Dwarf galaxies contain less than ten percent of the stars in the Milky Way. Find out how their behaviour in a region called the Fomax Cluster challenges dark matter’s existence.
Here in the Northern Hemisphere, now is the perfect time of year to view the Milky Way. I’m looking forward to enjoying this spectacular, translucent band of stars in the southern sky over the next month or two.
When we gaze at what we think of as the Milky Way, we’re looking into the core of our galaxy. Our Milky Way galaxy is a spiral and it has a few companions.
The number of stars in these companion galaxies is less than ten percent of the Milky Way’s total, so astronomers call them dwarf galaxies. They can have as few as 1,000 stars although typically they have a few billion.
Dwarf Galaxies Can Have as Few as 1,000 Stars
By way of comparison, typical galaxies contain several hundred billion stars or more. People in the Southern Hemisphere can easily see two of our companion dwarf galaxies, called the Magellanic Clouds.
Europeans named them after Ferdinand Magellan. Antonio Pigafetta, a crew member on Magellan’s first voyage around the globe, noticed them as the southern constellations became visible from this ship.
Dwarf galaxies have proven to be a rich source of information for astronomers over the past twenty years. Improved telescopes and survey techniques have revealed dozens of them.
Findings Challenge the Standard Model of Cosmology
This week, the Monthly Notices of the Royal Astronomical Society published a new study regarding dwarf galaxies in a region known as the Fomax cluster. The study’s findings challenge the standard model of cosmology.
When astronomers study distant galaxies, they see more gravitational effects than they can explain based on the combined mass of the stars they hold. The current consensus is that a mysterious substance cosmologists call dark matter accounts for this discrepancy.
The conventional wisdom is that most galaxies have a halo of dark matter particles around them. Although we can’t see these halos, scientists refer to them to explain the strong gravitational pull we observe galaxies exerting on one another.
“Dwarf Galaxies Are Disturbed by Gravitational Tides”
The new study’s lead author is PhD candidate Elena Asencio of the University of Bonn. As she explains, “We introduce an innovative way of testing the standard model based on how much dwarf galaxies are disturbed by gravitational tides from nearby larger galaxies.”
By “tides,” Ms Asencio means the way gravity from one body has different effects on the various parts of another body. For example, the moon pulls harder on the side of the Earth that’s facing it than on the opposite side, causing our ocean tides.
The Fomax Cluster has a high density of dwarf galaxies. A number of these dwarfs are misshapen, apparently by the combined gravity of the cluster as a whole, which puzzles astronomers.
“Not Expected According to the Standard Model”
“Such perturbations in the Fornax dwarfs are not expected according to the Standard Model,” explained co-author Professor Pavel Kroupa, of the University of Bonn and Charles University in Prague. “This is because, according to the standard model, the dark matter halos of these dwarfs should partly shield them from tides raised by the cluster.”
The extent of the dwarf galaxies’ distortion varies with their contents and how far they are from a cluster’s centre of gravity. Dwarf galaxies that aren’t as dense or that are close to the cluster’s centre become more distorted and may even be destroyed.
“If one wants to explain the observations in the standard model, the Fornax dwarfs should already be destroyed by gravity from the cluster center.” Ms Asencio said. This explained that this should have happened “even when the tides it raises on a dwarf are sixty-four times weaker than the dwarf’s own self-gravity.”
Dark Matter Consensus Doesn’t Explain Distorted Shapes
Yet, the dwarf galaxies are there for all to see. So, the study’s authors concluded that the dark matter consensus doesn’t explain the distorted shapes of the dwarf galaxies in the Fomax Cluster.
The researchers then tried applying the principles of a controversial theory called Milgromian Dynamics (MOND). According to MOND, the gravitational discrepancies in distant galaxies are due to exceptions to the law of gravity instead of dark matter.
“We were not sure that the dwarf galaxies would be able to survive the extreme environment of a galaxy cluster in MOND, due to the absence of protective dark matter halos in this model,” said co-author Professor Indranil Banik of the University of St Andrews. “Our results show a remarkable agreement between observations and the MOND expectations for the level of disturbance of the Fornax dwarfs”.
“Incompatabilities Between Observations and Dark Matter”
Professor Kroupa added, “The number of publications showing incompatibilities between observations and the dark matter paradigm just keeps increasing every year. It is time to start investing more resources into more promising theories.”
Although the dark matter theory explains the discrepancy between the gravity we observe in distant galaxies and their total visible mass, it has a fundamental weakness. Nobody has ever been able to observe dark matter or explain what it’s made of.
If dark matter exists, it must have mass without interacting with light. Since all other matter consists of particles, scientists speculated that the basic unit of dark matter must be some sort of “weakly interacting massive particle” (WIMP).
Gravitational Discrepancy Remains an Unsolved Mystery
The trouble is, nobody has been able to observe anything remotely resembling a WIMP or its effects. Without evidence of dark matter’s existence, the gravitational discrepancy remains an unsolved mystery.
I’ve never been fond of the term “dark matter.” Although we see more gravity than we can account for, it doesn’t necessarily follow that a mysterious form of imperceptible matter reconciles the difference.
“Major Implications for Fundamental Physics”
Our story of how the universe works and our place in it will always have gaps unless we can explain the way distant galaxies behave. Gravity is one of the fundamental forces of nature, so we need a more complete understanding of it.
Co-author Professor Hongsheng Zhao of the University of Saint Andrews wrapped things up, adding, “Our results have major implications for fundamental physics. We expect to find more disturbed dwarfs in other clusters, a prediction which other teams should verify.”
We always have more to learn if we dare to know.
No Trace of Dark Matter Halos
The distribution and morphologies of Fornax Cluster dwarf galaxies suggest they lack dark matter
Gravitomagnetism: Does It Explain Away Dark Matter?
Galaxies Without Dark Matter Might Form From Collisions
Mass Discrepancy: Is Dark Matter Hiding in Plain Sight?