The Galaxy with No Dark Matter
Dark matter is the invisible substance that makes up most of the mass in the universe and holds galaxies together. Without dark matter, galaxies would not exist. But there is a galaxy out there that has no dark matter, and it contradicts our current theories of how galaxies form and evolve. It is NGC 1277, a rare and ancient galaxy that is considered a “relic” from the dawn of the universe. How can this galaxy exist without dark matter? How did it come to be? And what does it mean for our understanding of the universe? These are some of the questions that scientists are trying to answer, and they are not easy ones.
What is Dark Matter and Why is it Important for Galaxies?
Dark matter is a type of matter that we cannot see or detect directly, but we know it exists because of the gravitational effects it has on normal matter, such as stars and gas. Dark matter makes up about 27% of the total matter and energy in the universe, while normal matter only accounts for about 5%. The rest is dark energy, which is responsible for the accelerated expansion of the universe.
Dark matter is believed to reside in “halos” surrounding galaxies, and it plays a crucial role in shaping their structure and dynamics. Without dark matter, galaxies would not be able to form or hold together, because the gravity of their visible matter would not be enough to overcome the centrifugal force of their rotation. This was first realized by Swiss astronomer Fritz Zwicky in 1933, who studied the Coma cluster of galaxies and found that they were moving too fast to be bound by their observable mass. He inferred that there must be some invisible mass providing extra gravity, and he called it “dark matter”.
Since then, many observations have confirmed the existence and importance of dark matter in galaxies. One of the most convincing evidence comes from galaxy rotation curves, which plot the rotational speed of stars and gas as a function of their distance from the galactic center. According to Newton’s law of gravity, we would expect these curves to decline as we move away from the center, because there is less mass inside a given orbit. However, what we actually observe is that these curves remain flat or even rise at large distances, indicating that there is more mass than we can see. This extra mass is attributed to dark matter.
Dark matter also affects how galaxies form and evolve over time. According to the current cosmological model, dark matter halos are the seeds for galaxy formation. They grow by merging with other halos and by accreting gas from the intergalactic medium. The gas then cools and condenses into stars and planets within the halos. The properties of dark matter halos, such as their mass, shape, density and concentration, determine how galaxies look and behave. For example, more massive halos tend to host larger and more luminous galaxies, while less massive halos tend to host smaller and fainter galaxies.
The Discovery of NGC 1277 and Why it is Puzzling
NGC 1277 is a peculiar galaxy located about 220 million light-years away from us in the constellation Perseus. It is part of a cluster of galaxies called Perseus Cluster, which is one of the most massive structures in the nearby universe. NGC 1277 is very small compared to other galaxies in its cluster, with a diameter of only about 20,000 light-years (the Milky Way has a diameter of about 100,000 light-years). It is also very dense, with a mass of about 120 billion times that of our sun packed into a small volume. It has very few young stars and gas, indicating that it stopped forming new stars long ago.
But what makes NGC 1277 truly remarkable is that it has no dark matter. At least, not within its observable radius of 20,000 light-years. This was revealed by measuring its rotation curve using an instrument called MUSE (Multi Unit Spectroscopic Explorer) on the Very Large Telescope (VLT) in Chile. Unlike other galaxies that have flat or rising rotation curves due to dark matter halos, NGC 1277 has a declining rotation curve that matches exactly its visible mass distribution. This means that there is no evidence for any extra mass beyond what we can see in stars and gas. The team estimated that the maximum amount of dark matter that NGC 1277 could have within its radius is less than 5% of its total mass, while the observations are consistent with zero dark matter.
This finding is astonishing, because it contradicts the current cosmological model that predicts that massive galaxies like NGC 1277 should have a significant amount of dark matter. In fact, NGC 1277 is the first galaxy of its kind to show no signs of dark matter. Previous candidates for dark matter-free galaxies, such as NGC 1052-DF2 and NGC 1052-DF4, were much smaller and less massive than NGC 1277, and their results were more uncertain and controversial. NGC 1277, on the other hand, is a clear and robust case of a galaxy with no dark matter.
The Possible Explanations for How NGC 1277 Came to Be
So how can we explain this puzzling discovery? How can a galaxy exist without dark matter? Well, there are two possible scenarios that the team suggested in their paper. One is that NGC 1277 was stripped of its dark matter by the gravitational interaction with the surrounding medium within the Perseus Cluster. The other is that NGC 1277 never had much dark matter to begin with, because it formed by the merging of protogalactic fragments that expelled the dark matter during the process.
The first scenario is based on the idea that dark matter halos are more extended and less bound than normal matter, and therefore more susceptible to tidal forces from other objects. When a galaxy moves through a cluster of galaxies, it experiences a drag force from the hot gas and a gravitational force from the other galaxies in the cluster. These forces can strip away some of the galaxy’s outer layers, including its dark matter halo. This process is known as ram pressure stripping or tidal stripping, depending on whether the dominant force is gas or gravity.
However, this scenario faces some challenges when applied to NGC 1277. For one thing, NGC 1277 is not moving very fast through the cluster, only about 1,000 kilometers per second. This is not enough to cause significant ram pressure stripping. For another thing, NGC 1277 is very close to the center of the cluster, only about 5,000 light-years away from the giant elliptical galaxy NGC 1275, which dominates the cluster’s mass. This means that NGC 1277 should experience strong tidal forces from NGC 1275, which should distort its shape and disrupt its stars. However, NGC 1277 appears to be very round and undisturbed.
The second scenario is based on the idea that dark matter halos are not essential for galaxy formation, and that some galaxies can form without them or with very little of them. This could happen if galaxies formed by the merging of smaller protogalactic fragments that had very low concentrations of dark matter. These fragments could have been heated up by supernova explosions or radiation from massive stars, which would have pushed out the gas and dark matter from their centers. When these fragments collided and merged, they would have formed a compact and dense galaxy with very little or no dark matter.
This scenario seems more plausible for NGC 1277, because it can explain why it is so small and dense compared to other galaxies in its cluster. It can also explain why it stopped forming stars long ago, because it used up all its gas in a short burst of star formation. However, this scenario also has some problems. For one thing, it requires a very special set of initial conditions and merger events to produce a galaxy like NGC 1277. For another thing, it does not agree with the current cosmological model that predicts that dark matter halos are ubiquitous and necessary for galaxy formation.
Conclusion
NGC 1277 is a galaxy that has no dark matter, and it challenges our current theories of how galaxies form and evolve. It is also very old and rare, considered a “relic” from the dawn of the universe. How this galaxy came to be and why it is so different from other galaxies are still open questions that scientists are trying to answer. This discovery has opened up new possibilities and puzzles for our understanding of the universe and how diverse and surprising nature can be.
