The “Ghost” Galaxy

Scientists from Yale University found out a mysterious galaxy with no dark matter, has created a puzzling situation amongst everyone. The name of the galaxy is  NGC 1052-DF2, or DF2 for short.

Galaxy can never exist without dark matter. It is the most dominant part of a galaxy. Galaxy has dark matter was the general perspective of every scientist until DF2 was found devoid of dark matter. It changed the vision of how a galaxy works and led to the point that galaxy can be formed in more than one way.

Now the question arises-

What is DF2 and how it has been formed?

Several years ago, DF2 was discovered in the constellation of Cetus, the whale and it lies 65 million lights years away. The unusual thing was not only the absence of dark matter but also a very low mass. The difference between DF2 and Milky Way is the number of stars.

A challenge was faced by researchers to get an idea of mass. Using standard telescopes, it was not possible to detect as stars are spread out in a vast area. So a telescope was invented, called the Dragonfly Telephoto Array to cut through the guts of the mysterious galaxy. The sparks of light on other surveys came to a conclusion as big bright blob. They found out that the galaxy contains exceptionally bright spheres of stars known as globular clusters.

The luminous star clusters, the giant blob of a galaxy, and the lack of dark matter is completely a new thing for the scientists. To get an idea of the mass of the galaxy, Keck telescope was used and by seeing how fast 10 of these clusters are moving. Now, if they move fast then there's a lot of mass holding things together, and if they move slowly then there's not a lot of mass holding things together. So clusters play an important role in determining the mass of the mysterious galaxy. The researchers did their calculation and came to the conclusion that the galaxy had a total mass of 350 million solar masses, and most of that was distributed among the stars, leaving little room for any dark matter. But generally, the galaxies which have low mass like this are usually dominated by dark matter.

Generally, a galaxy should have 400 times more dark matter than luminous matter. Normally, the galaxy has spiral arms or it's a big elliptical galaxy, but the dark matter can nowhere be seen. Professor van Dokkum added that he barely had any idea of how the galaxy is actually formed. In the paper, they provided with a probable explanation but none can actually lead to the object. But Ken Freeman who is an Australian astronomer and is known for his work in dark matter and galaxy evolution has a completely different idea. He said that objects like DF2 which has no dark matter are unusual, but it doesn't make it unique and there should be a reasonable explanation for how it formed.

DragonFly Telescope

Dark matter is all over, but sometimes there are galaxies that have been made by some other routes.

Professor Freeman said DF2 could be a type of galaxy called a tidal dwarf. "They basically don't have any dark matter at all because it's just ordinary matter that's been ripped out of a galaxy," Professor Freeman, who is based at the Australian National University, said. DF2 could, for example, have formed from a nearby giant elliptical galaxy called NGC 1052. But for this case, there's just no proof of the interaction of this thing with anything else or past interaction from where the team could see tidal leftovers. The stars also didn't match the stars from a galaxy with a larger mass.

Other possibilities which were considered are that the galaxy was formed from gas swept up by the tidal galaxy hypothesis — that DF2 does have dark matter, but it had not yet been detected. DF2 could be more like a large mass galaxy like our own quasar winds or that it was gas that never made it to its larger neighbor where the dark matter dominates the outer edges. There will be no way of knowing why the dark matter is there. But the probable answer can be that there is no data on the motions far out in the galaxy.

Only when they were able to measure motions in the outer parts of the Milky Way, could they see how dominant the dark matter really is. That is why dark matter in large galaxies remained undiscovered until the 1970s.

Future of  the “ghost” galaxy

Professor van Dokkum said the next step to get into further studies about its mass will be by the use of Keck Telescope.

There are lots of hustle for the measurement of the velocity of clusters but that will be worth the effort. The team further wants to pin down the age of galaxy by Hubble Telescope and probably that could lead to any clue of how the galaxy was formed.

At the very moment, the only thing that came to the sight is that the galaxy is older than 10 billion years, but a further search is going on to know if it is even older big bang.

If the galaxy was formed 10 billion years ago, it would have formed at the same time as other galaxies in that group and it would be possible if it could be gas that was sidetracked from a bigger galaxy next door. If it's 13 then it's a wholly different story. The team also plans to continue the hunt for more galaxies like DF2 using the Dragonfly telescope.

Links:

https://www.space.com/40119-ghostly-galaxy-almost-no-dark-matter.html

http://www.abc.net.au/news/science/2018-03-29/galaxy-with-no-dark-matter-puzzles-astronomers/9596840

https://www.nasa.gov/feature/goddard/2018/dark-matter-goes-missing-in-oddball-galaxy