Astronomers discover two new candidate satellites of the Milky Way

For years, astronomers have puzzled over how to explain why the Milky Way has fewer satellite galaxies than the standard dark matter model predicts. This is called the “missing satellites problem”.

To bring us closer to solving this problem, an international team of researchers used data from the Subaru Strategic Program (SSP) Hyper Suprime-Cam (HSC) to discover two entirely new satellite galaxies.

These results were published in Publications of the Astronomical Society of Japan on June 8, 2024 by a team of researchers from Japan, Taiwan and America.

We live in a galaxy called the Milky Way, which has other smaller galaxies orbiting it called satellite galaxies. Studying these satellite galaxies can help researchers unravel the mysteries surrounding dark matter and better understand how galaxies evolve over time.

“How many satellite galaxies does the Milky Way have? This has been an important question for astronomers for decades,” notes Masahi Chiba, a professor at Tohoku University.

The research team recognized the possibility that there are likely many undiscovered small satellite galaxies (dwarf galaxies) that are far away and difficult to detect. The powerful capability of the Subaru Telescope — which sits atop an isolated mountain above the clouds in Hawaii — is well-suited to finding these galaxies. In fact, this research team previously found three new dwarf galaxies using the Subaru telescope.

Now, the team has discovered two new dwarf galaxies (Virgo III and Sextans II). With this discovery, a total of nine satellite galaxies have been found by different research teams. This is still far fewer than the 220 satellite galaxies predicted by the standard dark matter theory.

However, the HSC-SSP footprint does not cover the entire Milky Way. If the distribution of those nine satellite galaxies across the Milky Way is similar to what was found in the track captured by HSC-SSP, the research team calculates that there may actually be closer to 500 satellite galaxies. Now, we are facing a “too many satellites problem”, rather than a “missing satellites problem”.

To better characterize the current abundance of satellite galaxies, more high-resolution images and analyzes are required. “The next step is to use a more powerful telescope that captures a wider view of the sky,” explains Chiba. “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.”