H II galaxy
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
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.
halo of galaxy
hâle-ye kakekašân (#)
Fr.: halo de galaxie
halo of the Galaxy
hâle-ye kakekašân (#)
Fr.: halo de la Galaxie
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.
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.
kahkešân-e mizbân (#)
Fr.: galaxie hôte
A usually faint galaxy in which a remarkable phenomenon, such as a → supernova event, occurs.
hot dust-obscured galaxy (HDOG)
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.
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.
Irr I galaxy
kahkašân-e bisâmân-e gune-ye I
Fr.: galaxie irrégulière de type I
Irr II galaxy
kahkašân-e bisâmân-e gune-ye I
Fr.: galaxie irrégulière de type II
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.
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.
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).
late-type galaxy (LTG)
kahkešân-e gune-ye farjâmin
Fr.: galaxie de type tardif
Fr.: galaxie amplificatrice
A galaxy that acts as a → gravitational lens. The effect can also be due to a cluster of galaxies.
kahkešân-e adasvâr (#)
Fr.: galaxie lenticulaire
A lens-shaped galaxy, which is an enormous grouping of old stars with very little internal structure.
low surface brightness galaxy (LSBG)
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.
Fr.: galaxie de faible masse
A galaxy with stellar masses ≤ 109 → solar masses (Dawn K. Erb, 2015, Nature, 9 July).
luminous infrared galaxy (LIRG)
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.