The faintest dwarf satellite galaxy in the Milky Way has been discovered by an international team of researchers.
Included in the Subaru Strategic Survey, which used the Hyper Suprime-Cam (HSC), the discovery was published in The Astrophysical Journal and hinted at a big number of dwarf satellites in the Milky Way's halo, waiting to be detected and identified. Called Virgo I, the faintest dwarf satellite galaxy is located about 280,000 light-years away from the sun.
Identifying Dwarf Satellite Galaxies
At the moment, there are about 50 satellite galaxies to the Milky Way that have been detected and identified, with about 40 diffuse and faint, belonging to a category known as "dwarf spheroidal galaxies." However, many of the recently discovered satellite galaxies are even fainter, registering absolute luminosity below 0.8. These are referred to as "ultra-faint dwarf galaxies" and the faintest dwarf satellite galaxy, Virgo I, just discovered is classified as one.
Earlier searches only used telescopes that could spot satellites that are relatively closely located to the sun or were sensitive enough to detect absolute luminosity at higher magnitudes. This is why Virgo I was only detected when the Subaru Telescope was used. Given the telescope had a larger aperture (and it was being used alongside the expansive field of view of the HSC), it was able to spot the faintest dwarf satellite galaxy in the Milky Way.
Aside from the Virgo I, the only other dwarf satellite galaxies to be categorized as ultra-faint are the Segue I and the Cetus II, which were discovered through the Sloan Digital Sky Survey and the Dark Energy Survey, respectively. However, the Cetus II has not been confirmed. With the Segue I having an absolute luminosity of 1.5, it is brighter than the recently discovered dwarf satellite galaxy, making the Virgo I the faintest of them all.
Virgo I Discovery Implications
According to Masashi Chiba, the study's lead, the Virgo I's discovery implies the existence of hundreds of faint dwarf satellites in the halo of the Milky Way, just waiting to be discovered.
"How many satellites are indeed there and what properties they have, will give us an important clue of understanding how the Milky Way formed and how dark matter contributed to it," he said.
Based on the cold dark matter theory, small dark halos in the hundreds orbit a dark halo the size of the Milky Way and are accompanied by luminous satellites. As more of the universe is observed and more satellite galaxies like the Virgo I are found, more light should be shed on the history of the Milky Way.
