The heart of the galaxy alone is full of dark matter

Isolated for billions of years, astronomers have identified a galaxy containing more dark matter than expected, from data from NASA's Chandra X-ray observatory.

The galaxy, known as Markarian 1216 (abbreviated Mrk 1216), contains stars whose age is less than 10%, that is, almost as old as the universe. himself. Scientists have discovered that it has evolved differently than conventional galaxies, both in terms of stars and invisible dark matter that, thanks to gravity, keeps the galaxy together. Dark matter accounts for about 85% of the matter in the universe, although it has only been detected indirectly.

Mrk 1216 belongs to a family of elliptically shaped galaxies that are more densely populated with stars than most other galaxies. Astronomers think they come down from red and compact galaxies, called "red nuggets", which formed about a billion years after the big bang, but whose growth has slowed down there are about 10 billion years old.

If this explanation is correct, Mark 1216 Dark Matter and its galactic cousins ​​must also be packaged. To test this idea for the first time, two astronomers studied the X-ray brightness and the temperature of hot gases at different distances from the center of Mrk 1216, in order to "weigh" the amount of dark matter that exists in the middle of the galaxy. .

"When we compared Chandra's data to our computer models, we found that it required a much higher concentration of dark matter than in other galaxies of similar total mass," said David Buote of the University of California at Irvine. "This tells us that Mrk 1216's story is very different from the typical galaxy, basically all of its stars and dark matter were assembled a long time ago with little added in the last 10 billion years." 39, years. "

According to the new study, a halo, or fuzzy sphere, of dark matter would have formed around the center of Mrk 1216 about 3 or 4 billion years after the big bang. This halo should have spread over a region larger than the stars of the galaxy. The formation of such a galaxy of red nuggets was typical of a wide range of elliptical galaxies that we see today. However, unlike Mrk 1216, most giant elliptical galaxies continued to grow progressively as smaller galaxies merged with them over cosmic time.

"The ancient ages and the dense concentration of stars in compact elliptic galaxies such as Mrk's relatively close 1216 views provided the first essential evidence that they are the descendants of red nuggets seen at great distances," he said. co-author Aaron Barth, also from the University of California at Irvine. "We think that the compact size of the dark matter halo seen here solves the problem."

Previously, astronomers estimated that the supermassive black hole in Mrk 1216 was larger than expected for a galaxy of this mass. This recent study, however, concluded that the black hole probably weighs less than 4 billion times the mass of the Sun. It sounds like a lot, but it may not be huge for a galaxy the size of Mrk 1216.

The authors also searched for signs of the supermassive black hole explosion in the center of the galaxy. They saw traces of cavities in the hot gas similar to those seen in other massive galaxies and clusters of galaxies like Perseus, but more data is needed to confirm their presence.

The Mrk 1216 data also provides useful information on dark matter. Because dark matter has never been directly observed, some scientists wonder if it exists. In their study, Buote and Barth interpreted Chandra's data using both standard "Newtonian" gravity models and an alternative theory known as Modified Newtonian Dynamics, or "MOND" designed to suppress the need for material. black in typical galaxies. The results showed that the two theories of gravity required about the same extraordinary amount of dark matter in the center of Mrk 1216, thus eliminating the need for the explanation of MOND.

"In the future, we hope to go further and study the nature of dark matter," Buote said. "The dense accumulation of dark matter in the middle of Mrk 1216 can be an interesting test for non-standard theories that predict a less concentrated dark matter, for example dark matter particles that interact by other means than gravity. . "

An article describing these results was published in the June 1, 2019 issue of The Astrophysical Journal.