Astronomers have discovered two new satellite galaxies of the Milky Way, and the findings could help us better understand dark matter – the mysterious stuff that makes up about 85% of the matter in the universe, yet remains effectively invisible to us.
The findings also bring scientists one step closer to solving a persistent problem with the standard model of cosmology, or the “lambda cold matter model,” also known as “ΛCDM”, in which the word “cold” assumes that dark matter consists of particles moving slower than the speed of light.
The newly discovered small star clusters are designated Sextans II and Virgo III. They join about 60 known dwarf galaxies that orbit our much larger spiral galaxy at a maximum distance of 1.4 million light years. The most famous and largest of these dwarf galactic satellites of 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 of the Milky Way’s small dwarf galactic satellites remain undiscovered due to their distant and faint nature, but Chiba and colleagues were determined to begin 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 satellites of the Milky Way.
Help! Our dwarf galaxies are missing
Dark matter is a persistent cosmological problem because it does not interact with light, nor does it interact with the ordinary matter that makes up stars, planets, moons, and us. And, well, if it interacts with those things, those interactions are too weak for us to notice.
This means that dark matter may consist of particles that are currently undetected, although there are possible explanations that do not require extensions to particle physics. For example, scientists have explored the idea that dark matter may consist of tiny primitive black holes left over immediately after the Big Bang.
However, dark matter does interact with gravity, which can affect the motion and dynamics of light and everyday matter. This has allowed scientists to infer the presence of dark matter and ultimately determine that large galaxies are surrounded by large halos of this mysterious substance. Those haloes are believed to extend far beyond the galactic disks and haloes of visible matter.
of ΛThe CDM predicts that these dark matter halos have played an important role in the evolution of galaxies. In the early universe, they formed gravitational wells into which star-forming gas and dust were drawn within galaxies. Eventually, these halos also merged, forming 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, simulations using ΛThe CDM predicts that our neighboring galaxy, Andromeda, should be surrounded by about 500 satellite galaxies. However, astronomers have only seen 39 dwarf galaxies orbiting Andromeda.
For the Milky Way, some simulations based on the Standard Model of cosmology show that our galaxy should be orbited by about 220 dwarf galaxies, but scientists can’t figure out where they all are. The discovery of Sextans II and Virgo III helps to correct this balance. However, the results drawn from these findings may present cosmologists with the opposite problem they were facing before.
Not enough dwarf galaxies or too many?
Although the number of Milky Way galaxies identified is still much lower than the predicted 220 dwarf galaxies, the team behind this research took into account the fact that Subaru cannot see the entirety of the night sky above Earth.
They combined the distribution of dwarf galaxies that Subaru has been able to see with its “footprint” in the night sky to calculate an estimate of how many satellites should, in reality, they surround our galaxy. This led to the calculation that 500 galaxies surround the Milky Way—more than twice 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 no. 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 found by Donatiello himself, they found that the distribution of these galaxies, which are located about 11.5 million light-years from Earth, was uneven. . . In other words, the small galaxies appeared to have a “favored direction,” with more galaxies lying on one side of the Sculptor galaxy than the other.
If there is also a favored direction for dwarf galaxies around the Milky Way, and the Subaru telescope was looking that way, then the estimates based on the Subaru observations would be inflated.
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The team behind these findings of Milky Way dwarf galaxies now aims to further investigate the actual number of satellite galaxies surrounding 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 Publications of the Astronomical Society of Japan.