
In theory, however, there should be a second, rarer population of low mass galaxies. When gravitational interactions occur between galaxies, they can disrupt the structure of the galaxy. Normal matter and dark matter can be ripped out in streams due to tidal forces, and while dark matter simply travels through the universe, normal matter can fall back and form stars without dark matter. However, the lack of dark matter makes them easy to destroy through further gravitational interactions, and therefore they should only live for a short time. In theory.
Zw II 96 in the constellation Delphinus, the dolphin, is an example of interacting galaxies. Note . . .  that stars can be torn out of these galaxies, either by forming new stars when gas is present, or simply removing material from a bound structure when the tidal effects are large and extensive enough.
The first observations. In recent years a new set of instruments has gone online that can measure the intricate properties of a greater number of low mass galaxies farther away from us than ever before. A few tens of million light-years away is a large galaxy called NGC 1052 at the center of a modestly large group of galaxies. Many of these galaxies are small, but some also have interesting shapes: the ultra-diffuse dwarf galaxies. They are faint, made up of older stars, and have a variety of properties.
However, two of them turned out to be interesting objects: NGC 1052-DF2 (called DF2 for short) and the DF4 mentioned above. According to previous measurements, both are satellite galaxies of NGC 1052, both are populations of old stars (in which not many new ones have formed in billions of years) and yet the existing stars as the globular clusters that surround them are present around – move incredibly slowly. It’s as if these galaxies somehow have less gravity holding them together in relation to their size than any other galaxy at all. Not only can we infer a much lower ratio of dark matter to normal matter than in other galaxies, but both galaxies did not agree with dark matter at all.
The expected relationship between the velocity dispersion of a galaxy (y-axis) and the amount of mass in . . .  its stars (x-axis). Note that for very low masses on the far left there is a multitude of velocity dispersions as there can be a tremendous amount of dark matter inside. If a massive galaxy has very little dark matter, it shouldn’t be long-lived.
The riddle. The problem is that these ultra-diffuse dwarf galaxies DF2 and DF4 are in a rich galaxy group that is close to other galaxies. If they really have very little or no dark matter, the gravitational effects of the nearby galaxies should tear them apart. To understand why, think of a galaxy as a sphere and imagine a nearby, more massive galaxy as a mass that simply exists at a point slightly distant. This « point » will exert a gravitational force on every part of your spherical galaxy, but different parts of the sphere will experience slightly different forces.
We can think about it by imagining that the center of the spherical galaxy experiences the average force. Parts that are closer to the outer mass experience above-average force, while parts that are further away experience below-average force. Parts that are « north » experience a slight « south » force; Parts that are « down » experience a slight « up » force, and so on. . The different parts of the same galaxy experience a different force: a tidal force that rids the galaxy of its matter, with the strongest stripping occurring at the edge of the galaxy.
At any point along an object that is attracted to a single point mass, the force of gravity (Fg) is . . . [ ] different. The average force for the point in the center defines how the object accelerates, which means that the entire object accelerates as if it were subjected to the same total force. If we subtract this force (Fr) from each point, the red arrows show the tidal forces that appear at different points along the object. If these forces become large enough, they can distort and even tear apart individual objects, including entire galaxies.
If these galaxies are both diffuse (meaning they occupy a large volume) but have no dark matter (meaning they have very little mass), tidal stripping should be very easy. In fact, it should be so simple that galaxies with the properties DF2 and DF4 are supposed to have should not persist for more than a billion years in environments like that around NGC 1052. As the galaxies move, the tugs of other galaxies should tear the stars out of them over time, and without a large, massive halo of dark matter clinging to them, the entire object should dissolve quickly.
However, from the stars inside, we know that these galaxies not only lasted for billions of years, but did not form new stars in about 7 billion years! If these galaxies have the properties that we have observed and inferred from, they should by no means still be there. Something doesn’t have to be right, or something about dark matter and structure formation in the universe needs to be questioned.
This wide-field view shows the galaxy NGC 1052 (top left) and the nearby galaxy NGC 1042 (center). . While . . .  These two galaxies appear nearby, they are actually separated by about 20 million light-years, with the ellipticals wider and the spiral closer. Galaxy DF2 is likely closer and has more dark matter than originally thought. DF4 may not be closer but has virtually no dark matter.
Better observations. Fortunately, one of the burden of proof for an exceptional claim like this one is to independently confirm and verify that the properties of these objects are what we believe they are. If you look at these galaxies DF2 and DF4, one of the things that could affect our measurements is misidentifying which large galaxy (or group of galaxies) they are tied to. For example, there are two other large galaxies near NGC 1052: NGC 1042 and NGC 1035, which are closer to us than NGC 1052. Most importantly, they are in the same line of sight so it’s easy to confuse which galaxy these ultra-diffuse dwarfs are tied to.
