Sagittarius Dwarf Spheroidal Galaxy

Sagittarius Dwarf Spheroidal Galaxy. It is a small satellite galaxy of the Milky Way, discovered in 1994, and is currently in the process of being torn apart and absorbed by our larger spiral galaxy. This ongoing disruption makes it a critical laboratory for studying galactic cannibalism, dark matter, and the assembly history of the Local Group. Observations of its extensive tidal streams have revolutionized our understanding of how large galaxies grow by accreting smaller companions.

Discovery and observation

The galaxy was identified in 1994 by a team of astronomers including Rodrigo Ibata, Gerard Gilmore, and Michael J. Irwin during a survey of the Milky Way halo using the UK Schmidt Telescope. Its discovery was a significant surprise, as it was found to be located on the far side of the Galactic Center, obscured by dense regions of interstellar dust. Follow-up studies using instruments like the Two Micron All-Sky Survey and the Sloan Digital Sky Survey have been crucial in mapping its structure. Key observational campaigns have also been conducted with the Hubble Space Telescope and the Gaia astrometry mission, which precisely tracks the motions of its constituent stars.

Characteristics

Morphologically, it is classified as a dwarf spheroidal galaxy, exhibiting a low surface brightness and an ellipsoidal shape devoid of significant recent star formation. Its stellar population is predominantly old and metal-poor, similar to those found in classical globular clusters like M54, which resides at its core. The galaxy has an estimated absolute magnitude around -13.5, with a total stellar mass of only a few hundred million solar masses. However, dynamical measurements suggest it is embedded in an extensive halo of dark matter, with a total mass potentially exceeding one hundred billion solar masses, indicating a high mass-to-light ratio.

Interaction with the Milky Way

The galaxy is engaged in an extreme gravitational interaction with the much more massive Milky Way. Over multiple orbits spanning billions of years, tidal forces from our galaxy have stretched it into long, filamentary structures known as tidal streams that wrap around the Galactic halo. These streams, which contain stars stripped from the dwarf, have been mapped across the entire sky and are often called the Sagittarius Stream. The process is a clear example of minor merger and provides direct evidence for the Lambda-CDM model of hierarchical galaxy formation. The orbit of the dwarf has been traced using data from the Gaia mission.

Significance in galactic astronomy

The discovery and study of this galaxy have had profound implications for galactic astronomy. Its tidal debris has been used to probe the shape and mass distribution of the Milky Way's dark matter halo, offering constraints competing with those from the Magellanic Clouds. The accretion event is thought to have triggered star formation episodes in the Milky Way disk and influenced the structure of the Virgo Supercluster's local filament. It serves as a nearby analog for similar accretion events observed around distant galaxies like Andromeda and has reshaped the census of the Local Group.

Future evolution

The ultimate fate of the galaxy is complete dissolution into the Milky Way halo. Simulations, such as those run on supercomputers at the University of California, Irvine, predict that its remaining core will be fully disrupted within the next one to two billion years. Its stars and dark matter will become indistinguishable from the native population of the Galactic halo, contributing to the growth of the Milky Way's stellar and dark matter content. This ongoing merger provides a preview of the future collision between the Milky Way and the Andromeda Galaxy.

Category:Dwarf spheroidal galaxies Category:Sagittarius (constellation) Category:Local Group