An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



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Number of Results: 107 Search : Galaxy
ultra-diffuse galaxy (UDG)
  کهکشان ِ اولتر-پخشیده   
kahkešân-e ultar-paxšide

Fr.: galaxie ultra-diffuse   

A galaxy of low stellar density, defined to have low central → surface brightness (> 24 mag arcsec-2) and an → effective radius (Re) of over 1.5 kpc. The question of whether UDGs represent a separate class of galaxies is still under debate. Currently, known UDGs that have been discovered in clusters, in groups, and in the field can have Re as large as 5 kpc which is comparable to that of giant Milky Way like galaxies. This fact has been used to suggest that UDGs are "failed" giants. As Re captures (at most) the central parts of giant galaxies, whether this radius can be used to fairly compare the sizes of UDGs to the more massive galaxies is questionable (see, e.g., Chamba et al., 2020, A&A 633, L3).

Term proposed by van Dokkum et al. (2015), arXiv: 1410.8141v2; → ultra-; → diffuse; → galaxy.

ultracompact dwarf galaxy (UCD)
  کهکشان ِ کوتوله‌ی ِ اولتر-همپک   
kahkešân-e kutule-ye ultar-hampak

Fr.: galaxie naine ultracompacte   

A type of very bright compact → stellar system (-14 ≤ MV≥ -12) that is intermediate between → globular clusters (GCs) and → compact elliptical galaxies (cEs). With masses of M > 2 × 106 Msun and radii > 10 → parsecs (pc), UCDs are among the densest stellar systems in the Universe. Nevertheless, the nature and origin of these objects is still widely debated. Early interpretations suggested that UCDs could be the most massive GCs or possibly the → tidally stripped remnants of → dwarf galaxies. However, there is evidence that both formation mechanisms could contribute to the UCD population. → Supermassive black holes (SMBHs) have been confirmed in most UCDs with masses M > 107 Msun. The most massive UCD discovered to date, M59-UCD3 (M* ~ 2 × 108 Msun, radius ~ 25 pc), hosts a SMBH (Ahn et al., 2018, arxiv/1804.02399, and references therein).

ultracompact; → dwarf; → galaxy.

ultraluminous infrared galaxy (ULIRG)
  کهکشانِ فروسرخ ِ اولتر-تابان   
kahkešân-e forusorx-e ultar-tâbân

Fr.: galaxie ultralumineuse en infrarouge   

A galaxy that emits more than 90% of its energy in the infrared (8-1000 µm) and whose infrared luminosity exceeds 1012 solar luminosities. → luminous infrared galaxy (LIRG). Quasars can also have such high or even higher bolometric luminosities. However LIRGs and ULIRGs emit the bulk of their energy in the infrared. Most of ULIRGs are found in merging and interacting galaxy systems. It is thought that their luminosity results from galactic collisions, which increase the rate of star formation.

ultraluminous; → infrared; → galaxy.

violent galaxy
  کهکشان ِ سورا   
kahkašân-e surâ

Fr.: galaxie violente   

A type of galaxy that releases a tremendous amount of energy, on the average 1058 ergs, compared with a supernova release of 1049 ergs. Violent galaxies include quasars and exploding galaxies. About 1 percent of the galaxies are classified as violent. The nearest violent galaxy is Cen A.

violent; → galaxy.

weak arm spiral galaxy
  کهکشان ِ مارپیچ با بازوی ِ نزار   
kahkešân-e mârpic bâ bâzu-ye nezâr

Fr.: galaxie spirale à faibles bras   

A gas-rich galaxy that has weak stellar → spiral arms with → interarm gas and star formation more important than a typical → spiral galaxy, such as NGC 4414.

weak; → arm; → spiral; → galaxy.

Whirlpool galaxy (M51)
  کهکشان ِ گرداب   
kahkešân-e Gerdâb (#)

Fr.: galaxie du Tourbillon   

A large → spiral galaxy of type Sc seen → face-on in the constellation → Canes Venatici and lying about 31 million → light-years away. Also known as M51 and NGC 5194. It is interacting with a small irregular galaxy NGC 5195.

whirlpool; → galaxy.

Wolf-Rayet galaxy
  کهکشان ِ وُلف-رایه   
kahkešân-e Wolf-Rayet

Fr.: galaxie Wolf-Rayet   

A subset of → starburst galaxies whose integrated spectra show broad emission features attributed to the presence of hundreds to thousands → Wolf-Rayet stars. The most massive stars formed in the burst evolve rapidly into a substantial population of Wolf-Rayet stars in aggregations of ionized gas.

Wolf-Rayet star; → galaxy.

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