Dwarf elliptical galaxy

Last updated

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.

Contents

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]

Relation to other elliptical galaxy types

Dwarf elliptical galaxies have blue absolute magnitudes within the range −18 < MV < −14 : fainter than ordinary elliptical galaxies.

The surface brightness profiles of ordinary elliptical galaxies was formerly approximated using de Vaucouleur's model, while dEs were approximated with an exponentially declining surface brightness profile. However, both types fit well by a 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] This is interpreted as showing that dwarf elliptical and ordinary elliptical galaxies belong to a single sequence.

An even-fainter type of elliptical-like galaxies, called dwarf spheroidal galaxies, may be a genuinely distinct class.

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 large spiral galaxies, which in turn are thought to merge to form giant ellipticals.

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 coherent 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

Related Research Articles

Galaxy Gravitationally bound astronomical structure

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass.

Hubble sequence Galaxy morphological classification scheme invented by Edwin Hubble

The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is often colloquially known as the Hubble tuning fork diagram because the shape in which it is traditionally represented resembles a tuning fork.

Elliptical galaxy Galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile

An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the three main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work The Realm of the Nebulae, along with spiral and lenticular galaxies. Elliptical (E) galaxies are, together with lenticular galaxies (S0) with their large-scale disks, and ES galaxies with their intermediate scale disks, a subset of the "early-type" galaxy population.

Spiral galaxy Galaxy having a number of arms of younger stars

Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.

Virgo Cluster galaxy cluster in the constellation Virgo

The Virgo Cluster is a large cluster of galaxies whose center is 53.8 ± 0.3 Mly away in the constellation Virgo. Comprising approximately 1300 member galaxies, the cluster forms the heart of the larger Virgo Supercluster, of which the Local Group is a member. The Local Group actually experiences the mass of the Virgo Supercluster as the Virgocentric flow. It is estimated that the Virgo Cluster's mass is 1.2×1015M out to 8 degrees of the cluster's center or a radius of about 2.2 Mpc.

Lenticular galaxy Type of galaxy intermediate between an elliptical and a spiral galaxy

A lenticular galaxy is a type of galaxy intermediate between an elliptical and a spiral galaxy in galaxy morphological classification schemes. It contains a large-scale disc but does not have large-scale spiral arms. Lenticular galaxies are disc galaxies that have used up or lost most of their interstellar matter and therefore have very little ongoing star formation. They may, however, retain significant dust in their disks. As a result, they consist mainly of aging stars. Despite the morphological differences, lenticular and elliptical galaxies share common properties like spectral features and scaling relations. Both can be considered early-type galaxies that are passively evolving, at least in the local part of the Universe. Connecting the E galaxies with the S0 galaxies are the ES galaxies with intermediate-scale discs.

Galactic bulge Tightly packed group of stars within a larger formation

In astronomy, a galactic bulge is a tightly packed group of stars within a larger star formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies. Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy. It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies.

Dwarf galaxy Small galaxy composed of up to several billion stars

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 sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition.

Messier 86 Elliptical galaxy in the constellation Virgo

Messier 86 is an elliptical or lenticular galaxy in the constellation Virgo. It was discovered by Charles Messier in 1781. M86 lies in the heart of the Virgo Cluster of galaxies and forms a most conspicuous group with another large galaxy known as Messier 84. It displays the highest blue shift of all Messier objects, as it is, net of its other vectors of travel, approaching the Milky Way at 244 km/s. This is due to both galaxies falling roughly towards the center of the Virgo cluster from opposing ends.

Messier 88 Spiral galaxy in the constellation Coma Berenices

Messier 88 is a spiral galaxy about 50 to 60 million light-years away from Earth in the constellation Coma Berenices. It was discovered by Charles Messier in 1781.

Interacting galaxy Galaxies whose gravitational fields result in the disturbance of one another.

Interacting galaxies are galaxies whose gravitational fields result in a disturbance of one another. An example of a minor interaction is a satellite galaxy disturbing the primary galaxy's spiral arms. An example of a major interaction is a galactic collision, which may lead to a galaxy merger.

