An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



<< < act dwa H I Lym reg ult > >>

Number of Results: 107 Search : Galaxy
H II galaxy
  کهکشان ِ H II   
kahkešân-e H II

Fr.: galaxie H II   

A low-mass and → metal-poor galaxy (1/30-1/3 Zsun), experiencing strong episodes of → star formation, characterized by the presence of bright → emission lines on a faint → blue continuum. The fact that H II galaxies are metal poor and very blue objects seems to suggest that they are young. Nevertheless, several studies show the existence of an → old stellar population underlying the present → star burst in most of these galaxies. This fact indicates that these objects are not young systems forming their first generation of stars. Same as → blue compact dwarf galaxy.
Spectroscopically, H II galaxies are essentially identical to the → giant H II regions found in nearby → irregular and → late-type galaxies. The correlation among structural parameters (→ H-beta → luminosity, → velocity dispersion, → linewidths) and between these parameters and the → chemical composition favors the interpretation of H II galaxies as giant H II regions in distant → dwarf irregular galaxies similar to the ones found nearby. Some examples of H II galaxies are: I Zw 18, SBS 0335-052, II Zw 33, UM 408.

H II; → galaxy.

halo of galaxy
  هاله‌ی ِ کهکشان   
hâle-ye kakekašân (#)

Fr.: halo de galaxie   

The diffuse, nearly spherical cloud of stars and → globular cluster s that surrounds a → spiral galaxy.

halo; → galaxy.

halo of the Galaxy
  هاله‌ی ِ کهکشان   
hâle-ye kakekašân (#)

Fr.: halo de la Galaxie   

The → halo of galaxy belonging to our → Milky Way.

halo; → galaxy.

Haro galaxy
  کهکشان ِ هارو   
kahkešân-e Hâro (#)

Fr.: galaxie de Haro   

A type of galaxies characterized by strong emission in the blue and violet regions of the spectrum. They are often elliptical or lenticular.

Named after the Mexican astronomer Guillermo Haro (1913-1988), who first compiled a sample of these objects; → galaxy.

head-tail galaxy
  کهکشان ِ "سر-دم"   
kahkešân-e sar-dom

Fr.: galaxie tête-queue, ~ têtard   

A member of the class of radio galaxies (→ radio galaxy) that have a strong radio emission coming from a bright "head" and a more diffuse emission from a "tail." They are often found in clusters.

head; → tail; → galaxy.

host galaxy
  کهکشان ِ میزبان   
kahkešân-e mizbân (#)

Fr.: galaxie hôte   

A usually faint galaxy in which a remarkable phenomenon, such as a → supernova event, occurs.

host; → galaxy.

hot dust-obscured galaxy (HDOG)
  کهکشان ِ تیره با غبار ِ داغ   
kahkešân-e tiré bâ qobâr-e dâq

Fr.: galaxie obscure à poussière chaude   

A member of the most extreme galaxies in terms of their luminosities and unusual hot → dust temperatures. The → infrared emission from HDOGs is dominated by obscured accretion onto a central → supermassive black hole (SMBH), in most cases without significant contribution from → star formation. The large contrast between the underlying → host galaxy and the hyper-luminous emission from the → active galactic nucleus (AGN) implies that either the SMBH is much more massive than expected for the stellar mass of its host, or is radiating well above its → Eddington limit. The most extreme of these remarkable systems known is → W2246-0526.

hot; → dust; → obscure; → galaxy.

  اَبَرکهکشان، هیپرکهکشان   
abarkahkašân, hiperkahkašân

Fr.: hypergalaxie   

A system consisting of a dominant → spiral galaxy associated with → dwarf satellite galaxies and intergalactic matter. Examples in the → Local Group are our Galaxy and the → Andromeda galaxy.

hyper- + → galaxy.

infrared galaxy
  کهکشان ِ فروسرخ   
kahkešân-e forusorx (#)

Fr.: galaxie infrarouge   

A galaxy that emits most of its energy in the infrared region of the spectrum. Such galaxies are thought to have unusually high rates of star formation and are also described as → starburst galaxies.

infrared; → galaxy.

Irr I galaxy
  کهکشان ِ بی‌سامان ِ گونه‌ی ِ I   
kahkašân-e bisâmân-e gune-ye I

Fr.: galaxie irrégulière de type I   

An → irregular galaxy that shows a hint of a spiral arm or bar, and can be placed at the far end of spirals in the → Hubble sequence.

irregular; → galaxy.

