It’s uglyly small, at least by astronomical standards. You might think it hardly deserves any attention. But dwarf galaxies – a kind of miniature version of the Milky Way – are the thorn in the side of cosmology. They display misunderstood characteristics. They behave differently than expected. In short: it does not fit well with the standard picture of the evolution of the universe. Nobody knows how to get rid of these problems.
Large spiral galaxies such as our own Milky Way and Andromeda – disk-shaped assemblies of several hundred billion stars – are well understood today. This also applies to elliptical systems, which have the shape of a kiwi or mandarin orange: slightly elongated or slightly flattened. It forms when two spiral galaxies collide and merge, something that will also happen to the Milky Way and Andromeda within a few billion years.
But dwarf galaxies are different. Much smaller, at first, with no more than a few hundred thousand stars sometimes. The shape is often irregular, although dwarf ovals are also found. And they are quite a lot: dozens have already been discovered in the immediate vicinity of the Milky Way, and other large galaxies are also surrounded by it, just like large cities surrounded by agricultural villages.
Ellen Tolstoy of the University of Groningen has been searching for Milky Way dwarfs for years. Its biggest headache profile is Reticulum II, an elongated tuft of stars about 100,000 light-years away, discovered only in 2015. “The stars in this dwarf contain an unexpected amount of heavy elements like europium,” Tolstoy says. Why is this unknown. Catastrophic star collisions may have occurred that created these elements. Other astronomers have picked up high-energy gamma rays from the dwarf galaxy – and there’s no good explanation for that either.
But Reticulum II is only one of many transverse dwarfs. “A lot of new galaxies have been discovered recently, and they’re all weird,” says Tolstoy. “Stars in dwarf galaxies have very different properties and often have a different chemical composition than stars in the Milky Way.” Striking, because it’s widely believed that big boys like the Milky Way have grown over billions of years by outperforming their smaller companions.
In general, this picture of the so-called hierarchical growth is correct: the European Gaia space telescope discovered long chains of stars in the Milky Way – dwarfs that were completely torn apart by tidal forces. “But somehow we’re missing something,” Tolstoy says. “Anyway, dwarf galaxies as we see them today were not the original building blocks of the Milky Way.”
This is what the famous cosmic standard model claims. According to that theory, called complex Lambda-CDM, the evolution of the universe is dominated by mysterious dark energy (represented by the Greek letter lambda) and cold dark matter (“cold dark matter”: CDM). Dark energy is causing the universe to expand faster and faster; Dark matter – a mysterious particle whose true nature is still unknown – wants to keep things together.
Using supercomputers, you can simulate the cosmic evolution of the Lambda-CDM universe at an accelerating pace, and the results of that simulation match surprisingly well with the real universe around us. At least, if you do not pay much attention to dwarf galaxies, because suddenly you will encounter all kinds of anomalies and problems.
Computer simulations paint a clear picture. Soon after the Big Bang, invisible dark matter begins to clump together into small, somewhat spherical clouds. These dark “halos” also attract ordinary gas atoms with their gravity. Stars are formed from this gas and thus countless dwarf galaxies are born. They soon merge into larger galaxies, such as the Milky Way and Andromeda. These then continue to grow to this day by constantly eating new dwarves.
It sounds good, but reality doesn’t seem to care much about the theory. At least when it comes to dwarf galaxies. First of all, they are not many enough. If the simulations are to be believed, hundreds of them should be found around our Milky Way by now, rather than a few dozen. Rara where are they?
Creative theorists have come up with a solution to this “missing dwarf problem”: they may be there, but they’re all dark matter, and for some reason stars haven’t formed in them. “It could be so,” Tolstoy says, “but it is of course difficult to verify.” Astronomers have been trying to detect invisible dark matter halos through the influence of gravity, but so far to no avail.
Flying art and work
The second mystery is that the velocity distribution of stars in dwarf galaxies does not match expectations. Because of the gravitational pull of lumpy dark matter, you’d expect stars closer to the center to move much faster than they move a little farther, but in fact there’s a kind of speed plateau. In some theoretical and aeronautical artwork this can be explained, but it doesn’t seem quite right.
