Astronomers have discovered a previously unknown pair of potential satellite galaxies orbiting the Milky Way. The location of these dwarf galaxies suggests that hundreds of other unknown mini-galaxies may be lurking around us—a potential challenge to our understanding of the galactic edges.
A satellite galaxy is a cluster of stars, either in a circular spot or halo-like shape, that orbit the Milky Way independently of the rest of the galaxy. The largest known satellite of our galaxy is Large Magellanic Cloud, which contains about 30 billion stars and can be seen with the naked eye. Other known satellites contain only a few hundred thousand or a few million stars.
A Census 2020 of known satellite galaxies suggests that scientists have discovered a maximum of about 60 satellite galaxies orbiting the Milky Way. However, there is some uncertainty about the true number of satellite galaxies, largely because scientists disagree about how large these star clusters must be and how far they must lie from the galactic center to be considered true satellites, according to NASA.
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But most astronomers agree that there must be many more, currently unknown, satellite galaxies. Based on our current understanding of dark matter — which does not react with light but interacts gravitationally with visible matter and makes up about 27% of the mass of the universe — researchers have long assumed that the Milky Way must have about 220 satellite galaxies. Our inability to spot many more of them is often referred to as the “missing satellite problem.”
In a new study published June 8 in the journal Publications of the Astronomical Society of Japan, researchers discovered two new potential satellite galaxies, Sextans II and Virgo III. The satellites are located about 411,000 and 492,000 light-years from Earth, respectively, and are likely both ultracompact dwarf (UCD) galaxies—collections of old stars packed tightly together, making them brighter than other satellite galaxies.
However, the discoveries do not help solve the problem of missing satellites. Instead, the location and orientation of these potential satellites suggests that there are even more satellite galaxies than scientists originally realized. This raises a new problem that the researchers called the “too many satellites problem.”
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The researchers discovered the UCD using the Hyper Suprime-Cam (HSC) attached to Japan’s Subaru Telescope at the Mauna Kea Observatories in Hawaii. This instrument has spent the last few years searching a region of space about 33 light-years across. Based on the estimated 220 satellite galaxies postulated by the missing satellite problem, an area of this size should have an average of about four satellite galaxies, Universe Today reported recently.
However, the latest discoveries bring the total number of satellites detected by the HSC in this area to nine. If this concentration of satellites is constant around the Milky Way, it would mean that there could be at least 500 satellite galaxies around the Milky Way, the researchers wrote in statement.
In the past, scientists have proposed many solutions to the problem of missing satellites, including that some satellite galaxies are hiding behind larger satellites and that others are so diffuse as to be almost impossible to detect with current technology. However, it is unlikely that these factors can explain the abundance of stars, leaving scientists with no real way to explain the new results.
“The next step is to use a more powerful telescope that captures a wider view of the sky,” study co-author Masahi Chiba, an astronomer at Tohoku University in Japan, the statement said. This should help clarify how common satellite galaxies are, he added.
One such telescope is the upcoming one Vera C. Rubin Observatory — state-of-the-art facility equipped with the world’s largest digital camera, which is expected to come online in 2025, Chiba said. “I hope that many new satellite galaxies will be discovered.
