The Milky Way is not alone in its little corner of the universe.
Tiny, faint dwarf galaxies, many with only thousands of stars, move around our cosmic neighborhood in long, graceful orbits. It’s unclear exactly how many there are, but there should be a lot more than the 60 or so we’ve found so far.
Astronomers have recently identified two more of these small companions, but the news isn’t quite as pandering as you might think. Now there seem to be too many of them.
That’s because two new satellites, named Virgo III and Sextans II, have been discovered in a region of space already crowded with more dwarf galaxies than dark matter models predict.
“Including the four previously known satellites, there are a total of nine satellites in the track of HSC-SSP,” writes the team led by Daisuke Homma of the National Astronomical Observatory of Japan.
“This rate of discovery of ultra-faint dwarfs is much higher than predicted from recent models for the expected population of Milky Way satellites under cold dark matter models, suggesting that we are experiencing a ‘too many satellites’ problem.”
Dark matter is an invisible, unknown thing in the universe that contributes additional gravity that cannot be attributed to normal matter. Galaxies, including the Milky Way, are imbued and surrounded by this mysterious substance, giving a greater rate of galactic rotation and a greater gravitational pull to attract, hold, and eventually devour satellite galaxies.
Based on dark matter models of the Milky Way, astronomers expect that the galaxy should have many more satellites of dwarf galaxies than have been found so far. That doesn’t necessarily mean these galaxies aren’t out there, and scientists are leaving no stone unturned in their quest to find them in the dark.
Dark matter-based models also give us fairly detailed predictions about how many satellite galaxies we should expect in specific locations, and this is where Virgo III and Sextans II present a problem.
Homma and his colleagues studied data from the Subaru Strategic Program (SSP) Hyper Suprime-Cam (HSC) to study part of the universe and search for satellite galaxies of the Milky Way. According to dark matter models, there should be about four satellites of dwarf galaxies in this section of the sky.
The two new galaxies bring the total in the region to nine. Even before their discovery, the number of satellites was too high to explain.
Moving things around—for example, excluding the classic Sextans dwarf galaxy, 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 around 220 dwarf galaxies around the Milky Way. If the distribution found in the HSC-SSP trace were extrapolated to the rest of the space around our galaxy, the total number would actually be closer to 500 satellites.
However, it is possible that the HSC-SSP track contains a higher concentration of satellites than the average part of the universe. The only way to know if this is the case is to keep looking elsewhere in the sky and count the dwarf galaxies we find there.
“The next step is to use a more powerful telescope to capture a wider view of the sky,” says astronomer Masashi Chiba of Tohoku University. “Next year, the Vera C. Rubin Observatory in Chile will be used to fulfill this purpose. I hope that many new satellite galaxies will be discovered.”
The research was published in Publications of the Astronomical Society of Japan.