Dwarf elliptical galaxy

Dwarf elliptical galaxies, or dEs, are elliptical galaxies that are smaller than ordinary elliptical galaxies. They are quite common in galaxy groups and clusters, and are usually companions to other galaxies.

Examples

"Dwarf elliptical" galaxies should not be confused with the rare "compact elliptical" galaxy class, of which M32, a satellite of the Andromeda Galaxy, is the prototype. In 1944 Walter Baade confirmed dwarf ellipticals NGC 147 and NGC 185 as members of the Local Group by resolving them into individual stars, thanks to their relatively little distance. In the 1950s, dEs were also discovered in the nearby Fornax and Virgo clusters.[1]

Comparison with ordinary elliptical galaxies

Dwarf elliptical galaxies have blue absolute magnitudes within the range −18 mag < M < −14 mag, fainter than ordinary elliptical galaxies. The surface brightness profiles of ordinary elliptical galaxies used to be approximated with de Vaucouleur's model, while dEs were approximated with an exponentially declining surface brightness profile. However, both types can be well fit by the same more general function, known as Sersic's model, and there is a continuity of Sersic index - which quantifies the shape of the surface brightness profile - as a function of galaxy luminosity,[2] revealing that dwarf and ordinary elliptical galaxies belong to a single sequence. Still fainter elliptical-like galaxies, called dwarf spheroidal galaxies, may be genuinely distinct.

Origins

Dwarf ellipticals may be primordial objects. Within the currently favoured cosmological Lambda-CDM model, small objects (consisting of dark matter and gas) were the first to form. Because of their mutual gravitational attraction, some of these will coalesce and merge, forming more massive objects. Further mergers lead to ever more massive objects. The process of coalescence could lead to the present-day galaxies, and has been called "hierarchical merging". If this hypothesis is correct, dwarf galaxies may be the building blocks of today's ordinary galaxies.

An alternative suggestion[3] is that dEs could be the remnants of low-mass spiral galaxies that obtained a rounder shape through the action of repeated gravitational interactions with ordinary galaxies within a cluster. This process of changing a galaxy's morphology by interactions, and the removal of much of its stellar disk, has been called "galaxy harassment". Evidence for this latter hypothesis has been claimed due to stellar disks and weak spiral arms seen in some dEs. Under this alternative hypothesis, the anaemic spiral arms and disk are a modified version of the original stellar disk of the now transformed spiral galaxy.

At the same time, the galaxy harassment scenario can not be the full picture[4]. The highly isolated dwarf elliptical galaxy CG 611 possesses the same physical attributes as dE galaxies in clusters - such as rotation and faint spiral arms - attributes that were previously assumed to provide evidence that dE galaxies were once spiral galaxies prior to a transformation process requiring immersion with a cluster of galaxies. CG 611 has a gas disk which counter-rotates to its stellar disk[5], clearly revealing that this dE galaxy's disk is growing via accretion events. If CG 611 was to fall into a galaxy cluster, ram-pressure stripping by the cluster's halo of hot X-ray gas would strip away CG 611's gas disk and leave a gas-poor dE galaxy that immediately resembles the other dEs in the cluster. That is, no removal of stars nor re-shaping of the galaxy within the dense galaxy cluster environment would be required, undermining the idea that dE galaxies were once spiral galaxies.

See also

References

  1. ^ G. Reaves (1956), Dwarf galaxies in the Virgo cluster
  2. ^ A. Graham and R. Guzman (2003), HST Photometry of Dwarf Elliptical Galaxies in Coma, and an Explanation for the Alleged Structural Dichotomy between Dwarf and Bright Elliptical Galaxies
  3. ^ Moore, B. et al. (1996), Galaxy harassment and the evolution of clusters of galaxies
  4. ^ Janz, J. et al. (2017), Implications for the origin of early-type dwarf galaxies - the discovery of rotation in isolated, low-mass early-type galaxies
  5. ^ Graham, A.W. et al. (2017), Implications for the Origin of Early-type Dwarf Galaxies: A Detailed Look at the Isolated Rotating Early-type Dwarf Galaxy LEDA 2108986 (CG 611), Ramifications for the Fundamental Plane’s SK2 Kinematic Scaling, and the Spin-Ellipticity Diagram
Boom! (novel)

Boom! is a children's science fiction novel by Mark Haddon published in 2009. It is the revised version of Mark Haddon's Gridzbi Spudvetch!, which was published in 1992.

