An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A galaxy which has a uniformly arranged, symmetrical morphology such as a spiral or elliptical galaxy. Opposite of → irregular galaxy.

→ regular; → galaxy.

An old galaxy with faint emission lines whose ratios are similar to those of → LINERs, i.e. galaxies with low-ionization nuclear emission-line regions. All galaxies after consuming their → molecular clouds, where stars are formed, follow a "passive" evolution during which their → stellar populations simply get older and older. The old stellar populations contain hot post-→ AGB stars and → white dwarfs which are able to ionize the surrounding gas and produce spectra identical to those of LINERS.

Retired in the sense "withdrawn from or no longer occupied with one's business or profession," p.p. of retire, from M.Fr. retirer "to withdraw (something)," from → re- "back" + O.Fr. tirer "to draw;" → galaxy. The concept of retired galaxies was first proposed by G. Stasińska et al. (2008, MNRAS 391, L29) to name the final stages of galaxies that cease their star forming activity. The word "retired" is also to be taken by opposition to "active" in the sense of "containing an accreting black hole" (like Seyfert galaxies), since liners are often thought to be a scaled down version of Seyfert nuclei.

Bâznešasté "retired," literally "seated back, seated away," from bâz-→ re- + nešasté "seated," p.p. of nešastan "to sit;" Mid.Pers. nišastan "to sit;" O.Pers. nišādayam [1 sg.impf.caus.act.] "to sit down, to establish," hadiš- "abode;" Av. nišasiiā [1 sg.subj.acr.] "I shall sit down," from nihad- "to sit down," from ni- "down, in, into," → ni-, + had- "to sit;" PIE base *sed- "to sit;" cf. Skt. sad- "to sit," sidati "sits;" Gk. hezomai "to sit," hedra "seat, chair;" L. sedere "to sit;" O.Ir. suide "seat, sitting;" Welsh sedd "seat;" Lith. sedmi "to sit;" Rus. sad "garden;" Goth. sitan, Ger. sitzen; E. sit.

A galaxy with a ring-like appearance around the central luminous center. The ring consists of massive, relatively young bright stars. It is believed that ring galaxies result from the head-on collision of two different galaxies.

→ ring; → galaxy.

A satellite galaxy of the Milky Way discovered only in 1994 since most of it is obscured by the Galactic disc. At only 50,000 light years distant from our Galaxy's core, it is travelling in a polar orbit around the Galaxy. Our Galaxy is slowly devouring it, as evidenced by a filament which stretches around the Milky Way's core like a gossamer loop. It is only about 10,000 light-years in diameter, in comparison to the Milky Way's diameter of 100,000 light years. It is populated by old yellowish stars has four known globular clusters: M54, Arp 2, Terzan 7, and Terzan 8. It should not be confused with the → Sagittarius Dwarf Irregular Galaxy.

→ Sagittarius; → dwarf; → elliptical; → galaxy.

A dwarf irregular galaxy, discovered in 1977, that is a member of the Local Group of galaxies. It has a diameter of 1,500 light-years and lies about 3.5 million light-years away. SagDIG contains as much as about 10⁸ solar masses of H I gas and is one of the most metal-poor galaxies. It should not be confused with the → Sagittarius Dwarf Elliptical Galaxy.

→ Sagittarius; → dwarf; → irregular; → galaxy.

A galaxy that orbits a larger one due to gravitational attraction. The Milky Way has at least ten satellite galaxies: the Large Magellanic Cloud, the Small Magellanic Cloud, Ursa Minor Dwarf, Draco Dwarf, Sculptor Dwarf, Sextans Dwarf, Carina Dwarf, Fornax Dwarf, Ursa Major I, and → Sagittarius Dwarf Elliptical Galaxy.

→ satellite; → galaxy.

A → dwarf elliptical galaxy that is a satellite of our → Milky Way. It lies about 285,000 → light-years away in the constellation → Sculptor, and has an → absolute magnitude of -11.28 and a mass of about 3 million → solar masses. The Sculptor Dwarf is a → metal-deficient galaxy containing only 4 percent of the oxygen and carbon elements in our own Galaxy.

→ Sculptor; → dwarf; → elliptical; → galaxy.

A member of an important class of → active galaxies which are characterized by the presence of an intensely bright nucleus in the optical wavelengths (10⁹-10¹²Lsun) displaying emission spectral lines. The presence of these emission features, which are not seen in the spectra of normal galaxies, indicates a very high degree of ionization. Moreover, the nucleus radiates → non-thermal  → continuum emission extending over the whole electromagnetic spectrum. It is thought that a → massive black hole in the nucleus of a galaxy accretes gas (→ accretion) from its surrounding environment to power Seyfert galaxies. These galaxies are divided into two types according to the width of their spectral lines. Type 1 Seyfert (Sy 1) galaxies have very broad emission lines (10³- 10⁴ km s^-1), while Type 2 Seyferts (Sy 2) show relatively narrow lines (several hundred km s^-1). These spectral differences may be the result of viewing the nucleus from different angles. A Type 2 Seyfert galaxy may be a mostly edge-on view of matter spiraling in toward the supermassive black hole, whereas a Type 1 Seyfert provides a more pole-on view, allowing us to see the more turbulent region around the black hole.

