Astronomers Discover Two Ghostly Galaxies Devoid Of Dark Matter

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Scientists have discovered two galaxies, nicknamed DF2 and DF4 that appear to lack any significant amount of dark matter.Astronomers deduce the existence of dark matter mostly based on the fact that the rotation curves of galaxies are not what you would expect without some form of hidden mass spread throughout the entire galaxy.

Until recently, every galaxy ever discovered, had a sizeable chunk of the invisible matter, so when scientists found galaxies with no dark matter, it meant one of two things: either the galaxies never had any dark matter, to begin with, or it somehow managed to shed its dark matter during the course of its life.

According to Astronomer.com, DF2 was the very first galaxy to be found existing without dark matter, therefore when the galaxy was discovered, word spread quickly around the astronomical community. However, one group of researchers have claimed that the evidence for DF2's dark-matter deficiency doesn't hold up.

DF2 belongs to a unique class of galaxies called ultra-diffuse galaxies (UDGs). A group of researchers led by Pieter van Dokkum of Yale University have tried to examine the properties of DF2.  Astronomer.com, reports that the scientists identified 10 large, spherical group of old stars known as globular clusters within DF2 and measured how quickly they appear to orbit around it. They found that the globular clusters are moving at a snail's pace compared to what is seen in other galaxies that are filled with dark matter. This led them to conclude that DF2 contains, only aninfinitesimal amount of dark matter. They reasoned that the clusters would be moving much faster if it had more.

The scientists also examined DF4, the second galaxy which seemingly lacked dark matter.

They measured how DF4's light is distributed across the galaxy, and determined DF4's distance is similar to DF2's — about 65 million light-years. 

However, Ignacio Trujillo of the Instituto de Astrofisca de Canarias embarked on a study to disprove the distance to DF2. Trujillo and his team subsequently published their analysis of DF4's distance using the TRGB method. In the analysis, they conclude DF4 is just 44 million light-years away, according to their results, it would mean that DF4's globular clusters are more similar to those found in the Milky Way. "All in all," they wrote in the paper, "the proposition that both [DF2] and [DF4] are 'missing dark matter' is still far from being placed on sure footing."

"Something that caught my attention very early on was the fact that the galaxy was not only anomalous for not having dark matter, but also for having an extraordinarily bright population of globular clusters," says Trujillo “I remember thinking: 'Two anomalies at the same time really looks odd.'"

The scientists who made the initial discovery, led by Van Dokkum, say that their findings are open to scrutiny.

“The broader point is that these are fascinating galaxies, and all aspects of our findings should certainly be questioned and scrutinized," van Dokkum says.