Boom! tells the story of two best friends, Charlie and Jimbo (a nickname for James). When Jimbo's sister, Becky, says that the teachers are going to send him to a school for mentally ill children, Jimbo and Charlie sneaked into the staff room, where they hid a walkie-talkie to eavesdrop on the teachers' conversation, in order to confirm what Becky said was true. None of the information they hear means anything (which points to the fact that Becky was just trying to scare Jimbo), except for the surprising fact that their teachers both Mr. Kidd and Mrs. Pearce is speaking another language. After a while of dangerous investigating (for instance, sneaking into Mrs. Pearce's attic) they were approached by a man in a suit at a restaurant who told them to leave their teachers alone and then promptly burned a hole through the table they were sitting at with his finger. They disobeyed his order, however, and Charlie was kidnapped and taken to the Sagittarius Dwarf Elliptical Galaxy, where he had to fake happiness or else face certain death. But, Jimbo did not seem to know this. After a while of Jimbo investigating Charlie's "Spudvetch!" notebook (their secret notebook for gathering information) he discovered that his best bet to find him would be on the Isle Of Skye, in Scotland. He and his sister, who he had managed to convince to come with him, eventually arrived there, although with much difficulty, and inside an abandoned shack, a mysterious portal opened. Jimbo got sucked into Sagittarius Dwarf Elliptical Galaxy, but his sister stayed on Earth, unaware. He found Charlie and, with much difficulty, escaped.

After a while Mr. Kidd and Mrs. Pearce were fired from the school as a result of the kidnapping. They went for revenge on Charlie and tried to kill him. Luckily, the FBI already knew what was going on and had been watching them. They shot Mr. Kidd and Mrs. Pearce and then said that if they ever told anyone about Mr. Kidd and Mrs. Pearce, they would be hunted down and killed. They decided they did not want to be killed, so returned to their normal lives.

Boötes II (dwarf galaxy)

Bootes II or Boo II is a dwarf spheroidal galaxy situated in the Bootes constellation and discovered in 2007 in the data obtained by Sloan Digital Sky Survey. The galaxy is located at the distance of about 42 kpc from the Sun and moves towards the Sun with the speed of 120 km/s. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an approximately round shape with the half-light radius of about 51 pc.Bootes II is one of the smallest and faintest satellites of the Milky Way—its integrated luminosity is about 1,000 times that of the Sun (absolute visible magnitude of about −2.7), which is much lower than the luminosity of the majority of globular clusters. However the mass of the galaxy is substantial corresponding to the mass to light ratio of more than 100.The stellar population of Bootes II consists mainly of moderately old stars formed 10–12 billion years ago. The metallicity of these old stars is low at [Fe/H]=−1.8, which means that they contain 80 times less heavy elements than the Sun. Currently there is no star formation in Bootes II. The measurements have so far failed to detect any neutral hydrogen in it—the upper limit is only 86 solar masses.Bootes II is located only 1.5 degrees (~1.6 kpc) away from another dwarf galaxy—Boötes I, although they are unlikely to be physically associated because they move in opposite directions relative to the Milky Way. Their relative velocity—about 200 km/s is too high. It is more likely associated with the Sagittarius Stream and, therefore, with the Sagittarius Dwarf Elliptical Galaxy (SagDEG). Bootes II may be either a satellite galaxy of SagDEG or one of its star clusters torn from the main galaxy 4–7 billion years ago.

Canis Major Overdensity

The Canis Major Dwarf Galaxy (CMa Dwarf) or Canis Major Overdensity (CMa Overdensity) is a disputed dwarf irregular galaxy in the Local Group, located in the same part of the sky as the constellation Canis Major.

The supposed small galaxy contains a relatively high percentage of red giants and is thought to contain an estimated one billion stars in all.

The Canis Major Dwarf Galaxy is classified as an irregular galaxy and is now thought to be the closest neighboring galaxy to the Earth's location in the Milky Way, being located about 25,000 light-years (7.7 kiloparsecs) away from the Solar System and 42,000 ly (13 kpc) from the Galactic Center. It has a roughly elliptical shape and is thought to contain as many stars as the Sagittarius Dwarf Elliptical Galaxy, the previous contender for closest galaxy to our location in the Milky Way.