If you believe a galaxy is farther away than it actually is, you can infer a number of properties incorrectly, including:
Alternative methods of measuring DF2 and DF4 indicated that they may not be tied to NGC 1052 but are closer. For DF2 this would mean that there was at least a typical amount of dark matter, but DF4 still remained a problem. Even adjusting the distance would lead to this conundrum: there is too little dark matter to survive in this environment for so long.
Hubble data for the galaxy NGC 1052-DF4, taken in 2019 by the team of Danieli, Van Dokkum and others, . . .  goes eight times deeper than previous observations. The aim of the observations was to determine the distance and measure the properties of the stars and globular clusters surrounding them. However, larger field data was required to determine which components of starlight emerged from this galaxy compared to neighboring galaxies.
The ultimate explanation. While DF2 is likely tied to NGC 1042, DF4 is very close to the great galaxy NGC 1035. Remember how tidal forces work: more massive objects tear apart less massive objects by exerting different forces on different parts of the object. When DF4 is near a large galaxy, it is stretched along one dimension (towards the large galaxy) and compressed in the other, perpendicular dimension.
Also, the matter removed from this galaxy should be done from the outside in. The material on the galactic outskirts should be stretched first and hardest so that it is easiest to remove. The material that starts in the center of the object should survive the longest and remain undisturbed until the end. And remember: even in these tiny, ultra-diffuse dwarf galaxies, there should still be a halo of dark matter around them that is much larger and more diffuse than normal matter. While normal matter sticks together and sinks to the center, dark matter remains mostly on the outskirts.
On the left, the light from a number of stars and galaxies is shown as raw data. With the environment . . .  Light sources modeled and removed, galaxy NGC 1052-DF4 remains in the center (right) and clearly shows evidence of its tidal disturbance.
And this is where the key lies, according to Montes’ team. If DF4 were a typical ultra-diffuse dwarf galaxy – which last formed stars 7 billion years ago, which has practically no gas but has a large halo of dark matter – we could ask, « What would happen if it were in is close? a large, massive galaxy? « The answer is as follows:
If that were the case, you would have to remove about 90% of the dark matter before the stars are tidally disturbed. And thanks to brand new Hubble observations, part of the recently released paper (free version available here), we can clearly see that the stars are finally hit.
The structure of the stars in the galaxy NGC 1052-DF4 can be seen in three different wavelength bands . . .  extended along the line of sight towards the nearby large galaxy NGC 1035. After the starlight has been subtracted from the other galaxies in the field, the tidally disturbed core remains, suggesting a mundane, non-exotic physical explanation for this galaxy.
Although it currently only affects roughly 7% of the stellar mass, this tidal interaction with a large, massive neighbor is enough to solve this dark matter mystery. The reason its stars are so old is that it was created a long time ago; The reason there is practically no dark matter is that the dark matter is being actively removed from it. The reason it still survives today is because it is experiencing active disturbance and is likely to be destroyed in a short period of time, at least on cosmic timescales.
The whole point is: you cannot have a long-lived galaxy without dark matter. You can lose your dark matter through a tidal interaction that creates a star aggregation known as the intertidal dwarf galaxy. However, these are temporary: short-lived and easy to tear apart. The secret of DF4 is that it looks like an ultra-diffuse galaxy, not a tidal galaxy, as it was an ultra-diffuse galaxy until recently. The tidal disturbance first affected dark matter and only now – now that it has almost completely disappeared – the stars also begin to disturb. With this new discovery, the mystery can be completely solved, which teaches us why DF4 has no dark matter after all.
I am a Ph. D.. . Astrophysicist, author and science communicator who is committed to physics and astronomy at various universities. I have won numerous academic writing awards
I am a Ph. D.. . Astrophysicist, author and science communicator who is committed to physics and astronomy at various universities. Since 2008, I’ve received numerous academic writing awards for my blog, Starts With A Bang, including the Institute of Physics Best Scientific Blog Award. My two books Treknology: The Science of Star Trek From Tricorders to Warp Drive, Beyond the Galaxy: How Mankind Looked Beyond Our Milky Way and Discovered the Entire Universe, are available on Amazon. Follow me on Twitter @startswithabang.
Dark matter, galaxy, astronomy, NGC 1052-DF2, NGC 1052, galaxy formation and evolution, universe
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