Discovery of the faintest companion galaxy of the Milky Way, Ursa Major Dwarf Galaxy III (UNIONS 1) | Satellite portal website sorae

In order to accurately measure the number and mass of stars in the entire galaxy, it is necessary to find clusters of stars that are dark and difficult to see. Many of these star clusters orbit the main galaxy.Companion to the Milky Way(space galaxy)”.

A research team led by Simon E. T. Smith of the University of Victoria used data from UNIONS (Northern Optical Ultraviolet-Near-Infrared Survey) to determine which galaxy is a companion galaxy to the Milky Way.Ursa Major Dwarf Galaxy III / UNIONS 1(Ursa Major III / UNIONS 1)” (hereinafter referred to as “Ursa Major Dwarf Galaxy III”). The absolute brightness of the Ursa Major Dwarf Galaxy III is 2.2 magnitudeThe faintest known companion galaxy of the Milky WayHe is.

■“Companion galaxies” that exist in large numbers around galaxies

When you hear the word “galaxy,” you probably think of a cluster of stars. However, in the vicinity of actual galaxies, there are many star clusters far beyond the galaxy, such as the Large and Small Magellanic Clouds. A group of these stars is gravitationally bound and orbits the host galaxy. A galaxy that has a lunar relationship with a parent galaxy is called a galaxy.Companion to the Milky WayBecause of their small size, most companion galaxies are classified as dwarf galaxies.

Each companion galaxy is a dwarf galaxy with a lower mass and fewer stars than the parent galaxy, but since there are so many companion galaxies, we cannot ignore their combined effect. However, each companion galaxy is dark and located near a brighter galaxy, making observation difficult.

■A much darker companion galaxy, Ursa Major Dwarf Galaxy III, has been discovered.

Smith and his team were analyzing data from UNIONS, a collection of comet observation data from the Northern Hemisphere. UNIONS collects data from the Canada-France-Hawaii Telescope and Pan-STARRS, both telescopes based in Hawaii, USA, and mainly investigates the structure of the Milky Way used for this purpose.

Smith and his colleagues discovered a region of high concentration of stars toward the constellation Ursa Major, but it’s unclear whether this is truly a gravitationally bound group of stars, or just a high density of stars. . an act. Therefore, we estimated the direction and speed of the star’s motion from observational data from the WM Keck Observatory, also in Hawaii, and the European Space Agency’s space telescope, Gaia.

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As a result, the directions of movement of the stars and their velocities were found to be identical, and it was revealed that this was truly a group of stars bound by gravity. This group of stars is the third dwarf galaxy discovered in the direction of the Ursa Major constellation.Ursa Major Dwarf Galaxy III“, and because it was the first companion galaxy discovered from UNIONS data, it was named “”.Unions 1“It is also called.

Ursa Major Dwarf Galaxy III is a very small companion galaxy, with a diameter of about 20 light-years (50% of total luminosity) and only about 50 to 60 stars (57 +21 -19), with a total mass estimated to be about 16 times that of the Sun. . for this reason,The absolute magnitude of the Ursa Major Dwarf Galaxy III is 2.2.then,The faintest companion galaxy knownThis is what it means. The distance from the Sun to the Ursa Major Dwarf Galaxy III is estimated at about 33,000 light-years, making it an extremely faint object with an apparent magnitude of 17.2.

The Ursa Major Dwarf III galaxy revolves in an elliptical orbit about 42,000 light-years away from the center of the Milky Way Galaxy, and about 85,000 light-years away from it at its farthest point, and about 55,000 light-years away from the center. It is estimated that it passes through the galactic disk at a position.

In addition, the Ursa Major Dwarf Galaxy III is at least 11 billion years old, and its stars are estimated to have low metal (heavy element) content. This feature indicates that the Ursa Major III dwarf galaxy emanates from a “halo” surrounding the Milky Way.

■Is the discovery of Ursa Major Dwarf Galaxy III useful for estimating dark matter?

The discovery of “hard-to-see” companion galaxies such as the Ursa Major Dwarf Galaxy III also imposes limitations on the search to estimate the amount of “unseen” dark matter. Dark matter is an unknown substance that cannot be seen by electromagnetic waves like light, but whose existence is known indirectly through gravity.

Another study by Smith and others estimated that if there was no dark matter in the Ursa Major III dwarf galaxy, it would disappear in only about 400 million years, because it would be disintegrated by tidal forces from the Milky Way. This differs significantly from the estimated age of at least 11 billion years, meaning that Ursa Major Dwarf Galaxy III contains a significant amount of dark matter.

The true nature of dark matter is still a big mystery, but one possible candidate is that it is made of extremely heavy particles. If so, gamma rays may be emitted due to decay. A research team led by Milena Crnogorcevic and Tim Linden of Stockholm University examined 15 years of data from the Fermi Gamma-ray Space Telescope to see if gamma rays were produced by the decay of such heavy particles.

As a result, no excess gamma radiation was found. Based on these data, even if the dark matter is a heavy kinetic energy particle (hot WIMP), the mass of the particle is 1 to 4 TeV (1 to 4 trillion MeV/2 to 7 x 10 to the negative 24th power). ). kg). This is one piece of data that can be used to rule out candidates when searching for the true nature of dark matter.

Ursa Major Dwarf Galaxy III has just been discovered, and both the discovery report and the research papers based on it are preprints that have not yet been peer-reviewed. Additional observations are needed to confirm whether various data related to Ursa Major Dwarf Galaxy III are correct and to obtain more accurate data.

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Written by Riri Aya