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‘There’s more than one way to make a galaxy’

  • Published at 12:54 am April 30th, 2018
  • Last updated at 12:55 am April 30th, 2018
‘There’s more than one way to make a galaxy’
Lamiya completed her A Levels in Dhaka before gaining admission to Wellesley College in Massachusetts in 2009. She joined the doctoral program on galaxy formation and evolution at Yale University in 2014 and the Dragonfly team in 2015, the same year that the dark matter less NGC 1052-DF2 galaxy and other ultra-diffuse galaxies were first discovered through the Dragonfly Telescope.

Dark matter has always been one of the more mysterious and complicated particles defined in quantum mechanics. What exactly is dark matter?

Dark matter is indeed a very mysterious particle, so much so that even scientists know very little about it. According to current scientific theory, baryons, or visible matter, make up only 4% of the universe. Of the remainder, 23% is dark matter and 73% is dark energy. As such, you can see that the vast majority of the universe is not made of visible matter, but rather invisible dark matter and the mysterious dark energy. There are several dark matter candidates such as WIMPs (Weakly Interacting Massive Particles), MACHOs (Massive Astrophysical Compact Halo Objects) and Axions. The jury is still out on what dark matter is exactly.

There is a saying that goes ‘seeing is believing.’ If dark matter is invisible, then how do we know it really exists?

Just because you cannot see something doesn’t necessarily mean that it is not there. You can’t see the wind, but you can still feel it, can’t you? The existence of dark matter is proven in a kind of similar way. Baryons interact with electromagnetic waves, such as light, as well as gravitational fields. As they interact with EM waves, you can see them. Dark matter is unique in that it interacts with gravitational fields but not with electromagnetic waves. As such, you cannot see dark matter, but you can feel its weight in a sense.

How on earth do you measure the weight of something you can’t see, not to mention an entire galaxy?

To give another example, someone who is bigger has a much bigger influence on things around them than someone smaller; for example, they may be able to throw a ball at a higher speed because they have more energy. This comes from Newton’s second law of motion that force is equal to mass multiplied by acceleration. [caption id="attachment_262177" align="aligncenter" width="1024"] The Dragonfly Telescope discovered the NGC 1052-DF2, the first galaxy found by humans that has no dark matter Courtesy[/caption] In other words, if you can measure the speed at which something is moving around an object M, you can use the speed to derive the mass of M. In frictionless space, gravity is the dominant force acting on objects, and so we can use the speed of rotation of stars in a galaxy to calculate the galaxy’s mass.

To clarify, the speed at which the particles are moving in this galaxy is not what it should be?

Exactly! When we calculated the speed of rotation of globular clusters in the NGC 1052-DF2 galaxy, we found that they were moving much slower than they should have been. When we calculated the mass of the galaxy from this speed, the result we received was basically equal to the mass of the visible stars it contained. At most, NGC 1052-DF2 has 1/400 the amount of dark matter a galaxy of that size should have.

You seem very excited. What makes this discovery so groundbreaking?

What makes this discovery so exciting is that it shows there is more than one way for galaxies to form. We originally thought that galaxies formed when baryons and dark matter collected at a point and created a gravitational force strong enough to funnel gas in towards it and eventually form stars. But now that we have found a galaxy without dark matter, we know that galaxies do not always form this way. It really is an exciting time to be an astronomer!

What does this new galaxy look like?

NGC 1052-DF2 looks very different than traditional spiral or elliptical galaxies. Unlike our Milky Way, there are a very sparse number of stars and so it does not have much of a defined shape. It is almost transparent. This is one of the reasons ultra-diffuse galaxies like it were so hard to find, as the galaxy was thought to be “background noise” on telescope images. However, the Dragonfly Telescope is optimized to detect faint and large galaxies, and hence we were able to locate it and other ultra-diffuse galaxies in 2015. I should add that other ultra-diffuse galaxies tend to have a higher fraction of dark matter than NGC 1052-DF2. As such, even among ultra-diffuse galaxies NGC 1052-DF2 is unique.

Are there any theories on how this galaxy may have formed instead?

This is a very difficult question, and right now scientists are still mostly speculating on the possibilities rather than having any concrete theories. Some have suggested that the gas was funneled in without dark matter, while others are considering whether the galaxy may have originally had a dark matter halo which was knocked out in a collision. However, both of these are more trying to come up with ways to fit the galaxy into existing theory rather than theories on how NGC 1052-DF2 may have formed without dark matter.