Just as ripples in a pond indicate that someone threw a stone, ran a water meter, or jumped a frog, the existence of a mysterious substance - dark matter - is determined by its vast influence on space. Astronomers cannot observe it directly, but the gravity of dark matter determines the birth, shape, and motion of galaxies. This makes last year's discovery completely unexpected: in a strange, diffuse galaxy, no dark matter was found at all. Do you think that's all? No matter how it is.
Galaxies without dark matter
Several scholars welcomed this discovery. Others have voiced their doubts by criticizing the measurement of the galaxy's distances and motion. The stakes are high: if there is indeed a lack of dark matter in this galaxy, it will paradoxically confirm the existence of this matter.
And so, the original team has acquired additional evidence to support its original discovery. In addition, a second galaxy has been discovered with similar symptoms. Where there used to be one ultradiffuse galaxy free of dark matter (at first glance), now there are two.
“One object can always be written off as a unicorn, but as soon as you find two unicorns, you start to wonder about the possible existence of unicorns,” says Michael Boylan-Kolchin, an astronomer at the University of Texas at Austin, who was not involved in the study. "And then you start thinking about where they came from, what their properties are, and how common they are."
Finding unicorns
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The two galaxies of interest to us are very faint and far from Earth: photons from their star clusters began to travel towards our planet in the last days of the reign of the dinosaurs, about 65 million years ago. The first galaxy NGC 1052-DF2 is the size of the Milky Way, but contains 100 times fewer stars. The new galaxy NGC 1052-DF4 is located in the same region of the sky and is about the same in size and mass.
In March last year, scientists led by Shani Danieli and Peter van Dokkum of Yale University published a study that estimated the size of NGC 1052-DF2 by observing its starlight, as well as the movements of the star clusters that surround it. If NGC 1052-DF2 contained as much dark matter as astronomers would normally expect from it, dark matter would increase the orbital speeds of these stellar phenomena. But they move sluggishly, which suggests that there is no dark matter. Critics argue that the velocities of these star clusters were miscalculated - and even if they were correct, the sample size of just 10 star clusters was too modest to reliably determine NGC 1052-DF2's dark matter stock.
In October, Danieli decided to tackle this issue using a different technique. She took the Keck Cosmic Web Imager, a new instrument recently installed behind the giant 10-meter main mirror of the Keck telescope in Hawaii. This instrument can measure light from very faint objects with extremely high resolution, making it ideal for studying ultradiffuse galaxies such as NGC 1052-DF2. This tool was so good that Danieli no longer had to study the movements of the star cluster to determine the mass of the galaxy. Instead, she could get mass directly, directly using the starlight of the galaxy.
In terms of information, starlight contains a lot of it. By separating light into its constituent colors (this is called spectroscopy), scientists can determine the composition of a star, its age, direction in space, and speed. Most of this information is transmitted in the form of spectral lines - linear elements embedded in the spectrum of a star due to the emission or absorption of various chemical elements. Keck's instrument measured the spectrum of approximately 10 million stars in the DF2 galaxy. The size of the spread between the fastest and slowest stars in the galaxy gives an idea of how much matter interacts with them. The more matter - dark or some other kind - the greater the spread in the speeds of stars.
“To our own surprise, we measured extremely narrow spectral lines that leave very little room for more mass other than the mass that stars bring into the galaxy,” says Danieli. There is simply no place for dark matter.
Meanwhile, Erik Emsellem of the European Southern Observatory and his colleagues have been exploring the galaxy with the Very Large Telescope in Chile's Atacama Desert. They also discovered low-velocity dispersion that supports the missing dark matter scenario.
Nicholas Martin, an astronomer at the University of Strasbourg in France, was one of the critics of the original article. In a follow-up paper published last year, he argued that it is too difficult to estimate the mass of the DF2 galaxy based on the movements of the surrounding star cluster. But Martin says he has been reassured by the latest results from Danieli and Emsell.
“This has become possible thanks to new instruments that have arrived at the largest telescopes on the planet. And, to be honest, a year ago it was not obvious to me that this would be feasible. A year ago I was not ready to say that this system would be outright necessarily strange, because it seemed to me that measurements were not fully supported by the data. But now that there are two different teams that have measured the speed range of the stars themselves, I think it becomes obvious that there is a weirdness."
Danieli's findings were presented at a conference on dark matter at Princeton University and will be published in the Astrophysical Journal Letters.
Overall, her research suggests that there is a whole class of such dark matter-poor galaxies.
In search of the missing matter
Some astronomers are racking their brains over how such galaxies could form and where the dark matter went. Boylan-Kolchin says one possibility is the gravitational pull of a neighboring, much larger galaxy, separated from dark matter. Or DF2 and DF4 may not be galaxies at all, but modest collections of stars disguised as galaxies; in this case, these isolated groups of stars could form from colliding jets of gas escaping elsewhere. Or there may be even more boring scenarios, for example, the orientation of galaxies relative to the Earth, which adversely affects obtaining accurate spectral measurements of their movements.
One thing is for sure: if no weighty doubts arise, the absence of dark matter in galaxies will convincingly show that this matter is separable from stars, gas, dust and other ordinary matter. And that, in turn, will strengthen the argument for the existence of dark matter.
To date, no one has definitely discovered dark matter, despite decades of intense research. The lack of evidence has prompted some astrophysicists to look for alternative ways to sculpt galaxies and control their motion through the emergence of hypotheses like "emergent gravity" and "modified Newtonian dynamics." Proponents of these hypotheses argue that most astronomers believe that dark matter may be a phenomenon that arises from physics that we do not fully understand. But in this case, these strange galaxies will speak in favor of the fact that the alternatives are wrong and that dark matter may indeed be the cause.
Ilya Khel