Fr.: galaxie active
A galaxy that produces huge amounts of energy at its center, which cannot be attributed to normal processes from stars, interstellar medium, and their interactions. There are several types of active galaxies: → Seyfert galaxies, → quasars, and → blazars. All of these objects show brightness variations, some as short as 3 hours. These fluctuations indicate a relatively very small size for the central object, because an object cannot vary in brightness faster than light can travel across it. For example, an object that is one → light-year in diameter cannot vary significantly in brightness over a period of less than one year.
Andromeda galaxy (M31, NGC 224)
kahkašân-e Ândromedâ (#), ~ Zan-e bé Zanjir Basté (#)
Fr.: galaxie d'Andromède
The nearest → spiral galaxy to our own and a major member of the → Local Group. It lies in the constellation → Andromeda and is the most remote object normally visible to the naked eye. The earliest known reference to this galaxy is by the Iranian astronomer Sufi who called it "the little cloud" in his Book of Fixed Stars (A.D. 964).
barred spiral galaxy
kahkašân-e mârpic-e miledâr
Fr.: galaxie spirale barrée
biased galaxy formation
diseš-e varakdâr-e kahkašânhâ
Fr.: formation biaisée de galaxies
The theory that bright galaxies form preferentially from anomalously overdense perturbations in the → early Universe.
Fr.: galaxie binaire
A pair of galaxies in orbit around each other.
→ binary; → galaxy.
blue compact dwarf galaxy
kahkešân-e kutule-ye âbi-ye hampak
Fr.: galaxie naine bleue compacte
An small → irregular galaxy undergoing → violent star formation activity. These objects appear blue by reason of containing clusters of hot, → massive stars which ionize the surrounding interstellar gas. They are chemically unevolved since their → metallicity is only 1/3 to 1/30 of the solar value. Same as → H II galaxy.
broad-line radio galaxy (BLRG)
radio kahkašân-e pahn-xatt
Fr.: galaxie radio à raies larges
A radio galaxy that shows broad optical emission lines. → broad-line region.
kahkašân-e carx-e arrâbé
Fr.: galaxie de la roue de charette
A galaxy with a striking ring-like feature lying about 400 million → light-years away in the → constellation → Sculptor. The ring-like structure, over 100,000 light-years in diameter, is composed of regions of → star formation filled with very bright, → massive stars. The shape results from collision with another smaller galaxy.
Kahkašân, → galaxy. Carx-e arrâbé "cartwheel," from carx, → wheel + arrâbé "cart, chariot," maybe related to Mid.Pers. ras, ray "wheel," O.Pers./Av. raθa- "wheel," Khotanese rrha- "car," Skt. ratha- "wheel," L. rota "wheel," PIE base *rotos "wheel."
Fr.: galaxie grumeleuse
An irregularly shaped, clumpy star-forming galaxy that does not fall anywhere on the → Hubble sequence and appears frequently at → redshifts z ~ 1-4. Galaxies at z ≤ 1 with similar morphologies and enhanced → star formation have been identified, but become less common with decreasing redshift. Clumpy galaxies are prominent in the → early Universe.
Fr.: galaxie compacte
A galaxy with no disk or nebulous background and a high surface brightness that appears only barely larger than a star-like point on a sky survey photograph.
compact massive galaxy (CMG)
kahkešân-e porjerm-e hampak
Fr.: galaxie massive compacte
A galaxy with a stellar mass of M ≥ 1011Msun and an → effective radius of Re ≤ 1.5 kpc. Many studies have shown that massive galaxies with low → star formation rates were remarkably compact at a → redshift of z≥ 2. At fixed stellar mass of Mstars ≅ 1011Msun, quiescent galaxies are a factor of ~ 4 smaller at z = 2 than at z = 0. As the stellar mass of the galaxies also evolves, the inferred size growth of individual galaxies is even larger. It is unlikely that all massive galaxies in the present-day Universe had a compact progenitor. However, the vast majority of CMGs that are observed at z = 2 ended up in the center of a much larger galaxy today. Their size growth after z = 2 is probably dominated by minor → mergers. Such mergers are expected because other mechanisms cannot easily produce the observed scaling between size growth and mass growth (P. G. van Dokkum1 et al., 2015, ApJ 813, 23).
core elliptical galaxy
kahkešân-e beyzigun-e maqzedâr
Fr.: galaxie elliptique à coeur
An → elliptical galaxy that displays a → surface brightness profile with a distinct break from a steep outer slope to a shallower inner → cusp. Core profiles mainly occur in very luminous elliptical galaxies and are considered the result of dissipation-less → mergers of two galaxies that have central → supermassive black holes (S. P. Rusli et al., 2013, AJ 146, 160).
