In 2018, astronomers were taken aback when they discovered a galaxy that was nearly entirely
devoid of dark matter.
As the first galaxy ever found without the elusive substance, which is believed to account
for 85% of the universe’s mass, the news sent shockwaves across the scientific world.
Some researchers were enthralled, while others were understandably skeptical.
According to a study conducted in 2018, scientists established that a ghost galaxy located 60
million light-years away known as NGC 1052-DF2 (or in short DF2) contains virtually no dark
matter.
In addition, another second research conducted in 2019 discovered yet another dim and hazy
galaxy that does not have dark matter, dubbed DF4.
Taken together, these findings reveal that DF2 is not alone, but instead part of a larger,
previously unknown population of galaxies that have seemingly broken free from the shackles
of dark matter.
The second study could have major implications for current theories regarding the formation
and evolution of galaxies, as well as the true nature of dark matter.
So, recently there has been another discovery of another galaxy with no dark matter.
An international team of astronomers led by researchers from the Netherlands detected
no indication of dark matter in the galaxy AGC 114905, despite taking comprehensive measurements
for forty hours with state-of-the-art telescopes.
After the researchers discovered six galaxies with little to no dark matter, they were advised
to measure again to see if they would detect dark matter within the galaxy.
However, after forty hours of comprehensive observations with the Very Large Array (VLA)
in New Mexico (United States), the evidence for a dark matter-free galaxy only grew stronger.
The AGC 114905 is about 250 million light-years away.
It is classed as an ultra-diffuse dwarf galaxy, with the term ‘dwarf galaxy’ referring
to the luminosity of the galaxy rather than its size.
The galaxy is about the same size as our own Milky Way, but there are a thousand times
fewer stars in it.
The popular belief is that all galaxies, including ultra-diffuse dwarf galaxies, can only exist
if they are held together by dark matter.
The researcher used the VLA telescope to acquire data on the rotation of gas in AGC 114905
for 40 hours between July and October 2020.
They then created a graph with the distance of the gas from the galaxy’s center on the
x-axis and the gas’s rotation speed on the y-axis.
This is a standard method of detecting the presence of dark matter.
The graph illustrates that ordinary matter can completely describe the motions of the
gas in the galaxy.
Of course, this is what they expected and hoped for, as it validates their earlier measurements.
But now, the problem is that the theory predicts that dark matter must exist in the galaxy,
but their findings show that it does not.
In fact, the difference between theory and observation is widening all the time.
The researchers listed all possible causes for the lack of dark matter in their scientific
publication.
For instance, AGC 114905 may have been stripped of dark matter by large neighboring galaxies,
but there are none.
They would also have to incorporate extreme parameter values that are far beyond the normal
range in the most highly regarded galaxy formation framework, the so-called cold dark matter
model.
They also can’t duplicate the motions of the gas within the galaxy using modified Newtonian
dynamics, an alternative theory to cold dark matter.
There is one additional assumption, according to the researchers, that might influence their
conclusions.
That’s the angle at which they’re looking at the galaxy.
That angle has to deviate significantly from their estimate before there is room for dark
matter again.
Meanwhile, the team is delving further into a second ultra-diffuse dwarf galaxy.
If no evidence of dark matter is found in that galaxy again, the case for dark matter-poor
galaxies becomes even much stronger.
The fact that we’re seeing celestial bodies that have no dark matter is fascinating.
No one knew that such galaxies existed, and the best thing in the world for an astronomer
is to discover an object, whether it’s a planet, a star, or a galaxy, that no one knew
about or even thought about.