The Gaia Sausage or Gaia Enceladus is the remains of a

dwarf galaxy A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is ...

(the Sausage Galaxy, or Gaia-Enceladus-Sausage, or Gaia-Sausage-Enceladus) that merged with the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

about 8–11 billion years ago. At least eight

globular cluster A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of membe ...

s were added to the Milky Way along with 50 billion

solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

es of stars, gas and

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...

. It represents the last major merger of the Milky Way.

Etymology

The "Gaia Sausage" is so-called because of the characteristic sausage shape of the population in a chart of velocity space, in particular a plot of radial (

\boldsymbol_r

) versus azimuthal velocity (

\boldsymbol_\theta

) of stars (See

spherical coordinate system In mathematics, a spherical coordinate system is a coordinate system for three-dimensional space where the position of a point is specified by three numbers: the ''radial distance'' of that point from a fixed origin, its ''polar angle'' measu ...

), using data from the

Gaia Mission ''Gaia'' is a space observatory of the European Space Agency (ESA), launched in 2013 and expected to operate until 2025. The spacecraft is designed for astrometry: measuring the positions, distances and motions of stars with unprecedented preci ...

. The stars that have merged with the Milky Way have orbits that are highly elongated. The outermost points of their orbits are around 20

kiloparsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...

s from the

Galactic Center The Galactic Center or Galactic Centre is the rotational center, the barycenter, of the Milky Way galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a compact rad ...

at what is called the "halo break". These stars had previously been seen in Hipparcos data and identified as originаting from an accreted galaxy. The name "Enceladus" refers to the mythological giant

Enceladus Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most refle ...

, who was buried under

Mount Etna Mount Etna, or simply Etna ( it, Etna or ; scn, Muncibbeḍḍu or ; la, Aetna; grc, Αἴτνα and ), is an active stratovolcano on the east coast of Sicily, Italy, in the Metropolitan City of Catania, between the cities of Messina a ...

and caused earthquakes. Thus this former galaxy was buried in the Milky Way, and caused the puffing up of the thick disc.

Components

Globular clusters

The globular clusters firmly identified as former Sausage members are Messier 2,

Messier 56 Messier 56 (also known as M56 or NGC 6779) is a globular cluster in the constellation Lyra. It was discovered by Charles Messier in 1779. It is angularly found about midway between Albireo (Beta (β) Cygni) and Sulafat (Gamma (γ) Lyrae). In a ...

Messier 75 Messier 75 or M75, also known as NGC 6864, is a giant globular cluster of stars in the southern constellation Sagittarius. It was discovered by Pierre Méchain in 1780 and included in Charles Messier's catalog of comet-like objects that same ...

Messier 79 Messier 79 (also known as M79 or NGC 1904) is a globular cluster in the southern Lepus constellation, constellation Lepus. It was discovered by Pierre Méchain in 1780 and is about 42,000 light-years away from Earth and 60,000 light years from t ...

, NGC 1851, NGC 2298, and NGC 5286.

NGC 2808: Globular cluster or old core?

NGC 2808 NGC 2808 is a globular cluster in the constellation Carina (constellation), Carina. The cluster belongs to the Milky Way, and is one of our home galaxy's most massive clusters, containing more than a million stars. It is estimated to be 12.5-bi ...

is another globular-like cluster of Sausage. This cluster is composed of three generations of stars, all born within 200 million years of the formation of the cluster. One theory to account for three generations of stars is that NGC 2808 is the former core of Sausage. This is also an explanation for the number of stars, more than a million, which is unusually large for a globular cluster.

Stars

The stars from this dwarf orbit the Milky Way core with extreme eccentricities on the order of about 0.9. Their

metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...

is also typically higher than other halo stars, with most having e/H> −1.7 dex, i.e., at least 2% of the solar value The "Gaia Sausage" reconstructed the Milky Way by puffing up the

thin disk The thin disk is a structural component of spiral and S0-type galaxies, composed of stars, gas and dust. It is the main non-centre (e.g. galactic bulge) density, of such matter. That of the Milky Way is thought to have a scale height of around ...

to make it a

thick disk The thick disk is one of the structural components of about 2/3 of all disk galaxies, including the Milky Way. It was discovered first in external edge-on galaxies. Soon after, it was proposed as a unique galactic structure in the Milky Way, diff ...

, whilst the gas it brought into the Milky Way triggered a fresh round of star formation and replenished the thin disk. The debris from the dwarf galaxy provides most of the metal-rich part of the

galactic halo A galactic halo is an extended, roughly spherical component of a galaxy which extends beyond the main, visible component. Several distinct components of galaxies comprise the halo: * the stellar halo * the galactic corona (hot gas, i.e. a plasma) ...

References

External links

* YouTube * * {{Milky Way Milky Way Dwarf galaxies