Fr.: galaxie cœur-halo
A radio galaxy characterized by an emission "halo" surrounding a more intense "core". About 20% of the known extended radio sources are of the core-halo type.
Fr.: galaxie à disque
dwarf elliptical galaxy
kahkašân-e beyzigun-e kutulé (#)
Fr.: galaxie elliptique naine
A galaxy that is much smaller than other members of the elliptical class; it is designated as dE. A subtype of dwarf ellipticals is called a → dwarf spheroidal galaxy (dSph). The basic characteristics of the class are low surface brightness and smooth light distribution. They range in luminosity from that of the faintest dSph galaies MV ~ -9 to about -17. In the → Local Group there are 19 known dEs. They are very common in → galaxy clusters.
kahkešân-e kutulé (#)
Fr.: galaxie naine
A small, low luminosity galaxy that is associated with a larger spiral galaxy and may make up part of a galactic halo. There are many of them in the Local Group, and often orbit around larger galaxies such as the Milky Way and the Andromeda Galaxy. There are three main types of them: → dwarf elliptical galaxy (dE), → dwarf irregular galaxy (dI), and → dwarf spiral galaxy (dSA).
dwarf irregular galaxy
kahkašân-e bisâmân-e kutulé
Fr.: galaxie irrégulière naine
An irregular galaxy that is much smaller than other irregulars. Dwarf irregulars are generally metal poor and have relatively high fractions of gas. They are thought to be similar to the earliest galaxies that populated the Universe, and are therefore important to understand the overall evolution of galaxies.
dwarf spheroidal galaxy (dSph)
kahkašân-e korevâr-e kutulé (#)
Fr.: galaxie sphéroïdale naine
A subtype of dwarf ellipticals (→ dwarf elliptical galaxy), which are companion to the Milky Way and other similar galaxies. The first example of such objects was discovered by Harlow Shapley (1938) in the constellation → Sculptor. Nine such galaxies are known currently to orbit the Milky Way. Nearby galaxy clusters such as the Virgo, Fornax, Centaurus, and Coma clusters contain hundreds to thousands of individual dSph galaxies. These galaxies have very low surface brightnesses, as low as only 1% that of the night sky (faintest MV ~ -9). They are also among the smallest, least luminous galaxies known. Most of the radiation from dSph galaxies is emitted by stars in the optical portion of the electromagnetic spectrum. The lack of strong emission lines, infrared, or radio radiation suggests that these galaxies are generally devoid of an → interstellar medium. The velocities of stars within dSph galaxies are so high that these galaxies must be disrupting. However, the bulk of mass in these galaxies might be undetected. Dynamical models that include → dark matter do adequately explain the → velocity dispersion of the stars in all dSph systems. In the most extreme cases, only 1% of the mass of the galaxy is visible. Many of the → Local Group dSph galaxies show evidence for star formation more recent than 10 Gyr.
dwarf spiral galaxy
kahkašân-e mârpic-e kutulé (#)
Fr.: galaxie spirale naine
A galaxy that belongs to the spiral class but is significantly smaller.
early-type galaxy (ETG)
kahkešân-e gune-ye âqâzin
Fr.: galaxie de type précoce
In the → Hubble classification, galaxies on the left part of the → Hubble sequence. Early-type galaxies tend to have redder colors, higher average surface brightnesses, and lower → neutral hydrogen content than → late-type galaxies. This terminology is based on the obsolete and erroneous idea that → elliptical and → lenticular galaxies might be evolutionary precursors to → spiral and → barred spiral galaxies.