Missing Dwarf Galaxies Found Near Milky Way in Worst Place

The Milky Way is not alone in its little corner of the Universe.

Small faint dwarf galaxies, many with a few to a thousand stars, orbit our cosmic neighborhood in long graceful circles. It’s unclear exactly how many there are, but there must be many more than the 60 or so we’ve found so far.

Astronomers have recently identified two more of these fierce companions, but the news isn’t as problematic as you might think. Now, there seems to be a lot.

That’s because the two new satellites, called Virgo III and Sextans II, were discovered in a region of space already filled 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 track,” 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 that predicted by recent models for the expected population of Milky Way satellites under cool dark matter models, thus suggesting that we we encounter a “too many satellites” problem.

Dark matter is an invisible, unknown entity in the Universe that contributes extra gravity that cannot be attributed to normal matter. Galaxies, including the Milky Way, are filled with and surrounded by this mysterious matter, giving more speed to the galactic rotation and more gravitational oomph to attract, hold and eventually eat the satellite galaxies.

Based on models of the Milky Way’s dark matter, astronomers expect that the galaxy should have many more satellites of dwarf galaxies than have been found to date. That doesn’t necessarily mean those galaxies aren’t out there, and scientists are leaving no cosmic stone unturned in their quest to find them in the dark.

Dark matter-based models also give us fairly detailed predictions of how many satellite galaxies we should expect to see in specific locations, and this is where Virgo III and Sextans II are presenting a problem.

Homma and his colleagues studied data from the Subaru Strategic Program (SSP) Hyper Suprime-Cam (HSC) to study a segment of space, searching for satellite galaxies of the Milky Way. According to dark matter models, there should be about four dwarf galactic satellites in that part of the sky.

The two new galaxies bring the total in that region to nine. Even before their discovery, the number of satellites there was too high to explain.

Moving things around – excluding the classic dwarf galaxy Sextans, for example, or adopting a different model to predict the number of satellites we should see – also doesn’t solve the problem.

The best model currently predicts that there should be about 220 dwarf galaxies orbiting the Milky Way. If the distribution found in the HSC-SSP track is extrapolated to the rest of the space around our galaxy, this total would in reality be closer to 500 satellites.

However, it is possible that the HSC-SSP track contains a higher concentration of satellites than the space average. 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,” says 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 research was published in Publications of the Astronomical Society of Japan.