The third puzzle has nothing to do with the properties of dwarves, but with their distribution in the universe. According to computer simulations, large galaxies should be surrounded on all sides by the entourage of statues. In fact, dwarfs orbit more or less in one flat plane – not only around our Milky Way, but also around the Andromeda galaxy and the elliptical system Centaurus A.
Now the computer simulation is, of course, just a computer simulation. “They are always surrounded by uncertainty,” says Timur Seifollah, an Iranian-born astrophysicist who hopes to do his Ph.D. in Groningen this summer. In addition, most simulations are primarily designed to provide insight into the behavior of dark matter – which is much easier to capture in the calculations than the behavior of gas and stars, because with dark matter you don’t have to deal with differences in temperatures and electric and magnetic charges.
In any case, the search for dwarf galaxies plays a key role in testing the Lambda-CDM model and in the continuous improvement of existing computer simulations, Saifullah said. And according to his promoter Reynier Peletier, we still know very little about him. “There is not nearly enough monitoring data available,” says Pelletier. This is, of course, because dwarfs are so small and called faint: astronomers can really only study them in detail in our cosmic backyard.
So Peltier and Saifullah are very skeptical about the amazing research by Peter van Dokkum of Yale University in the distant dwarf galaxies DF2 and DF4. According to van Dokkum and colleagues, these galaxies are 65 million light-years away and contain virtually no dark matter, which at first glance contradicts the cosmological standard model.
Recent measurements using the Hubble Space Telescope seem to confirm the distance measurements made by the US team, but Pelletier does not believe this. “The Spanish astronomer Ignacio Trujillo, with whom I worked and trusted completely, analyzed the same Hubble measurements and came to a much closer distance to the two galaxies, only 45 million light-years away.” This means that van Dokkum and colleagues’ conclusions are in doubt, he wants to say.
When the popular image of cosmology is under attack, feelings sometimes run high. Trujillo and van Dokkum have been at odds with each other for years. According to Timur Saifullah, it’s time for more astronomers to turn their attention to the mysterious dwarfs DF2 and DF4. “However, it’s totally weird, and it can tell us more about the behavior of dark matter anyway,” he says.
Even if it turns out that the two dwarf galaxies don’t really contain dark matter, according to Elaine Tolstoy, this can still be explained within the Standard Model. “Who knows, they appeared in a completely different way than ordinary dwarves,” she says. Anyway, she hasn’t said the last word on the matter yet.
Tolstoy also stresses the importance of more and better observations, especially of dwarf galaxies. For example, using the sensitive Weave spectrometer, which was built in part in the Netherlands and recently installed on the William Herschel telescope on the Canary Island of La Palma. The European Vesta telescope in Chile will also be equipped with such a sophisticated instrument in a few years, and the Euclid space telescope, also European, is scheduled to launch at the end of 2022, something Reynier Peletier is especially looking forward to. .
There is no doubt that these new projects will provide more information about the runaway sprawl of the universe. But Tolstoy doesn’t know what that means for the mysteries that still surround dwarf galaxies. Have we not just studied it in sufficient detail? Or is there something fundamentally wrong with our ideas about the evolution of the universe? This is always the question.
The two most famous companions of the Milky Way are the Magellanic Clouds, which can only be seen from the tropics or the Southern Hemisphere. It is much larger than “ordinary” dwarf galaxies: the Large Magellanic Cloud contains about thirty billion stars. Clen about three billion. Moreover, it has now been established with certainty that the two galaxies are rather coincidental transients, as they flew through the Milky Way for the first time at a relatively small distance. The Magellan Clouds are named after the Portuguese explorer Fernando de Magellan, who described them in 1519. American astronomer Harlow Chapley discovered the first true dwarf companions of the Milky Way in the second half of the 1930s. The vast majority have only been found in the past fifteen years.
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