After analyzing images from the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS), an international team of astronomers discovered a new compact moon of the Milky Way, called Ursa Major III/UNIONS 1. The newly discovered object turned out to be the smallest satellite. The brightest known satellite of the Milky Way. The discovery was reported in an article published on the preprint server on November 16. arXiv.

Dozens of smaller gravitationally bound galaxies or star clusters are known to orbit the Milky Way. Although the list of identified moons is relatively long, astronomers believe that some dwarf and faint galaxies remain undiscovered.

One of the astronomical probes capable of finding such satellites is UNIONS, which observes 4,800 square degrees of sky in the northern hemisphere. This is a collaboration between two observatories in Hawaii: the Canada-France-Hawaii Telescope (CFHT) and the Pan-Star Survey Telescope and Rapid Response System (Pan-STARRS). One of the main goals of the research is to study the composition and structure of the Milky Way.

A team of astronomers led by Simon ET Smith from the University of Victoria in Canada examined the UNIONS data as part of the search for new galaxies in the Local Group. As a result, a satellite of the Milky Way that was not reported in previous studies was found.

“Ursa Major III/UNIONS 1 (UMa3/U1) was discovered during the ongoing search for faint Local Group systems in UNIONS,” the researchers write.

The team first identified Ursa Major III/UNIONS 1 as a spatially resolved stellar overdensity in the UNIONS data. They then obtained radial velocities from the Keck telescope and eigenmotions from ESA’s Gaia satellite; This confirmed that the system was a consistent system.

Ursa Major III/UNIONS 1 has an absolute V-band magnitude of +2.2, making it the faintest Milky Way moon discovered to date. The previous record holder was Kim 3, an ultra-faint V-band star cluster with a total absolute magnitude of +0.7.

According to the article, Ursa Major III/UNIONS 1 is very compact, has a half-light radius of about 10 light-years, and contains only about 50-60 stars. Therefore, its total mass is also estimated to be very small – about 16 solar masses.

The results show that Ursa Major III/UNIONS 1 has a circumcenter of 41,700 light-years and passes through the disk of the Milky Way at a distance of approximately 52,100 light-years from the center of the Galaxy. The data also show that Ursa Major III/UNIONS 1 is at least 11 billion years old and that its stellar population is metal-poor.

When it comes to its origin and nature, Ursa Major III/SUZI 1 may have been added to the Milky Way halo and may be a dwarf galaxy or star cluster. Therefore, more observations are needed to reveal the true nature of this compact and faint moon. Source