Messier 110

Messier 110 or M110, also known as NGC 205, is a dwarf elliptical galaxy that is a satellite of the Andromeda Galaxy. Although Charles Messier never included the galaxy in his list, it was depicted by him, together with M32, on a drawing of the Andromeda galaxy; a label on the drawing indicates that Messier first observed NGC 205 on August 10, 1773. The galaxy was independently discovered by Caroline Herschel on August 27, 1783; her brother William Herschel described her discovery in 1785. The suggestion to assign the galaxy a Messier number was made by Kenneth Glyn Jones in 1967.This galaxy has a morphological classification of pec dE5, indicating a dwarf elliptical galaxy with a flattening of 50%. M110 is designated peculiar because there are patches of dust and young blue stars located near the center. This is unusual for dwarf elliptical galaxies in general, and the reason for this peculiarity is unclear. Unlike M32, NGC 205 does not (as of 2005) show evidence for a supermassive black hole at its center.The interstellar dust in M110 has a mass of (1.1–1.8)×104 M☉ with a temperature of 18–22 K, and the interstellar gas has (4–7)×106 M☉. The inner regions of M110 show a deficiency in the interstellar medium (IM) materials, which most likely were ejected by supernova explosions. Tidal interactions with M31 may have stripped away a significant fraction of the expelled gas and dust, leaving the galaxy as a whole deficient in its IM density.A few novae have been detected in this galaxy, including one discovered

in 1999 by Johnson and Modjaz, and another detected in 2002, by Nakano and Sumoto. The latter, designated EQ J004015.8+414420, had also been captured in images taken by the Sloan Digital Sky Survey (SDSS) in October, 2002.About half of the Andromeda's satellite galaxies are orbiting the host galaxy along a highly flattened plane, with 14 out of 16 following the same sense of rotation. One theory proposes that these objects once belonged to a subhalo surrounding NGC 205, then the group was broken up by tidal forces during a close encounter with Andromeda.

Messier 32

Messier 32 (also known as M32 and NGC 221) is a dwarf "early-type" galaxy located about 2.65 million light-years from Earth, appearing in the constellation Andromeda. M32 is a satellite galaxy of the Andromeda Galaxy (M31) and was discovered by Guillaume Le Gentil in 1749. M32 measures 6.5 ± 0.2 thousand light-years in diameter at the widest point.The galaxy is a prototype of the relatively rare, compact elliptical (cE) galaxy class.

Half the stars concentrate within an effective radius of only 100 parsecs.

Densities in the central stellar cusp increase steeply, exceeding 3×107 M⊙ pc−3 at the smallest radii resolved by HST, and the half-light radius of this central star cluster is around 6 parsec.

Like more ordinary elliptical galaxies, M32 contains mostly older faint red and yellow stars with practically no dust or gas and consequently no current star formation. It does, however, show hints of star formation in the relatively recent past.

Mike Irwin

Michael J. Irwin is a British astronomer. He is the director of the Cambridge Astronomical Survey Unit and one of the discoverers of the Cetus Dwarf galaxy and the Sagittarius Dwarf Elliptical Galaxy.

NGC 185

NGC 185 (also known as Caldwell 18) is a dwarf spheroidal galaxy located 2.08 million light-years from Earth, appearing in the constellation Cassiopeia. It is a member of the Local Group, and is a satellite of the Andromeda Galaxy (M31). NGC 185 was discovered by William Herschel on November 30, 1787, and he cataloged it "H II.707". John Herschel observed the object again in 1833 when he cataloged it as "h 35", and then in 1864 when he cataloged it as "GC 90" within his General Catalogue of Nebulae and Clusters. NGC 185 was first photographed between 1898 and 1900 by James Edward Keeler with the Crossley Reflector of Lick Observatory. Unlike most dwarf elliptical galaxies, NGC 185 contains young stellar clusters, and star formation proceeded at a low rate until the recent past. NGC 185 has an active galactic nucleus (AGN) and is usually classified as a type 2 Seyfert galaxy, though its status as a Seyfert is questioned. It is possibly the closest Seyfert galaxy to Earth, and is the only known Seyfert in the Local Group.

NGC 3226

NGC 3226 is a dwarf elliptical galaxy that is interacting with the spiral galaxy NGC 3227. The two galaxies are one of several examples of a spiral with a dwarf elliptical companion that are listed in the Atlas of Peculiar Galaxies. Both galaxies may be found in the constellation Leo.

NGC 4121

NGC 4121 is a dwarf elliptical galaxy in the constellation Draco.

NGC 4323

NGC 4323 is a lenticular or dwarf elliptical galaxy galaxy located about 52.5 million light-years away in the constellation Coma Berenices. It was discovered in 1882 by astronomer Wilhelm Tempel. It is a companion of Messier 100. It is also a member of the Virgo Cluster.

