The Milky Way is not alone in its little corner of the universe.
Small, pale dwarf galaxies, many with fewer than a thousand stars, drift around our cosmic neighborhood in long, graceful circles. It’s not clear exactly how many there are, but it must be a lot more than the 60 or so we’ve found so far.
Astronomers have recently identified two more of these tiny satellites, but the news isn’t as groundbreaking as you might think. Now it seems there are too many.
That’s because the two new moons, called Virgo III and Sextans II, were discovered in a region of space already crowded with more dwarf galaxies than dark matter models predict.
“Including four previously known satellites, there are a total of nine satellites in the HSC-SSP footprint,” wrote a team led by Daisuke Homma of the National Astronomical Observatory of Japan.
“This detection rate of ultra-faint dwarfs is much higher than predicted by recent models for the expected population of Milky Way satellites within cold dark matter models, thus suggesting that we are facing a problem with ” too many satellites’.
Dark matter is an invisible, unknown thing in the universe that contributes extra gravity that cannot be attributed to normal matter. Galaxies, including the Milky Way, are imbued and surrounded by this mysterious stuff, which gives a greater speed to the galactic rotation and more gravitational force to attract, hold, and eventually eat satellite galaxies.
Based on models of the Milky Way’s dark matter, astronomers expect that the galaxy should have many more dwarf galaxy companions than have been discovered so far. That doesn’t necessarily mean these galaxies aren’t out there, and scientists are leaving no cosmic stone unturned in their efforts to find them in the dark.
Dark matter-based models also give us fairly detailed predictions about how many satellite galaxies we should expect to see in certain locations, and this is where Virgo III and Sextans II present a problem.
Homma and colleagues examined data from the Subaru Strategic Program’s (SSP) Hyper Suprime-Cam (HSC) to survey a segment of space looking for satellite galaxies of the Milky Way. According to dark matter models, there should be about four satellites of a dwarf galaxy in this part of the sky.
The two new galaxies bring the total in this region to nine. Even before they were discovered, the number of satellites there was too great to explain.
Shucking things around – ruling out the classic Sextans dwarf galaxy, for example, or adopting a different model to predict the number of satellites we should see – doesn’t solve the problem either.
The best model currently predicts that there should be about 220 dwarf galaxies around the Milky Way. If the distribution found in the HSC-SSP footprint is extrapolated to the rest of the space around our galaxy, this total would actually be closer to 500 satellites.
However, it is possible that the HSC-SSP footprint contains a higher concentration of satellites than the middle part of space. The only way to determine if this is the case is to keep looking at other parts of the sky and count the dwarf galaxies we find there.
“The next step is to use a more powerful telescope that captures a wider view of the sky,” said astronomer Masashi Chiba of Tohoku University. “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 study was published in Publications of the Astronomical Society of Japan.
