Dwarf spheroidal galaxy
Encyclopedia
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
applied to low luminosity galaxies that are companion
Companion
Companion may refer to:* A friend or acquaintance you associate yourself with* Companion , a nurse assistant or similar professional who assists a patient one-on-one* Companion , an architectural feature of ships...
s to the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
and to the similar systems that are companions to the Andromeda Galaxy M31
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...
. While similar to dwarf elliptical galaxies
Dwarf elliptical galaxy
Dwarf elliptical galaxies, or dEs, are elliptical galaxies that are much smaller than others. They are classified as dE, and are quite common in galaxy groups and clusters, and are usually companions to other galaxies.- Examples :...
in appearance and properties such as little to no gas or dust
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
or recent star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
, they are approximately spheroid
Spheroid
A spheroid, or ellipsoid of revolution is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters....
al in shape, generally lower luminosity, and are only recognized as satellite galaxies in the Local Group
Local Group
The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...
.
While there were nine "classical" dSph galaxies discovered up until 2005, the Sloan Digital Sky Survey
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...
has resulted in the discovery of 11 more dSph galaxies—this has radically changed the understanding of these galaxies by providing a much larger sample to study.
Recently, as growing evidence has indicated that the vast majority of dwarf ellipticals have properties that are not at all similar to elliptical galaxies, but are closer to irregular and late-type spiral galaxies, this term has been used to refer to all of the galaxies that share the properties of those above. These sorts of galaxies may in fact be the most common type of galaxies in the universe, but are much harder to see than other types of galaxies because they are so faint.
Because of the faintness of the lowest luminosity dwarf spheroidals and the nature of the stars contained within them, some astronomers suggest that dwarf spheroidals and globular cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
s may not be clearly separate and distinct types of objects. Other recent studies, however, have found a distinction in that the total amount of mass inferred from the motions of stars in dwarf spheroidals is many times that which can be accounted for by the mass of the stars themselves. In the current predominantly accepted
Cold Dark Matter cosmologyLambda-CDM model
ΛCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter, which is also known as the cold dark matter model with dark energy...
, this is seen as a sure sign 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...
, and the presence of dark matter is often cited as a reason to classify dwarf spheroidals as a different class of object from globular clusters (which show little to no signs of dark matter). Because of the extremely large amounts of dark matter in these objects, they may deserve the title "most dark matter-dominated galaxies"
See also
- GalaxyGalaxyA galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
- Dwarf galaxyDwarf galaxyA dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200-400 billion stars...
- Dwarf elliptical galaxyDwarf elliptical galaxyDwarf elliptical galaxies, or dEs, are elliptical galaxies that are much smaller than others. They are classified as dE, and are quite common in galaxy groups and clusters, and are usually companions to other galaxies.- Examples :...
- Galaxy morphological classification
- Galaxy formation and evolutionGalaxy formation and evolutionThe study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby...
- Groups and clusters of galaxies
- Irregular galaxyIrregular galaxyAn irregular galaxy is a galaxy that does not have a distinct regular shape, like a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a...
- Local groupLocal GroupThe Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...
- List of nearest galaxies
- Dark galaxyDark galaxyA dark galaxy is a hypothetical galaxy composed of dark matter. Dark galaxies receive their name because they have no detectable stars and are theoretically invisible...