The two galaxies did not find dark matter. What's happening?
Just like ripples in a pond it indicates that someone threw a stone, ran a water meter or a frog jumped, the existence of the mysterious substances - dark matter - is defined by its vast influence on the space. Astronomers can not observe it directly, but the gravity of dark matter determines birth, shape, and motion of galaxies. This makes the opening of last year completely unexpected: in a strange, diffuse galaxy did not find dark matter. You think that's all? Certainly not.
Galaxy without dark matter
Some scholars have welcomed the discovery. Others expressed their doubts, criticizing the measurement of distances and motions of galaxies. The stakes are high: if this galaxy does lack dark matter, it is, paradoxically, to confirm the existence of this substance.
So, the original team has acquired additional evidence of its initial discovery. It was also discovered a second galaxy with similar symptoms. Where once there was one ultradiffuznaya galaxy free of the dark matter (at first glance), now there are two.
"One object can always be written off as a unicorn, but once you find the two unicorns, you start to think 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 think about where they came from, what are their properties and how they are distributed."
In Search of Unicorns
Two galaxies of interest to us are very dull and far from Earth: the photons from their star clusters 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 of the Milky Way, but contains 100 times fewer stars. The new galaxy NGC 1052-DF4 is in the same area of the sky and on the size and weight is about the same.
In March last year, scientists led by Shani Daniel and Pieter van Dokkum of Yale University published a study that estimated the size of NGC 1052-DF2, watching his starlight and the motion of star clusters that surround it. If NGC 1052-DF2 contain as much dark matter, how much of it is usually expected, astronomers, dark matter would increase the orbital velocities of these stellar phenomena. But they move sluggishly, which suggests that there is no dark matter. Critics argue that the rate of star clusters were incorrectly calculated - and even if their calculations are correct, the sample size of only 10 star clusters was too modest for a reliable determination of the stock of dark matter in NGC 1052-DF2.
In October, the Danieli has decided to address this issue by using a different technique. She took Keck Cosmic Web Imager, a new tool for the newly installed giant 10-meter primary mirror of the telescope Keck in Hawaii. This instrument can measure the light from very faint objects with extremely high resolution, which makes it an ideal tool for studying ultradiffuznyh 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, it could get a lot of direct directly using the starlight of the galaxy.
In the universe can always find something really nice!
In terms of information, starlight contains a lot of it. By separating the light into its component colors (this is called spectroscopy), scientists can determine the composition of stars, her age, the direction of movement in space and speed. Much 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 different chemical elements. Tool Keck spectra of about 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 even some - the greater the variation in the velocity of the star.
"To our own surprise, we measured an extremely narrow spectral lines, which leave little room for a greater mass than the mass contributed by the stars in the galaxy," said Daniel. Dark Matter is simply no room.
Meanwhile, Eric Emsellem of the European Southern Observatory and his colleagues studied the galaxy using the Very Large Telescope in Chile's Atacama Desert. They also found a low-velocity dispersion, which 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 subsequent paper published last year, he argued that too difficult to estimate the mass of the galaxy DF2 based on movements of the surrounding star cluster. But Martin said that it calmed the latest results of the Danieli and Emsellem. "This is made possible thanks to the new devices, which arrived on the largest telescopes in the world. And, frankly, a year ago, it was not obvious to me that this would be feasible. I was not ready to say a year ago that this system will be straight necessarily strange, because I thought that the measurements are not supported by the data fully. But now that there are two different teams, which measured the speed range of the stars themselves, I think it became apparent that there is an oddity. "
The results of the Danieli were presented at the Conference on dark matter at Princeton University, and will be published in Astrophysical Journal Letters.
In general, her research suggests that there is a whole class of such poor dark matter galaxies.
In search of the missing matter
Some astronomers scratching their heads over how such galaxies could have formed and where did the dark matter. Boylan-Kolchin said that one of the possibilities is the gravitational pull of a nearby, much larger galaxies separated from dark matter. Or DF2 and DF4 are not galaxies at all, and modest collections of stars, galaxies masquerading; in this case, these isolated groups of stars can be formed from the colliding jets of gas flowing from the other location. Or may be even more boring scenario, for example, the orientation of the galaxies relative to the Earth, which adversely affects the accurate spectral measurements of their movements.
One thing is clear for sure: if there are no weighty question does not appear, the lack of dark matter in galaxies clearly show that this substance is separated from the stars, gas, dust, and other ordinary matter. And this, in turn, strengthen the argument for the existence of dark matter. To date, no one really discovered dark matter, despite decades of active research. Lack of evidence of astrophysicists led some to seek alternative ways of modeling of galaxies and control their movement due to the emergence of hypotheses like "emergent gravity" and "modified Newtonian dynamics." Proponents of the hypothesis argue that most astronomers believe that dark matter may be a phenomenon that arises from physics we do not understand fully. But in this case these strange galaxy will speak in favor of the fact that the alternatives are incorrect and that the dark matter can actually be the cause.
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