Fr.: rotation galactique
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.
Fr.: structure galactique
The global shape and the arrangement of the various parts or constituents of a galaxy.
Fr.: système galactique
Same as → galactic coordinates.
Fr.: vent galactique
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.
Fr.: fenêtres galactiques
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.
sâl-e kahkešâni (#)
Fr.: année galactique
The time taken for the Sun to revolve once around the center of the Milky Way, amounting to about 220 million years.
ostacân bâ marpel-e kahkešâni
Fr.: flot à l'échelle galactique
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.
Of or relative to the center of a galaxy.
Fr.: distance galactocentrique
The distance from the center of a galaxy.
Fr.: facteur de Gaunt
In the atomic theory of spectral line formation, a quantum mechanical correction factor applied to the absorption coefficient in the transition of an electron from a bound or free state to a free state.
Gaunt, after John Arthur Gaunt (1904-1944), English physicist born in China, who significantly contributed to the calculation of continuous absorption using quantum mechanics; → factor
Fr.: réfraction géodésique
The limiting case of → astronomical refraction when the light path is entirely within the Earth's atmosphere.
žirandegi-ye zamin-meqnâtisi, ~ zamin-meqnâti
Fr.: activité géomagnétique
Fr.: attraction gravitationnelle
The force that pulls material bodies toward one another because of → gravitation.
Fr.: contraction gravitationnelle
Decrease in the volume of an astronomical object under the action of a dominant, central gravitational force.
Fr.: interaction gravitationnelle
Mutual attraction between any two bodies that have mass.
Fr.: Grand Attracteur
A hypothesized large concentration of mass (about 1016 → solar masses), some hundred million → light-years from Earth, in the direction of the → Centaurus → supercluster, that seems to be affecting the motions of many nearby galaxies by virtue of its gravity.
Hickson Compact Group (HCG)
goruh-e hampak-e Hickson
Fr.: groupe compact de Hickson
A list of 100 compact groups of galaxies that were identified by a systematic search of the → Palomar Observatory Sky Survey red prints. Each group contains four or more galaxies, has an estimated mean surface brightness brighter than 26.0 magnitude per arcsec2 and satisfies an isolation criterion.
šekast-e ofoqi (#)
Fr.: réfraction horizontale
The angular distance of an object below the horizon when it appears to lie on the horizon.
Imaging Atmospheric Cherenkov Technique (IACT)
tašnik-e tasvirgiri-ye Čerenkov-e javvi
Fr.: téchnique d'imagerie Čerenkov atmosphérique
The method used to detect very brief flashes of → Cherenkov radiation generated by the → cascade shower of → relativistic charged particles produced when a very high-energy → gamma ray (in the range 50 GeV to 50 TeV) strikes the atmosphere at a typical altitude of 10 km. Owing to this technique, it possible to discriminate cosmic gamma rays from the cosmic ray background and to determine their energy and source direction. More specifically, the incoming gamma-ray photon undergoes → pair production in the vicinity of the nucleus of an atmospheric molecule. The electron-positron pairs produced are of extremely high energy and immediately radiate in a → bremsstrahlung process. This radiation is itself extremely energetic, with many of the photons undergoing further pair production. A cascade of charged particles ensues which, due to its extreme energy, produces a flash of Cherenkov radiation lasting between 5 and 20 nano-seconds. The total area on the ground illuminated by this flash corresponds to many hundreds of square meters, which is why the effective area of IACT telescopes should be large.
Fr.: impact, collision
A collision between two bodies. In the case of solar system objects, when one is much smaller than the other (like a meteoroid colliding with the Earth), a crater may be produced on the larger body.
From L. impactus, p.p. of impingere "to drive into, strike against," from → in- "in" + pangere "to fix, fasten."
Barxord, verbal noun of barxordan "to collide, clash, dash against each other," from bar- "on, upon, up" (Mid.Pers. abar; O.Pers. upariy "above; over, upon, according to;" Av. upairi "above, over," upairi.zəma- "located above the earth;" cf. Gk. hyper- "over, above;" L. super-; O.H.G. ubir "over;" PIE base *uper "over") + xordan "to hit, strike," originally "to eat, drink, devour," and by extension "to destroy," from Mid.Pers. xvardan "to eat, enjoy (food)," Av. xvar- "to consume, eat;" Laki dialect hovârden "to eat;" Proto-Iranian *huar- "to consume, eat."