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

The plane in which the → disk of a → spiral galaxy, such as our → Milky Way, lies.

→ galactic; → plane.

The point on the sky, north or south, at which the galaxy's rotation axis would meet the celestial sphere.

→ galactic; → pole.

A diffuse radio signal that originates outside the solar system. It is strongest in the direction of the Galactic plane.

→ galactic; → radio; → noise.

The revolving of the gaseous and stellar content of a galaxy around its central nucleus. The rotation is not uniform, but differential. One revolution of the Sun within our own Galaxy takes about 220 million years, or one cosmic year.

→ galactic; → rotation.

The global shape and the arrangement of the various parts or constituents of a galaxy.

→ galactic; → structure.

Same as → galactic coordinates.

→ galactic; → system.

An outflow of hot gas, analogous to the → solar wind, from a galaxy that has recently undergone a high → burst of star formation or has an → active galactic nucleus. Galactic winds are streams of high speed charged particles blowing out of galaxies with speeds of 300 to 3,000 km s^-1. In the case of starbursts, galactic winds are powered by → stellar winds driven by → massive stars and → supernova explosions. Galactic winds contain a mixture of extremely hot metal-enriched supernova ejecta and cooler entrained gas and dust. Outflowing material has been observed at great distances from galaxies (10 to 100 kpc). In some cases they escape the galaxy potential well and pollute the → intergalactic medium with → heavy elements. A prominent example is the → superwind of the starburst galaxy M82.

→ galactic; → wind.

The regions near the Galactic plane where there is low absorption of light by interstellar clouds so that some external galaxies may be seen through them.

→ galactic; → window.

The time taken for the Sun to revolve once around the center of the Milky Way, amounting to about 220 million years.

→ galactic; → year.

The enormous amounts of → mass and → energy released from active galaxies into the → intergalactic medium. → Supermassive black holes, believed to exist at the centres of active galaxies (→ active galaxy), → accrete matter and liberate huge quantities of energy. The energy output is often observed as → active galactic nuclei (AGN) outflows in a wide variety of forms, e.g. → collimated  → relativistic jets and/or huge overpressured cocoons in → radio, → blueshifted broad → absorption lines in the → ultraviolet and → optical, → warm absorbers and ultrafast outflows in → X-rays, and → molecular gas in → far infrared. Moreover, the processes of → star formation and → supernova explosions release mass/energy into the surroundings. This → stellar feedback heats up, ionizes and drives gas outward, often generating large-scale outflows/→ winds. Galactic outflows are observed at low redshifts reaching a velocity as large as 1000 km s^-1 and at high-z up to z ~ 5, sometimes extending over distances of 60-130 kpc. Galactic-scale outflows may be a primary driver of galaxy evolution through the removal of cool gas from star-forming regions to a galaxy's → halo or beyond.

→ galactic; → scale; → outflow.

Of or relative to the center of a galaxy.

From galacto-, combining form of → galaxy + centric, adj. of rarr; center.

The distance from the center of a galaxy.

→ galactocentric; → distance.

1) Generally, a large body of → gas, → dust, and → stars held together by their mutual → gravitational attraction and ranging in mass from about 10⁶ to 10¹³ Msun. If a galaxy also contains → dark matter its mass will be much larger. Galaxies are grouped into three main categories: → spiral galaxy, → elliptical galaxy, and → irregular galaxy (→ Hubble classification).
2) With capital G, the galaxy to which our Sun belongs; → Milky Way galaxy.
See also:
→ active galaxy, → Andromeda galaxy, → barred spiral galaxy, → biased galaxy formation, → binary galaxy, → blue compact dwarf galaxy, → broad-line radio galaxy, → bulge of a galaxy, → Cartwheel Galaxy, → compact galaxy, → core-halo galaxy, → disk galaxy, → dwarf elliptical galaxy, → dwarf galaxy, → dwarf irregular galaxy, → dwarf spheroidal galaxy, → early-type galaxy, → edge-on galaxy, → face-on galaxy, → field galaxy, → flocculent spiral galaxy, → galaxy bimodality, → galaxy cluster, → galaxy formation, → galaxy harassment, → galaxy main sequence, → gas-poor galaxy, → gas-rich galaxy, → grand design spiral galaxy, → green pea galaxy, → halo of galaxy, → halo of the Galaxy, → Haro galaxy, → host galaxy, → hypergalaxy, → infrared galaxy, → Irr I galaxy, → Irr II galaxy, → isolated galaxy, → late-type galaxy, → lensing galaxy, → lenticular galaxy, → low surface brightness galaxy, → luminous infrared galaxy, → Lyman break galaxy, → Markarian galaxy, → metagalaxy, → metal-deficient galaxy, → metal-poor galaxy, → parent galaxy, → passive galaxy, → passively evolving galaxy, → peculiar galaxy, → primordial galaxy, → progenitor galaxy, → protogalaxy, → radio galaxy, → receding galaxy, → retired galaxy, → ring galaxy, → Sagittarius Dwarf Elliptical Galaxy, → Sagittarius Dwarf Irregular Galaxy, → satellite galaxy, → Sculptor Dwarf Elliptical Galaxy, → Seyfert galaxy, → shell galaxy, → Sombrero galaxy, → starburst galaxy, → strong arm spiral galaxy, → submillimeter galaxy, → superthin galaxy, → superwind galaxy, → tidal dwarf galaxy, → Triangulum galaxy, → ultraluminous infrared galaxy, → violent galaxy, → weak arm spiral galaxy, → Whirlpool galaxy, → Wolf-Rayet galaxy.

