Like the ripples in a pond indicates that someone has thrown a stone, ran the water meter or jumped a frog, the existence of a mysterious substance — dark matter — is determined by its extensive influence on the space. Astronomers can’t observe it directly, but the gravity of dark matter determines the birth, the shape and motion of galaxies. This makes the opening last year completely unexpected: a strange, diffuse galaxy did not find dark matter. I think that’s all? As it is not so.
Galaxy without dark matter
Some scientists have welcomed this discovery. Others expressed their doubts, criticizing the measurement of the distances and motions of the galaxy. The stakes are high: if this galaxy really is not enough dark matter, it paradoxically confirms the existence of this substance.
And now, the original team has acquired additional evidence to support its initial discovery. In addition, it was discovered a second galaxy with similar symptoms. Where once there was one untraditional galaxy free of dark matter (at first glance), and now two of them.
“A single object can always be written off as a unicorn, but once you find two unicorns, you begin 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 this study. “And then you start to wonder about where they came from, what their properties are and how they are common.”
In search of unicorns
Two of the galaxies are very dim and are far from the Land: the photons from the star clusters they began to travel in the direction of our planet in the last days of the reign of the dinosaurs, about 65 million years ago. The first galaxy NGC 1052-DF2-size milky Way, but contains 100 times fewer stars. New galaxy NGC 1052-DF4 is in the same area of sky and the size and weight is about the same.
In March last year, scientists headed by Johann Daniel and Peter van Dokkum of Yale University published a study which evaluated the size of NGC 1052-DF2, watching his stellar light, and the movement of star clusters that surround it. If NGC 1052-DF2 contain so much dark matter, so astronomers normally expect, dark matter would increase the orbital velocity of these stellar phenomena. But they are moving slowly, which suggests that dark matter is missing. Critics object to the fact that the speed of these star clusters have been calculated incorrectly — and even if their calculations are correct , the sample size of only 10 star clusters was too modest to reliably determine the factor of dark matter in NGC 1052-DF2.
In October, Daniel decided to address this issue using a different technique. She took Keck Cosmic Web Imager, a new instrument, recently installed a giant 10-metre primary mirror of the telescope, the Keck in Hawaii. This device can measure light from very faint objects with extremely high resolution, making it the perfect tool for learning ultradefrag galaxies, such as NGC 1052-DF2. This tool was so good that Daniel no longer had to study the motion of a star cluster to determine the mass of the galaxy. Instead, she was able to obtain mass directly, using the stellar light of the galaxy.
From the point of view of information, starlight contains a lot of it. Separating light into its component colours (this is called spectroscopy), scientists can determine the composition of the star, its age, the direction of movement 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 radiation or absorption of various chemical elements. Tool Cake measured a range of approximately 10 million stars in the galaxy DF2. The size of the spread between the fastest and slowest stars in the galaxy gives an idea of how matter interacts with them. The more matter — dark or some — more variation in the velocities of stars.
“To our own surprise, we measured an extremely narrow spectral lines that leave very little space for the larger mass, in addition to the mass contributed by stars in the galaxy,” says Daniel. Dark matter is simply no room.
Meanwhile, Eric Emsellem from the European southern Observatory and his colleagues explored the galaxy using the Very Large Telescope in the Chilean Atacama desert. They also found low speed dispersion, which supports the scenario of the missing dark matter.
Nicholas Martin, an astronomer at the University of Strasbourg in France, was one of the critics of the original article. In subsequent work, published last year, he claimed that is too difficult to estimate the mass of the galaxy DF2 based on the movements of the surrounding star clusters. But Martin says he was reassured recent results of Daniel and Emsellem.
“This became possible thanks to new devices that arrived on the biggest telescopes on the planet. And, to be honest, a year ago I was not clear that it will be feasible. A year ago I was not ready to say that this system will direct definitely strange, because it seemed to me that the measurements are not supported by the data in full. But now that there are two different teams, which measured the velocity range of the stars, I think it became apparent that strangeness is.”
The results of the work Daniel was presented at a conference on dark matter at Princeton University and will be published in Astrophysical Journal Letters.
Overall, her research suggests that there is a whole class of these poor dark matter galaxies.
In search of the missing matter
Some astronomers puzzled over how such galaxies could be formed and where did the dark matter. Boylan-Kolchin says that one possibility is the gravitational pull of a nearby and much larger galaxy, separated from the dark matter. Or DF2 and DF4 can be no galaxies at all, and the modest collections of stars, masquerading as a galaxy; in this case, these isolated groups of stars can form from colliding jets of gas emanating from another place. Or may be even more boring scenario, for example, the orientation of the galaxies relative to the Earth, which is detrimental to obtaining accurate measurements of their movements.
One thing is clear for sure: if any substantial doubt does not appear, the absence of dark matter in galaxies convincingly shown that this substance is inseparable from stars, gas, dust, and other normal matter. And this, in turn, will strengthen the argument in favor of existence of dark matter.
To date, nobody has detected dark matter, despite decades of intensive searches. The lack of evidence has led some astrophysicists to find an alternative way of modeling galaxies and control their movement due to the emergence of hypotheses like “emergent gravity” and “modified Newtonian dynamics”. The proponents of these hypotheses claim that the majority of astronomers believed that dark matter may be a phenomenon that arises from physics that we don’t understand fully. But in this case these strange galaxies will speak in favor of the fact that the alternative is wrong and what dark matter really could be the cause.
Well, let’s wait and see. Don’t forget to subscribe to our channel with the news to see dark matter first.