NGC 4436

NGC 4436 is a lenticular or dwarf elliptical galaxy located about 60 million light-years away in the constellation of Virgo. NGC 4436 was discovered by astronomer William Herschel on April 17, 1784. The galaxy is a member of the Virgo Cluster.

NGC 4468

NGC 4468 is a dwarf elliptical galaxy located about 55 million light-years away in the constellation of Coma Berenices. The galaxy was discovered by astronomer William Herschel on January 14, 1787. It is a member of the Virgo Cluster.

NGC 4482

NGC 4482 is a dwarf elliptical galaxy located about 60 million light-years away in the constellation Virgo. NGC 4482 was discovered by astronomer William Herschel on March 15, 1784. It was rediscovered by astronomer Arnold Schwassmann on September 6, 1900 and was listed as IC 3427. It is a member of the Virgo Cluster.

NGC 4489

NGC 4489 is a dwarf elliptical galaxy located about 60 million light-years away in the constellation of Coma Berenices. It was discovered by astronomer William Herschel on March 21, 1784. NGC 4489 is a member of the Virgo Cluster.

NGC 4627

NGC 4627 is a dwarf elliptical galaxy in the constellation Canes Venatici.

Sagittarius Dwarf Spheroidal Galaxy

The Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph), also known as the Sagittarius Dwarf Elliptical Galaxy (Sgr dE or Sag DEG), is an elliptical loop-shaped satellite galaxy of the Milky Way. It consists of four globular clusters, the main cluster having been discovered in 1994. Sgr dSph is roughly 10,000 light-years in diameter, and is currently about 70,000 light-years from Earth, travelling in a polar orbit (i.e. an orbit passing over the Milky Way’s galactic poles) 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). In its looping, spiraling path, it has passed through the plane of the Milky Way several times in the past. In 2018 the Gaia project of the European Space Agency showed that Sgr dSph had caused perturbations in a set of stars near the Milky Way's core, causing unexpected rippling movements of the stars triggered when it sailed past the Milky Way between 300 and 900 million years ago.

Sagittarius Stream

In astronomy, the Sagittarius Stream is a long, complex, structure made of stars that wrap around the Milky Way galaxy in an almost polar orbit. It consists of tidally stripped stars from the Sagittarius Dwarf Elliptical Galaxy resulting from the process of merging with the Milky Way over a period of billions of years.

Sculptor Dwarf Galaxy

The Sculptor Dwarf Galaxy (also known as Sculptor Dwarf Elliptical Galaxy or the Sculptor Dwarf Spheroidal Galaxy) is a dwarf spheroidal galaxy that is a satellite of the Milky Way. The galaxy lies within the constellation Sculptor. It was discovered in 1937 by American astronomer Harlow Shapley using the 24-inch Bruce refractor at Boyden Observatory. The galaxy is located about 290,000 light-years away from the Solar System. The Sculptor Dwarf contains only 4 percent of the carbon and other heavy elements in our own galaxy, the Milky Way, making it similar to primitive galaxies seen at the edge of the universe.

Segue 2

Segue 2 is a dwarf spheroidal galaxy situated in the constellation Aries and discovered in 2009 in the data obtained by Sloan Digital Sky Survey. The galaxy is located at the distance of about 35 kpc (35,000 parsecs (110,000 ly)) from the Sun and moves towards the Sun with the speed of 40 km/s. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an approximately round shape with the half-light radius of about 34 pc.The name is due to the fact that it was found by the SEGUE program, the Sloan Extension for Galactic Understanding and Exploration.

Segue 2 is one of the smallest and faintest satellites of the Milky Way—its integrated luminosity is about 800 times that of the Sun (absolute visible magnitude of about −2.5), which is much lower than the luminosity of the majority of globular clusters. However, the mass of the galaxy—about 550,000 solar masses—is substantial, corresponding to the mass to light ratio of about 650.The stellar population of Segue 2 consists mainly of old stars formed more than 12 billion years ago. The metallicity of these old stars is also very low at [Fe/H] < −2, which means that they contain at least 100 times less heavy elements than the Sun. The stars of Segue 2 were probably among the first stars to form in the Universe. Currently, there is no star formation in Segue 2.Segue 2 is located near the edge of Sagittarius Stream and at the same distance. It may once have been a satellite of Sagittarius Dwarf Elliptical Galaxy or its star cluster.In June 2013 The Astrophysical Journal reported that Segue 2 was bound together with dark matter.Circa 1,000 stars are supposed to exist within the galaxy.

Morphology
Structure
Active nuclei
Energetic galaxies
Low activity
Interaction
Lists
See also

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