Dark matter is everywhere in the universe, or so we thought. It is a fundamental part of space. It is this mysterious element that is supposed to hold galaxies together when gravity can no longer do the work. But, recently, astronomers have made an unexpected discovery: a galaxy that has virtually no dark matter.
The galaxy NGC 1052-DF2, which is 65 millions light years away from the Milky Way, has baffled the authors of a study published in Nature. It has much less dark matter than it should have for its size.
Researchers were scanning the sky using a telescope called Dragonfly, a clever assembly of 48 Canon 400mm wide-angle photographic lenses coupled with long-range astronomical sensors designed to detect very weak objects when they made the unusual discovery. From the very first observations, the object appeared unusual to them. At the observatory William Myron Keck on the island of Hawaii, they measured the mass of this galaxy. They realized that it is about equal to the sum of masses of the stars, gas and dust that constitute it. Logic? Absolutely not!
Speaking with Space.com, Pieter van Dokkum of Yale University explained that for a galaxy with a star mass of 200 million solar masses, the “normal” would be 80 billion solar masses of dark matter. However, the discovered galaxy has only 300 million solar masses.
In reality, for all that science knows today, the main component of a galaxy is neither its dust, nor its gas, nor even its myriads of stars, all things made of ordinary matter called baryonic, the one we all know. The main component of galaxies is the mysterious dark matter. Theoretically, it is thanks to it that their formation could have been initiated.
“For decades, we thought that galaxies started life as dark matter patches, after which everything happens: the gas falls into these halos of dark matter, turning into stars. They accumulate slowly and we end up with galaxies of the type of the Milky Way. NGC1052-DF2, however, challenges these standard ideas about how galaxies are formed,” says Pieter van Dokkum. Indeed, if the mass of DF2 is equal to the sum of the masses of its stars, gas, and dust, it’s because it contains no or almost no dark matter! Exactly 400 times less than it should. How is it possible ? How could this galaxy have formed? At this point, there is really no answer.
In addition, the absence of dark matter is not the only quirk of DF2. Approximately the size of our galaxy, the Milky Way is home to 200 times fewer stars. Unlike the classic spiral galaxies, DF2 does not have a dense central region, no spiral arms, no disc of matter. And, unlike typical elliptical galaxies, it does not have a central black hole either.
By observing it with the Hubble telescope, Pieter van Dokkum likens it to “a gigantic blob through which we can see”. “It’s so sparse that you can see all the galaxies behind it. It’s a literally a transparent galaxy. ”
DF2, being located in a cluster dominated by a giant elliptical galaxy, NGC 1052, is perhaps under the influence of the latter. Or would the current appearance of the galaxy be the result of a cataclysmic event within it? To understand more, Roberto Abraham and Pieter van Dokkum, armed with their amazing telescope, are already looking for other similar galaxies. “Dragonfly is good at finding these sorts of things, maybe we are only seeing the tip of the iceberg,” thinks Roberto Abraham.
The discovery may result in a change in what scientists currently believe, as indicated by Professor Roberto Abraham.
Before, scientists believed that galaxies were composed of stars, gases and dark matter, all mixed together. But dark matter was the majority.
Now, it seems that at least some galaxies exist with many stars and gases, and little dark matter. Which is pretty bizarre.