NGC 4216 Intermediate spiral galaxy in the constellation Virgo

NGC 4216 is a metal-rich intermediate spiral galaxy located not far from the center of the Virgo Cluster of galaxies, roughly 55 million light-years away. It is seen nearly edge-on.

Dwarf spiral galaxy Dwarf counterparts of spiral galaxies

A dwarf spiral galaxy is the dwarf version of a spiral galaxy. Dwarf galaxies are characterized as having low luminosities, small diameters, low surface brightnesses, and low hydrogen masses. The galaxies may be considered a subclass of low-surface-brightness galaxies.

NGC 4261 Elliptical galaxy in the constellation Virgo

NGC 4261 is an elliptical galaxy located around 100 million light-years away in the constellation Virgo. It was discovered April 13, 1784 by the German-born astronomer William Herschel. The galaxy is a member of its own somewhat meager galaxy group known as the NGC 4261 group, which is part of the Virgo Cluster.

NGC 4889 Supergiant elliptical galaxy in the constellation Coma Berenices

NGC 4889 is an E4 supergiant elliptical galaxy. It was discovered in 1785 by the British astronomer Frederick William Herschel I, who catalogued it as a bright, nebulous patch. The brightest galaxy within the northern Coma Cluster, it is located at a median distance of 94 million parsecs from Earth. At the core of the galaxy is a supermassive black hole that heats the intracluster medium through the action of friction from infalling gases and dust. The gamma ray bursts from the galaxy extend out to several million light years of the cluster.

Sersic profile

The Sérsic profile is a mathematical function that describes how the intensity of a galaxy varies with distance from its center. It is a generalization of de Vaucouleurs' law. José Luis Sérsic first published his law in 1963.

NGC 4651 Spiral galaxy in the constellation Coma Berenices

NGC 4651 is a spiral galaxy located in the constellation of Coma Berenices that can be seen with amateur telescopes, at a distance not well determined that ranges from 35 million light years to 72 million light years.

NGC 4647 Intermediate Spiral galaxy in the constellation Virgo

NGC 4647 is an intermediate spiral galaxy estimated to be around 63 million light-years away in the constellation of Virgo. It was discovered by astronomer William Herschel on March 15, 1784. NGC 4647 is listed along with Messier 60 as being part of a pair of galaxies called Arp 116; their designation in Halton Arp's Atlas of Peculiar Galaxies. The galaxy is located on the outskirts of the Virgo Cluster.

NGC 4522 Edge-on spiral galaxy in the constellation of Virgo

NGC 4522 is an edge-on spiral galaxy located about 60 million light-years away within the Virgo Cluster in the constellation Virgo. NGC 4522 is losing its molecular gas though ram-pressure stripping as it plows though the cluster at a speed of more than 10 million kilometres per hour. The galaxy was discovered by astronomer John Herschel on January 18, 1828.

LEDA 2108986 Isolated dwarf galaxy in the constellation Boötes

LEDA 2108986, also known by its Case Western Reserve University designation "Case Galaxy 611", is an extremely isolated, early-type dwarf galaxy with an embedded spiral structure residing in what is likely an intermediate-scale disk. The galaxy was discovered in 1987 by Sanduleak and Pesch, and is located at a distance of about 45.7 megaparsecs (149,000,000 ly) in the Boötes void and has no significant neighbours within 2.5 Mpc.

References

  1. G. Reaves (1956), Dwarf galaxies in the Virgo cluster
  2. Graham, A. & Guzman, R. (June 2003). "HST photometry of dwarf elliptical galaxies in Coma, and an explanation for the alleged structural dichotomy between dwarf and bright elliptical galaxies". The Astronomical Journal. 125 (6): 2936–2958. arXiv: astro-ph/0303391 . Bibcode:2003AJ....125.2936G. doi:10.1086/374992.
  3. Moore, B.; et al. (1996). "Galaxy harassment and the evolution of clusters of galaxies". Nature. Bibcode:1996Natur.379..613M.
  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". Monthly Notices of the Royal Astronomical Society. Bibcode:2017MNRAS.468.2850J.
  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 S2
    K
    kinematic scaling, and the spin-ellipticity diagram"
    . Astrophysical Journal. Bibcode:2017ApJ...840...68G.