The Milky Way is not alone in its little corner of the Universe.
Tiny, faint dwarf galaxies, many with as few as a thousand stars, swoop around our cosmic neighborhood on long graceful circuits. It's unclear exactly how many there are, but there should be a lot more than the 60 or so we've found to date.
Astronomers have recently identified two more of these itty-bitty companions, but the news isn't as problem-solving as you might think. Now, there appear to be too many.
That's because the two new satellites, named Virgo III and Sextans II, were discovered in a region of space already crowded with more dwarf galaxies than models of dark matter predict.
"Including four previously known satellites, there are a total of nine satellites in the HSC-SSP footprint," write a team led by Daisuke Homma of the National Astronomical Observatory of Japan.
"This discovery rate of ultra-faint dwarfs is much higher than that predicted from the recent models for the expected population of Milky Way satellites in the framework of cold dark matter models, thereby suggesting that we encounter a 'too many satellites' problem."
Dark matter is an invisible, unknown something in the Universe that contributes additional gravity that can't be attributed to normal matter. Galaxies, including the Milky Way, are imbued with and surrounded by this mysterious stuff, giving more speed to the galactic rotation and more gravitational oomph to attract, retain, 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 satellites 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 effort to find them in the gloom.
Dark matter-based models also give us fairly detailed predictions about how many satellite galaxies we should expect to see in specific places, and this is where Virgo III and Sextans II are posing a problem.
Homma and his colleagues studied data from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) to study a segment of space, looking for Milky Way satellite galaxies. According to dark matter models, there should be around four dwarf galaxy satellites in that slice 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 classical dwarf galaxy Sextans, for example, or adopting a different model to predict the number of satellites we should see – also doesn't resolve the problem.
The best model currently predicts that there should be around 220 dwarf galaxies orbiting the Milky Way. If the distribution found in the HSC-SSP footprint is extrapolated to the rest of the space around our galaxy, that total would in reality be closer to 500 satellites.
It is possible, however, that the HSC-SSP footprint contains a higher concentration of satellites than the average section of space. The only way to determine whether this is the case is to keep looking at other patches 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 fulfill that purpose. I hope that many new satellite galaxies will be discovered."
The research has been published in the Publications of the Astronomical Society of Japan.