A strange galaxy was spotted by a very special telescope in 2015 by astronomers. Nearly the same size of our very own Milky Way, this strange galaxy was named as Dragonfly 44 Galaxy. Because of its nearly identical size to that of our Milky Way, astronomers called it our twin galaxy but, they added a strange adjective to the word ‘Twin’. The adjective they added was ‘Dark’. So, Dragonfly 44 Galaxy is our ‘Dark Twin’.
Why Dragonfly 44 Galaxy is Called Dark Twin?
You smarty pants! You have hit the right question. Allow us to explain…
The galaxy is called Dark Twin of Milky Way because 99.99% of the galaxy is made of dark matter. Now, what really is dark matter? Um… we can’t say that because we don’t know and guess what? No one else can let you know this because no one knows. Scientists just know that it exists and 80% of the mass of our known universe is made of this dark matter. Also, scientists are very sure about its existence because they can clearly see the affect of dark matter on galaxies’ weights as well as on gravity.
So, dark matter is pretty enigmatic and we need to wait and wait and wait until someday, some brilliant mind from anywhere across the globe achieves a breakthrough and gives us a clear understanding of what dark matter really is.
Now back to the Dragonfly 44 Galaxy. Turns out that only 1/100th of 1% of our Dark Twin is made of what we call Visible Matter (it is the matter that we can see). Rest of the galaxy is simply not visible! This explains why it is called Dark Twin.
A Bit About Dragonfly 44 Galaxy
This Dragonfly 44 Galaxy sits in the Coma Cluster. Coma Cluster is actually a group of nearly 1,000 galaxies and is located some 300 million light years away from us (that is, from our Earth). Interestingly, this Coma Cluster has 47 ultradiffuse galaxies (aka ‘fluffy’ galaxies) and the Dark Twin is one of them. Of these 47 fluffy galaxies, the Dragonfly 44 is one of the largest and one of the brightest but it emits only 1% as much light as that is emitted by our Milky Way. So imagine how dim the rest of the 46 galaxies could be!
The Dark Twin was discovered by Yale University’s Pieter van Dokkum and some of his colleague. The telescope they used was Dragonfly Telephoto Array Telescope that is located in New Mexico. This is a specially designed telescope with 8 different telephoto lenses and camera. Its special design allows it to see those things in our space that are way too dim for any normal telescope to detect. This explains why telescopes like the Hubble Space Telescope that is capable of looking billions of light years far into the open space was unable to detect the Dark Twin which is only 300 million light years away!
Something Not Adding Up in Dragonfly 44 Galaxy
After discovering the Dragonfly 44 Galaxy, the discoverers came to a brutal realization. They realized that if the galaxy was really as big as our Milky Way and if it had really so few stars as spotted, there was no way the galaxy could have hold itself together. Why? Simple! There wouldn’t be enough gravity! The stars would have simply drifted away. So, there was something else that was holding the galaxy together. What else can be a better contender than the elusive and undetectable dark matter? This led the discoverers of the galaxy to determine exactly how much dark matter was present in the Dark Twin.
To find answers, the team headed to the W. M. Keck Observatory that is located in Mauna Kea, Hawaii. They used the Keck II Telescope. Specifically, they actually went for a specific tool in the telescope known as the DEIMOS or Deep Imaging Multi-Object Spectograph. DEIMOS has been designed to specifically study stars’ movements in galaxy.
According to Pieter van Dokkum, the movement of the stars in a galaxy allows scientists to determine the amount of matter present in the galaxy. The team observed that the stars in the Dark Twin moved really fast and that the total mass of the galaxy was way more compared to the total mass of all the stars in the galaxy taken together. Calculations showed that the stars made up just 0.01 percent of the total mass of the galaxy and that’s the visible matter (that contains everything from electrons, protons, neutrons etc.). The remaining mass of the galaxy came from something that couldn’t be seen. What could it possibly be? The DARK MATTER.
New Photographs of Dragonfly 44 Galaxy
Once the team determined the extent of dark matter present in the Dark Twin, they headed for Gemini Observatory, which too is located at Mauna Kea. They made use of the GMOS or Gemini Multi-Object Spectrometer in the observatory to shoot some color images of the Dragonfly 44 Galaxy.
The new photographs that the team managed to shoot revealed something really interesting. The Dark Twin, which is actually a spheroidal galaxy appeared to be a dirty smudge on the backdrop of deep space. Closer inspection of the GMOS images of Dragonfly 44 Galaxy also revealed a star cluster halo around the galaxy – just like one found around our Milky Way. According to some scientists, such halos are actually created by dark matter. If that is true, dark matter may not be dark after all.
Scientists are all working really hard to find and know more about the ever-elusive Dark Matter. If even a single particle of the matter can be found, it will lead to ground-breaking breakthroughs and who knows, we may have to eventually rewrite physics and change our understanding of everything around us.
Image Credit:P. van Dokkum, R. Abraham, J. Brodie