Astronomers have made a surprising discovery – two previously unknown potential satellite galaxies orbiting the Milky Way. These newly found dwarf galaxies, named Sextans II and Virgo III, are located around 411,000 and 492,000 light-years from Earth respectively. The discovery is significant because it suggests that many more satellite galaxies could be lurking around us, potentially challenging our understanding of the Milky Way’s edges.
Satellite galaxies are clumps of stars orbiting a larger galaxy, like the Milky Way. They can be circular or halo-like in shape and range in size from containing a few hundred thousand to a few billion stars. While we know of a handful of satellite galaxies orbiting the Milky Way, including the large Magellanic Cloud, which is visible to the naked eye, the majority remain undiscovered.
The existence of these unseen galaxies has been a puzzle for astronomers. Based on current theories about dark matter, which doesn’t interact with light but influences gravity, the Milky Way should have around 220 satellite galaxies. The fact that we’ve only identified a fraction of this predicted number is known as the ‘missing satellites problem.’
The newly discovered satellite galaxies, Sextans II and Virgo III, are considered ultra-compact dwarf galaxies (UCDs), which are tightly packed collections of old stars. These UCDs are brighter than other satellite galaxies, making them easier to detect. However, instead of solving the missing satellites problem, their location and orientation suggest there could be many more satellite galaxies than initially thought, leading to a new ‘too many satellites problem.’
This unexpected discovery was made using the Hyper Suprime-Cam (HSC) attached to Japan’s Subaru Telescope in Hawaii. The HSC has been scanning a region of space about 33 light-years across, searching for faint objects. Based on the predicted number of satellites, this area should have around four on average. However, the HSC has already found nine, including Sextans II and Virgo III. If this concentration of satellites is consistent across the Milky Way, it could mean that there are at least 500 satellite galaxies around us.
The ‘too many satellites problem’ is perplexing scientists. Previous explanations for the missing satellites problem, such as satellite galaxies being too faint or too far away to detect, don’t seem to apply here. The new findings have forced researchers to re-evaluate their understanding of satellite galaxy formation and distribution.
To better understand this new puzzle, researchers plan to use more powerful telescopes with wider fields of view, such as the upcoming Thirty Meter Telescope (TMT), which is expected to come online in 2025. These instruments will allow scientists to search for more satellite galaxies and shed light on the surprising abundance of these celestial objects around the Milky Way.
