Extinct dwarf galaxies discovered near Milky Way in worst place: ScienceAlert

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

Tiny, faint dwarf galaxies, some with only a thousand stars, orbit our cosmic neighborhood in long, graceful circuits. We don't know exactly how many there are, but there are likely to be many more than the 60 or so we've discovered so far.

Astronomers have recently identified two more of these tiny companions, but the news is not as encouraging as one might think. It seems there are now too many of them.

That's because the two new satellites, named Virgo III and Sextans II, were discovered in a region of space already populated by more dwarf galaxies than dark matter models predict.

“Including four already known satellites, there are a total of nine satellites in the HSC-SSP footprint,” writes 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 by recent models for the expected population of Milky Way satellites under cold dark matter models, suggesting that we have a 'too many satellites' problem.”

Dark matter is an invisible and unknown substance in the Universe that contributes additional gravity that cannot be attributed to normal matter. Galaxies, including the Milky Way, are permeated and surrounded by this mysterious matter, which accelerates galactic rotation and increases the gravitational force to attract, hold, and eventually devour satellite galaxies.

Based on models of the Milky Way's dark matter, astronomers expect that the galaxy is home to many more dwarf satellite galaxies than have been discovered so far. That doesn't necessarily mean that these galaxies don't exist, and scientists are working hard to find them in the darkness.

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

Homma and his colleagues studied data from the Hyper Suprime-Cam (HSC) Subaru Strategic Space Program (SSP) to survey a segment of space, looking for satellite galaxies of the Milky Way. According to dark matter models, there should be about four satellite dwarf galaxies in this slice of the sky.

These two new galaxies bring the total number of satellites in this region to nine. Even before their discovery, the number of satellites was too high to be explained.

Moving the pieces around – by excluding the classic dwarf galaxy Sextans, for example, or adopting a different model to predict how many satellites we should see – doesn't solve the problem either.

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

It is possible, however, that the HSC-SSP footprint contains a higher concentration of satellites than the average region of space. The only way to determine whether this is the case is to continue observing other areas of the sky and counting the dwarf galaxies we find there.

“The next step will be to use a more powerful telescope that can 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 for this purpose. I hope that many new satellite galaxies will be discovered.”

The research was published in the Publications of the Astronomical Society of Japan.