Named after Carl Keenan Seyfert (1911-1960), the American astronomer who first identified this type of galaxies in 1943; → galaxy.

An elliptical galaxy that is surrounded by thin shells of stars which are thought to have been ejected during a galaxy merger. Shell galaxies are different from ring galaxies in that the shells are much further away from the galaxy's centre and much fainter than the rings. Spectroscopy of the stars in the shell show that they are old whereas the stars in a ring galaxy are young.

→ shell; → galaxy.

A → spiral galaxy in the constellation → Virgo. It was the first galaxy whose rotation was detected. Also named M104 and NGC 4594.

Sp. sombrero "broad-brimmed hat," originally "umbrella or parasol," from sombra "shade," from L.L. subumbrare, from → sub- "under" + umbra "shade, shadow;" → galaxy.

Kahkešân, → galaxy; sombrero, Sp., as above.

Same as → NGC 5866.

→ spindle; → galaxy.

A galaxy with a prominent nuclear → bulge and luminous → spiral arms of gas, dust, and young stars that wind out from the nucleus. Masses span the range from 10¹⁰ to 10¹²→ solar masses.

→ spiral; → galaxy.

A galaxy that is located on the → galaxy main sequence in the plane relating → star formation rates to total stellar masses.

→ star; → formation; → galaxy.

A galaxy showing a short-lived intense period of star formation that is unsustainable over the → Hubble time due to the limited supply of gas within a galaxy. Starburst galaxies were first classified by Searle & Sargent (1972) and Searle et al. (1973), based on the blue colors produced by the → massive stars formed during the burst. In the local Universe, starbursts create approximately 10% of the radiant energy and 20% of the massive stars. At z = 1, starburst characteristics are found in 15% of galaxies, presumably attributable to the greater amounts of gas typically present in young galaxies and increased galactic interactions. The starburst's impact on a galaxy and the surrounding → intergalactic medium is primarily due to the consumption of gas that fuels the burst and the feedback from massive stars formed in the burst (McQuinn et al. 2010, astro-ph/1008.1589).

→ starburst; → galaxy.

A galaxy with prominent stellar → spiral arms and little star formation between stellar arms, such as M51.

→ strong; → arm; → spiral; → galaxy.

A member of an extremely luminous population of → high-redshift galaxies which are detected in → submillimeter waves (→ flux density at 850 μm ≥ 3 - 5 mJy). SMGs are powered primarily by star formation rather than an → active galactic nucleus (AGN). Because of their high → dust content, these galaxies emit almost all of their luminosity in the infrared, with a → bolometric luminosity ranging from 10¹²-10¹³ → solar luminosities. As such, SMGs resemble → ultraluminous infrared galaxies (ULIRGs), which are almost exclusively → merging galaxies. Indeed, many observations support a → merger origin for SMGs (see, e.g. C.C. Hayward et al. 2011 and references therein, astro-ph/1101.0002).

→ submillimeter; → galaxy.

A galaxy that appears as an extraordinary thin and long figure on the sky because of its → edge-on orientation, highly flattened stellar → disk, and absence of a → bulge component. Superthin galaxies are → gas-rich and have optically diffuse disks with little internal absorption, as well as low emission-line intensity ratios and slowly rising → rotation curves. They seem to be among the least evolved disk galaxies in the local Universe, having undergone only minimal dynamical heating, → star formation, and → angular momentum transport. Examples are: UGC 7321, UGC 3697, UGC 9242.

→ super-; → thin; → galaxy.

A galaxy with → superwind characteristics. M 82 and NGC 4666 are among superwind galaxy candidates.

→ superwind; → galaxy.

A self-gravitating entity which has been formed from tidal material expelled during interactions between larger galaxies. TDGs are typically found at the tip of tidal tails at distances between 20 and 100 kpc from the merging galaxies, of which at least one should be a gas-rich galaxy. They are gas-rich objects that can be as massive as the Magellanic Clouds, form stars at a rate which might be as high as in blue compact dwarf galaxies and seem dynamically independent from their parent galaxies.

→ tidal; → dwarf; → galaxy.

One of the prominent members of the → Local Group situated in the constellation → Triangulum. Also know as NGC 598. M33 is a type Sc → spiral galaxy seen nearly → face-on. It lies 2.8 million → light-years away and its diameter is 52,000 light-years. M33 is thought to be a satellite of the → Andromeda Galaxy.

→ Triangulum; → galaxy.