The Milky Way isn’t the only galaxy in its little corner of the universe.
Small, faint dwarf galaxies, many of which contain only a thousand stars, are speeding around our cosmic neighborhood on long, graceful circuits. It’s unclear exactly how many there are, but there should be many more than the 60 or so we’ve found so far.
Astronomers recently identified two more of these tiny companions, but the news isn’t as problem-solving as you might think. Now there seem to be too many of them.
That’s because the two new satellites, Virgo III and Sextans II, were discovered in a region of space that already contains more dwarf galaxies than dark matter models predict.
“Including four previously known satellites, a total of nine satellites are located in the HSC-SSP region,” wrote a team led by Daisuke Homma of the National Astronomical Observatory of Japan.
“The discovery rate of ultrafaint dwarf stars is much higher than predicted by recent models for the expected population of Milky Way satellites under cold dark matter models. This suggests that we are dealing with a ‘too many satellites’ problem.”
Dark matter is an invisible, unknown thing in the universe that adds extra gravity not attributable to normal matter. Galaxies, including the Milky Way, are saturated and surrounded by this mysterious stuff, giving more speed to the galactic rotation and more gravity to attract, hold on to, and eventually devour satellite galaxies.
Based on models of the Milky Way’s dark matter, astronomers expect the galaxy should have far more dwarf galaxy satellites than have been found so far. 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 darkness.
Dark matter models also give us pretty detailed predictions about how many satellite galaxies we can expect to find in specific places. And this is where Virgo III and Sextans II pose a problem.
Homma and his colleagues used data from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) to study a patch of space in search of Milky Way satellite galaxies. According to dark matter models, there should be about four dwarf galaxy satellites in that patch of 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.
Nor is the problem solved by moving things around – for example, excluding the classical dwarf galaxy Sextans or using a different model to predict the number of satellites we should see.
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 footprint is extrapolated to the rest of the space around our galaxy, that 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 average region of space. The only way to tell if this is the case is to continue 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 achieve that goal. I hope that many new satellite galaxies will be discovered.”
The research was published in the Publications of the Astronomical Society of Japan.