Sagittarius Dwarf Elliptical Galaxy
Encyclopedia
The Sagittarius Dwarf Elliptical Galaxy (Sag DEG) is an elliptical loop-shaped satellite galaxy
of the Milky Way Galaxy. It consists of four globular clusters, the main cluster being discovered in 1994. Sag DEG is roughly 10,000 light-year
s in diameter, and is currently about 70,000 light-years from Earth
, travelling in a polar orbit at a distance of about 50,000 light-years from the core of the Milky Way (about 1/3 the distance of the Large Magellanic Cloud
). It should not be confused with the Sagittarius Dwarf Irregular Galaxy
, or the Sag DIG, a small galaxy 3.4 million light-years distant.
, and Gerry Gilmore, Sag DEG was immediately recognized as being the nearest known neighbor to our Milky Way at the time. (Since 2003, the newly discovered Canis Major Dwarf Galaxy
is considered the actual nearest neighbor). Although it is one of the closest companion galaxies to the Milky Way, the main parent cluster is on the opposite side of the galactic core
from Earth, and consequently is very faint, although it covers a large area of the sky. Sag DEG appears to be an older galaxy, with little interstellar dust and composed largely of Population II stars, older and metal-poor, as compared to the Milky Way. No neutral hydrogen gas related to SagDEG was found by Burton & Lockman in 1999.
Further discoveries by astrophysics
teams from both the University of Virginia
and the University of Massachusetts
, drawing upon the 2MASS
Two-Micron All Sky Infrared Survey data, revealed the entire loop-shaped structure. In 2003 with the aid of infrared telescopes and super computers, Steven Majewski, Michael Skrutskie, and Martin Weinberg were able to help create a new star map, picking out the full Sagittarius Dwarf presence, position, and looping shape from the mass of background stars and finding this smaller galaxy to be at a near right angle to the plane of the Milky Way.
Sag DEG has four known globular clusters with one, M54
, apparently residing at its core. It is also dynamically linked to the "young" globular Terzan 7
as well as to Terzan 8 and Arp 2. Additionally, Palomar 12
is now generally thought to also be associated with Sag DEG (Cohen 2004, Sbordone et al. 2006) as well as Whiting 1 (Carraro et al. 2007).
old populations are metal poor ([Fe/H]=-1.6 ± 0.1) while its youngest populations have super-solar abundances.
of Sag DEG.
At first, many astronomers thought that Sag DEG had already reached an advanced state of destruction, so that a large part of its original matter was already mixed with that of the Milky Way. However, Sag DEG still has coherence as a dispersed elongated ellipse, and appears to move in a roughly polar orbit around the Milky Way as close as 50,000 light-years from the galactic core. Although it may have begun as a ball of stars before falling towards the Milky Way, Sag DEG is now being torn apart by immense tidal forces over hundreds of millions of years. Numerical simulations suggest that stars ripped out from the dwarf would be spread out in a long stellar stream
along its path, which were subsequently detected.
However, some astronomers contend that Sag DEG has been in orbit around the Milky Way for some billions of years, and has already orbited it approximately ten times. Its ability to retain some coherence despite such strains would indicate an unusually high concentration of dark matter
within that galaxy.
In 1999, Johnston et al. concluded that SagDEG has orbited the Milky Way for at least one Gya and that during that time its mass has decreased by a factor of two or three. Its orbit is found to have galactocentric distance
s that oscillate between ~13 and ~41 kpc with a period of 550 to 750 million years. The last perigalacticon
was approximately fifty million years ago. Also in 1999, Jiang & Binney found that it may have started its infall into the Milky Way at a point more than 200 kpc away if its starting mass was as large as ~1011 M☉
.
The models of both its orbit and the Milky Way's potential field could be improved by proper-motion observations of SagDEG's stellar debris. This issue is under intense investigation, with computational support by the MilkyWay@Home
project.
Satellite galaxy
A satellite galaxy orbits a larger galaxy due to gravitational attraction. Although a galaxy is made of a large number of objects which are not connected to each other, it has a center of mass, which represents a weighted average of the positions of each component object...
of the Milky Way Galaxy. It consists of four globular clusters, the main cluster being discovered in 1994. Sag DEG is roughly 10,000 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s in diameter, and is currently about 70,000 light-years from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, travelling in a polar orbit at a distance of about 50,000 light-years from the core of the Milky Way (about 1/3 the distance of the Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
). It should not be confused with the Sagittarius Dwarf Irregular Galaxy
Sagittarius Dwarf Irregular Galaxy
The Sagittarius Dwarf Irregular Galaxy or SagDIG is a dwarf galaxy in the constellation of Sagittarius. It lies about 3.4 million light-years away. SagDIG should not be confused with the Sagittarius Dwarf Elliptical Galaxy or SagDEG, a satellite galaxy of the Milky Way. It was discovered by...
, or the Sag DIG, a small galaxy 3.4 million light-years distant.
Features
Officially discovered in 1994, by Rodrigo Ibata, Mike IrwinMike Irwin
Michael Irwin is director of the Cambridge Astronomical Survey Unit and one of the discoverers of the Cetus Dwarf galaxy and the Sagittarius Dwarf Elliptical Galaxy.-References:...