Irr II galaxy
  کهکشان ِ بی‌سامان ِ گونه‌ی ِ II   
kahkašân-e bisâmân-e gune-ye I

Fr.: galaxie irrégulière de type II   

An amorphous, → irregular galaxy that does not appear to show any structure that can place it into the → Hubble sequence.

irregular; → galaxy.

irregular galaxy
  کهکشان ِ بی‌سامان   
kahkašân-e bisâmân

Fr.: galaxie irrégulière   

A galaxy with no spiral structure and no symmetric shape. Irregular galaxies are usually filamentary or very clumpy in shape and tend to smaller than others. Two types of irregular galaxies are defined, → Irr I galaxy and → Irr II galaxy.

irregular; → galaxy.

isolated galaxy
  کهکشان ِ وایوتیده   
kahkešân-e vâyutidé

Fr.: galaxie isolée   

A galaxy that is not a member of a dense aggregate. In other words, a galaxy that is formed in a low galactic density environment and has evolved without major interactions with other galaxies of similar mass.

isolated; → galaxy.

jellyfish galaxy
  کهکشان ِ مدوسا   
kahkešên-e medusâ

Fr.: galaxie méduse   

A type of galaxy exhibiting "tentacles" (tails) of material that appear to be stripped from the main body of the galaxy, making it resemble a jellyfish. Such type of galaxies occur in → galaxy clusters and are produced by a process called → ram pressure stripping. The mutual → gravitational attraction between galaxies causes them to fall at high speed into the clusters, where they encounter a hot → intracluster medium (ICM) with dense gas. The falling galaxy feels a powerful wind, forcing tails of gas out of the galaxy's disk and triggering → starbursts within it. Jellyfish galaxies have mainly been observed in nearby clusters (e.g., Virgo, Coma, A1367, A3627, Shapley). A few examples have been identified in clusters at → redshifts z ~ 0.2-0.4, and there is accumulating evidence for a correlation between the efficiency of the stripping phenomenon and the presence of shocks and strong gradients in the X-ray → intergalactic medium (Poggianti et al., 2016, AJ 151, 78).

jellyfish; → galaxy.

late-type galaxy (LTG)
  کهکشان ِ گونه‌ی ِ فرجامین   
kahkešân-e gune-ye farjâmin

Fr.: galaxie de type tardif   

In the → Hubble classification, a galaxy on the left part of the → Hubble sequence. See also → early-type galaxy.

late; → type; → galaxy.

lensing galaxy
  کهکشان ِ لنزنده   
kahkeâân-e lenzandé

Fr.: galaxie amplificatrice   

A galaxy that acts as a → gravitational lens. The effect can also be due to a cluster of galaxies.

lensing; → galaxy.

lenticular galaxy
  کهکشان ِ عدسی‌وار   
kahkešân-e adasvâr (#)

Fr.: galaxie lenticulaire   

A lens-shaped galaxy, which is an enormous grouping of old stars with very little internal structure.

Lenticular "resembling a lentil in form," → lens; → galaxy.

low surface brightness galaxy (LSBG)
  کهکشان با درخشندگی ِ رویه‌ای ِ کم   
kahkešân bâ deraxšandegi-ye ruye-yi-ye kam

Fr.: galaxie à faible brillance de surface   

A member of a particularly faint population of galaxies with a central → surface brightness below the brightness of the background sky. The central regions of many of them resemble a → dwarf galaxy, but most of the mass is contained in a large gaseous disk of low density that is observable only with long-exposure optical images or at radio wavelengths. Some are as massive as a large → spiral galaxy, for example Malin 1. The proportion of LSBGs relative to normal galaxies is unknown. They may however represent a significant fraction of mass in the Universe. LSBGs are thought to be primitive systems because they have total masses similar to normal galaxies, but have typically converted less than 10% of their gas into stars. Spiral LSBGs do not obey → Freeman's law.

low; → surface; → brightness; → galaxy.

low-mass galaxy
  کهکشان ِ کم‌جرم   
kahkešân-e kamjerm

Fr.: galaxie de faible masse   

A galaxy with stellar masses ≤ 109 → solar masses (Dawn K. Erb, 2015, Nature, 9 July).

low; → mass; → galaxy.

luminous infrared galaxy (LIRG)
  کهکشان ِ فروسرخ ِ تابان   
kahkešân-e forusorx-e tâbân

Fr.: galaxie lumineuse en infrarouge   

A galaxy that emits most of its energy in the infrared and whose infrared luminosity (in the 8-1000 µm range) is more than 1011 solar luminosities. → ultraluminous infrared galaxy.

luminous; → infrared; → galaxy.

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