An Etymological Dictionary of Astronomy and Astrophysics
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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.

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

→ halo; → galaxy.

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

→ halo; → galaxy.

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.

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.

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

→ host; → galaxy.

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.

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.

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.

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.

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

→ irregular; → galaxy.

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.

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.

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.

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

→ late; → type; → galaxy.

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

→ lensing; → galaxy.

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.

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.

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

→ low; → mass; → galaxy.

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.