, and Gerry Gilmore, Sag DEG was immediately recognized as being the nearest known neighbor to our Milky Way at the time. (Since 2003, the newly discovered Canis Major Dwarf Galaxy
Canis Major Dwarf Galaxy
The Canis Major Dwarf Galaxy is located in the same part of the sky as the constellation Canis Major. The galaxy contains a relatively high percentage of red giant stars, and is thought to contain an estimated one billion stars in all....
is considered the actual nearest neighbor). Although it is one of the closest companion galaxies to the Milky Way, the main parent cluster is on the opposite side of the galactic core
Galactic Center
The Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 8.33±0.35 kpc from the Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest...
from Earth, and consequently is very faint, although it covers a large area of the sky. Sag DEG appears to be an older galaxy, with little interstellar dust and composed largely of Population II stars, older and metal-poor, as compared to the Milky Way. No neutral hydrogen gas related to SagDEG was found by Burton & Lockman in 1999.
Further discoveries by astrophysics
Astrophysics
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
teams from both the University of Virginia
University of Virginia
The University of Virginia is a public research university located in Charlottesville, Virginia, United States, founded by Thomas Jefferson...
and the University of Massachusetts
University of Massachusetts
This article relates to the statewide university system. For the flagship campus often referred to as "UMass", see University of Massachusetts Amherst...
, drawing upon the 2MASS
2MASS
Observations for the Two Micron All-Sky Survey began in 1997 and were completed in 2001 at two telescopes located one each in the northern and southern hemispheres to ensure coverage of the entire sky...
Two-Micron All Sky Infrared Survey data, revealed the entire loop-shaped structure. In 2003 with the aid of infrared telescopes and super computers, Steven Majewski, Michael Skrutskie, and Martin Weinberg were able to help create a new star map, picking out the full Sagittarius Dwarf presence, position, and looping shape from the mass of background stars and finding this smaller galaxy to be at a near right angle to the plane of the Milky Way.
Globular clusters
Messier 54
Messier 54 is a globular cluster in the constellation Sagittarius. It was discovered by Charles Messier in 1778 and subsequently included in his catalog of comet-like objects....
, apparently residing at its core. It is also dynamically linked to the "young" globular Terzan 7
Terzan 7
Terzan 7 is a sparse and young globular cluster that is believed to have originated in the Sagittarius Dwarf Elliptical Galaxy and is physically associated with it. It is relatively metal rich with Fe/H] = -0.6 and an estimated age of 7.5 Gyr...
as well as to Terzan 8 and Arp 2. Additionally, Palomar 12
Palomar 12
Palomar 12 is a globular cluster in the constellation Capricornus that belongs to the halo of the Milky Way galaxy. First discovered on the Palomar Survey Sky plates by Robert G. Harrington and Fritz Zwicky,...
is now generally thought to also be associated with Sag DEG (Cohen 2004, Sbordone et al. 2006) as well as Whiting 1 (Carraro et al. 2007).
Metallicity
Sag DEG has multiple stellar populations, ranging in age from the oldest globular clusters (almost as old as the universe itself) to trace populations as young as several hundred million years (mya). It also exhibits an age-metallicity relationship, in that itsold populations are metal poor ([Fe/H]=-1.6 ± 0.1) while its youngest populations have super-solar abundances.
Geometry and dynamics
Based on its current trajectory, the Sag DEG main cluster is about to pass through the galactic disc of the Milky Way within the next hundred million years, while the extended loop-shaped ellipse is already extended around and through our local space and on through the Milky Way galactic disc, and in the process of slowly being absorbed into the larger galaxy, calculated at 10,000 times the massMass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
of Sag DEG.
At first, many astronomers thought that Sag DEG had already reached an advanced state of destruction, so that a large part of its original matter was already mixed with that of the Milky Way. However, Sag DEG still has coherence as a dispersed elongated ellipse, and appears to move in a roughly polar orbit around the Milky Way as close as 50,000 light-years from the galactic core. Although it may have begun as a ball of stars before falling towards the Milky Way, Sag DEG is now being torn apart by immense tidal forces over hundreds of millions of years. Numerical simulations suggest that stars ripped out from the dwarf would be spread out in a long stellar stream
Stellar stream
This is a list of stellar streams. A stellar stream is an association of stars orbiting a galaxy that was once a globular cluster or dwarf galaxy that has now been torn apart and stretched out along its orbit by tidal forces...
along its path, which were subsequently detected.
However, some astronomers contend that Sag DEG has been in orbit around the Milky Way for some billions of years, and has already orbited it approximately ten times. Its ability to retain some coherence despite such strains would indicate an unusually high concentration of dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
within that galaxy.
In 1999, Johnston et al. concluded that SagDEG has orbited the Milky Way for at least one Gya and that during that time its mass has decreased by a factor of two or three. Its orbit is found to have galactocentric distance
Galactocentric distance
Galactocentric distance is a star's distance from the center of a galaxy. For example, our Sun is about 27 kly from the center of the Milky Way. Galactocentric distance may also refer to a galaxy's distance from another galaxy....
s that oscillate between ~13 and ~41 kpc with a period of 550 to 750 million years. The last perigalacticon
Perigalacticon
Perigalacticon is an apsis. It is the point in a star's orbit around a galaxy when it lies closest to the galactic center. The Sun is presently near its perigalacticon. Perigalacticon may also refer to a galaxy's closest point as it orbits another galaxy....
was approximately fifty million years ago. Also in 1999, Jiang & Binney found that it may have started its infall into the Milky Way at a point more than 200 kpc away if its starting mass was as large as ~1011 M☉
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
.
The models of both its orbit and the Milky Way's potential field could be improved by proper-motion observations of SagDEG's stellar debris. This issue is under intense investigation, with computational support by the MilkyWay@Home
MilkyWay@Home
MilkyWay@home is a volunteer distributed computing project in astrophysics running on the Berkeley Open Infrastructure for Network Computing platform...
project.
External links
- http://www.space.com/scienceastronomy/galaxy_gobble_030924.html
- SagDEG (SEDS)
- SagDEG at solstation.com