Astronomers have discovered two new satellite galaxies in the Milky Way, and these discoveries could help us better understand dark matter, the mysterious stuff that makes up about 85% of the matter in the universe but remains effectively invisible to us.
The findings also bring scientists one step closer to solving a still-standing problem with the Standard Model of cosmology, or the “lambda cold matter model,” also known as “ΛCDM,” in which the word “cold” implies that dark matter is composed of particles moving slower than the speed of light.
The newly discovered small star clusters are designated Sextani II and Virgo III. They join the 60 or so known dwarf galaxies that swarm around our much larger home spiral galaxy at maximum distances of 1.4 million light-years. The most famous and largest of these companions of dwarf galaxies in the Milky Way are the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC).
“How Many Satellite Galaxies Does the Milky Way Have?” This has been an important question for astronomers for decades,” team leader Masahi Chiba, a professor at Tohoku University, said in a statement.
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Many small companions of dwarf galaxies in the Milky Way remain undiscovered due to their distant and faint nature, but Chiba and colleagues were determined to start finding these elusive objects. So they turned to the Subaru Telescope. This powerful ground-based telescope, located near the summit of Maunakea, Hawaii, is well-suited for hunting dwarf galaxies, and the same team previously used it to discover three new companions of the Milky Way.
Help! Our dwarf galaxies are missing
Dark matter is a persistent cosmological problem because it neither interacts with light nor with the ordinary matter that makes up stars, planets, moons, and us. And well, if it interacts with these things, those interactions are too weak to notice.
This means that dark matter may be composed of particles that are not currently detected, although there are potential explanations that do not require extensions to particle physics. For example, scientists have explored the idea that dark matter could be made from tiny primordial black holes left over just after the Big Bang.
However, dark matter does interact with gravity, which can affect the motion and dynamics of light and everyday matter. This allowed scientists to infer the presence of dark matter and eventually determine that large galaxies are surrounded by huge halos of this mysterious substance. These haloes are thought to extend far beyond the galactic discs and haloes of visible matter.
The ΛCDM predicts that these dark matter haloes have played a significant role in the evolution of galaxies. In the early universe, they formed gravitational wells into which the gas and dust that formed the stars in the galaxies were drawn. Eventually, these halos also coalesced to form large galaxies like the Milky Way.
This model also suggests that there must indeed be hundreds of satellite galaxies around the Milky Way and other large galaxies. For example, the use of simulations ΛCDM predicts that our neighboring galaxy Andromeda should be surrounded by about 500 satellite galaxies. Yet astronomers have seen only 39 dwarf galaxies orbiting Andromeda.
For the Milky Way, some simulations based on the Standard Model of cosmology show that there should be about 220 dwarf galaxies orbiting our galaxy, but scientists can’t figure out where they all are. The discovery of Sextants II and Virgo III helps restore this balance. Still, the results stemming from these discoveries could present cosmologists with the opposite problem they’ve faced before.
Not enough dwarf galaxies or too many?
Although the number of galaxies identified in the Milky Way is still well below the predicted 220 dwarf galaxies, the team behind this study took into account the fact that Subaru cannot see the entire night sky above Earth.
They combined the distribution of dwarf galaxies that Subaru was able to see with its “footprint” in the night sky to calculate an estimate of how many satellites Must, actually surround our galaxy. This led to the estimate that 500 galaxies surround the Milky Way—more than double the amount predicted by simulations based on ΛCDM.
So have scientists gone from a “not enough dwarf galaxies problem” to a “too many dwarf galaxies” problem?
Maybe not. Recently, amateur astronomer Giuseppe Donatiello discovered five new satellite galaxies around the Sculptor Galaxy, officially known as NGC 253.
When he and a team of astronomers looked at the distribution of satellite galaxies around the Sculptor Galaxy, including three previously discovered by Donatiello himself, they found that the distribution of these galaxies, which are about 11.5 million light-years from Earth, is uneven . In other words, the small galaxies seem to have a “preferred direction”, with more galaxies lying on one side of the Sculptor Galaxy than on the other.
If there is also a preferred direction to the dwarf galaxies around the Milky Way and the Subaru telescope happens to be looking that way, then the estimates based on the Subaru observations will be inflated.
The team behind these discoveries of dwarf galaxies in the Milky Way now intends to further investigate the actual number of satellite galaxies that surround us with another ground-based telescope.
“The next step is to use a more powerful telescope that captures a wider view of the sky,” Chiba concluded. “Next year, the Vera C. Rubin Observatory in Chile will be used to accomplish this goal. I hope that many new satellite galaxies will be discovered.”
The team’s results were published June 8 in the Proceedings of the Astronomical Society of Japan.