From L.L. galaxias "Milky Way," from Gk. galaxis (adj.), from gala (genitive galaktos) "milk."
In Gk. mythology, Jupiter, hoping to immortalize his infant son Hercules (who was born to a mortal woman), placed the baby on Hera's breast. Her milk spilled up, forming the Milky Way. A painting by Italian artist Jacopo Tintoretto (c. 1518-1594), called "The Origin of the Milky Way," depicts the legend describing how the Milky Way was formed.

Kahkešân, short for (râh-e) kahkešân literally "the (path of the) chaff-draggers" or "trail of chaff," from kah, kâh "chaff, straw, hay" (Mid.Pers. kâh "chaff, straw;" cf. Pali kattha- "a piece of wood;" Skt. kastha- "stick;" Gk. klados "twig;" O.Ir. caill "wood;" Ger. Holz "wood;" E. holt; PIE *kldo-) + kešân pr.p. of kešidan/kašidan "to carry, draw, protract, trail, drag" (Mid.Pers. kešidan "to draw, pull;" Av. karš- "to draw; to plow," karša- "furrow;" cf. Skt. kars-, kársati "to pull, drag, plow;" Gk. pelo, pelomai "to move, to bustle;" PIE base k^wels- "to plow"). The term (râh-e) kahkešân may be a popular corruption of Mid. Pers. (râh-i) Kâwôsân "the path of Kâwos" referring to the Persian mythological king Kay Kâwôs, who built an eagle-propelled throne to fly to China, as recounted in the Dênkard and the Shâhnâmé.

The division of galaxies into a "red sequence" and a "blue sequence" in the → color-magnitude diagrams of galaxies involving large statistical surveys. In both sequences, redder galaxies tend to be brighter. The blue sequence is truncated at the red magnitude ~ -22, while the red sequence extends to brighter magnitudes. The division between the two classes of galaxies is associated with a critical stellar mass ~ 3 × 10¹⁰ Msun. Galaxies below the critical mass are typically blue, star forming spirals and reside in the field. Galaxies above the critical mass are dominated by red spheroids of old stars and live in dense environments (Kauffmann et al, 2003, MNRAS 341, 33 & 54).

→ galaxy; → bimodality.

A → spheroidal region at the center of a → spiral galaxy which mostly contains → old stars. Galactic bulges are generally classified into two types: → classical bulges and → pseudo-bulges.

→ galaxy; → bulge.

An aggregation of galaxies, made up of a few to a few thousand members, which may or may not be held together by its own gravity. Same as → cluster of galaxies.

→ galaxy; → cluster.

The study dealing with the processes that gave rise to galaxies in a remarkably → early Universe. See also → structure formation, → protogalaxy

→ galaxy; → formation.

Frequent, high speed galaxy → encounters within → galaxy clusters. Harassment can disturb the morphologies of the galaxies involved, often inducing a new → burst of star formation. Asymmetrical galaxies, → warps, → bars, and → tidal tails can all be produced through galaxy harassment.

→ galaxy; → harassment.

The dominant member of the → Virgo cluster of galaxies, which contains some 2,000 galaxies. Also known as NGC 4486, it has an → apparent visual magnitude 9.6. Discovered in 1781 by Charles Messier, this → elliptical galaxy is located 55 million → light-years away from Earth in the constellation → Virgo. M87 is the home of several thousand billion stars, a → supermassive black hole (SMBH) and a family of roughly 15,000 → globular clusters. For comparison, our → Milky Way galaxy contains only a few hundred billion stars and about 150 globular clusters. M87 is characterized by a prominent kiloparsec scale → relativistic jet emitted by the central SMBH. As gaseous material from the center of the galaxy → accretes onto the black hole, the energy released produces a stream of subatomic particles that are accelerated to velocities near the → speed of light.

→ galaxy; → Messier catalog.

A scaling relation between the → star formation rate (SFR) in galaxies and the total stellar mass (M_*) of the galaxies. This relation, colloquially called the "galaxy main sequence," extends over several orders of magnitudes in M_* and out to → high redshifts, with a modest scatter of ~ 0.3 dex which includes both intrinsic scatter and measurement uncertainties. The existence of such tight scatter at all observed epochs suggests that most galaxies assembled their stellar mass fairly steadily rather than predominantly in → starburst episodes, implying that → mergers have a sub-dominant contribution to the global star formation history (Wuyts et al., 2011 ApJ 742, 96).

→ galaxy; → main; → sequence.