Faintest satellite galaxy of Milky Way discovered

Scientists have detected a dwarf satellite galaxy, believed to be the faintest yet found in the halo of the Milky Way, a discovery that provides important insights into galaxy formation and how dark matter contributes to it.
The satellite, named Virgo I, lies in the direction of the constellation Virgo.
Its discovery by researchers from Tohoku University in Japan suggests the presence of a large number of yet-undetected dwarf satellites in the halo of the Milky Way.
Currently, some 50 satellite galaxies to the Milky Way have been identified. About 40 of them are faint and diffuse and belong to the category of "dwarf spheroidal galaxies."

Many recently discovered dwarf galaxies are very faint with absolute luminosity in the optical waveband below minus 8 magnitude.
However, previous searches made use of telescopes with a diameter of 2.5 to four meters, so only satellites close to the Sun or those with higher magnitudes were identified.

Those that are more distant or faint ones in the halo of the Milky Way are yet to be detected.
The combination of the 8.2-metre Subaru Telescope and Hyper Suprime-Cam (HSC) instrument enabled an efficient search for very faint dwarf satellites over large areas of the sky.
"We have carefully examined the early data of the Subaru Strategic Survey with HSC and found an apparent over density of stars in Virgo with very high statistical significance, showing a characteristic pattern of an ancient stellar system in the colour-magnitude diagram," said Daisuke Homma, a graduate student at Tohoku University.

"Surprisingly, this is one of the faintest satellites, with absolute magnitude of minus 0.8 in the optical waveband," said Homma, who found Virgo I under the guidance of Masashi Chiba.
"This is indeed a galaxy, because it is spatially extended with a radius of 124 light years - systematically larger than a globular cluster with comparable luminosity," Homma said.
The faintest dwarf satellites identified so far was Segue I, discovered by Sloan Digital Sky Survey (SDSS) and Cetus II in Dark Energy Survey (DES). Cetus II is yet to be confirmed, as it is too compact as a galaxy.
Virgo I may ultimately turn out to be the faintest one ever discovered. It lies at a distance of 280,000 light years from the Sun, and such a remote galaxy with faint brightness has not been identified in previous surveys.

It is beyond the reach of SDSS, which has previously surveyed the same area in the direction of the constellation Virgo.
"This discovery implies hundreds of faint dwarf satellites waiting to be discovered in the halo of the Milky Way," Chiba said.
"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.