Lupus dark cloud
abr-e târik-e Gorg
Fr.: nuage sombre du Loup
Any of the several → dark clouds lying in the direction of the constellation → Lupus between → Galactic longitudes 334° < l < 352° and → Galactic latitudes +5° < b < +25°. In terms of angular extent the whole group is one of the largest low-mass star forming complexes on the sky, and it also contains one of the richest associations of → T Tauri stars. An average distance of about 150 pc places it among the nearest star forming regions, together with those in Corona Australis, Ophiuchus, Taurus-Auriga, and Chamaeleon (Comeron, 2008, in Handbook of Star Forming Regions Vol. II, PASP, Reipurth, ed.).
Fr.: Boucle du Loup
An large nonthermal radio source in the constellation → Lupus, identified as a very old supernova remnant. It is also an extended source of soft X-rays.
Fr.: 21 Lutetia, 21 Lutèce
A large → main belt → asteroid that belongs to a sub-type of hydrated → M-type asteroids. It is an elongated body with its longest side around 130 km. The → Rosetta space probe flew by Lutetia and gathered data on it in 2008. Lutetia was discovered on November 15, 1852, by Hermann Goldschmidt (1802-1866) from the balcony of his apartment in Paris.
Named → Lutetia from L. Lutetia Parisiorum, literally "Parisian swamps," the Gallo-Roman city that was the ancestor of present-day Paris.
Fr.: biais de Lutz-Kelker
A systematic error that can be introduced when → trigonometric parallaxes are used to calibrate a luminosity system. The bias arises when stars are selected by a lower limit in the observed parallax values. This favors the stars for which the measured parallax result is relatively too large.
Named after Th. Lutz & D.H. E. Kelker, 1973, PASP 85, 573; → bias.
SI unit of illumination equal to a luminous flux of 1 lumen per square meter. SI unit of luminous incidence or illuminance, equal to 1 lumen per square meter.
From Gk. lux "light," → lumen.
Theodore Lyman (1874-1954), an American physicist who was a pioneer in studying
the spectroscopy of the → extreme ultraviolet region of
the electromagnetic radiation.
Named for Th. Lyman, as above.
Lyman alpha blob (LAB)
A gigantic cloud of → hydrogen hydrogen gas emitting the → Lyman alpha line identified in → high redshift, → narrow band → surveys. LABs can span hundreds of thousands of → light-years that is larger than galaxies. Normally, Lyman alpha emission is in the ultraviolet part of the spectrum, but Lyman alpha blobs are so distant, their light is redshifted to (longer) optical wavelengths. The most important questions in LAB studies remain unanswered: how are they formed and what maintains their power? One of the largest LABs known is SSA22-LAB-01 (z = 3.1). Embedded in the core of a huge → cluster of galaxies in the early stages of formation, it was the very first such object to be discovered (in 2000) and is located so far away that its light has taken about 11.5 billion years to reach us. Recent observations of SSA22-LAB-01 using → ALMA shows two galaxies at the core of this object and they are undergoing a burst of → star formation that is lighting up their surroundings. These large galaxies are in turn at the centre of a swarm of smaller ones in what appears to be an early phase in the formation of a massive cluster of galaxies (see J. E. Geach et al. 2016, arXiv:1608.02941).
Lyman alpha emitting galaxy (LAEs)
kahkešân-e gosilande-ye Lyman-alpha
Fr.: galaxie émettrice de Lyman alpha
A galaxy belonging to an important population of low mass → star-forming galaxies at → redshift z > 2. Their number increases with redshift. A large fraction of the → dwarf starburst galaxies during the → reionization epoch may be intrinsic LAEs, but their Lyα photons can be scattered by the → neutral hydrogen (H I) in the → intergalactic medium (IGM), which makes Lyα line a powerful probe of reionization. These high-z LAEs have low → metallicity, low stellar masses, low dust → extinction, and compact sizes. The current best nearby analogs of high-z LAEs are → Green Pea galaxies (Yang et al, 2017, arxiv/1706.02819 and references therein).
Lyman alpha forest
jangal-e Lyman-alpha (#)
Fr.: forêt Lyman alpha
Lyman alpha line
xatt-e Lyman-âlfâ (#), tân-e ~ ~
Fr.: raie Lyman alpha
Lyman alpha nebula
Fr.: nébuleuse Lyman alpha
A huge gaseous nebula (≥ 50 kpc) lying at high → redshifts (z ~ 2-6) and strongly emitting radiation due to the → Lyman alpha line (luminosities of ≥ 1043 erg s-1) of hydrogen gas. Also called Lyman alpha blobs, they are thought to lie in massive (M ~ 1013 solar masses) → dark matter halos, which would subsequently evolve into those typical of rich → galactic clusters.
Fr.: bande de Lyman
A sequence of → permitted transitions in the → ultraviolet from an → excited state (B) of the → molecular hydrogen (H2) to the electronic → ground state, with ΔE > 11.2 eV, λ < 1108Å (first → band head). When a hydrogen molecule absorbs such a photon, it undergoes a transition from the ground electronic state to the excited state (B). The following rapid → decay creates an → absorption band in that wavelength range. See also → Werner band. → Lyman-Werner photon.
Fr.: coupure de Lyman
The dividing point in a galaxy's spectrum at wavelengths shorter than the → Lyman limit. Galaxies contain large amounts of → neutral hydrogen which is very effective at absorbing radiation shortward of 912 Å. Hence galaxies are virtually dark at these wavelengths.
Lyman break galaxy
kahkešân bâ bore-ye Lyman
Fr.: galaxie de la coupure de Lyman
A star-forming galaxy at → high redshift affected by the → Lyman break. Such a galaxy is detected in the red (R, → photometric band) but not in the blue (U and B bands). At those high redshfits (above 2.5), the → Lyman limit at 912 Å is shifted between the U and B bands.
peyvastâr-e Lyman (#)
Fr.: continuum de Lyman
A continuous range of wavelengths in the spectrum of hydrogen at wavelengths less than the → Lyman limit. The Lyman continuum results from transitions between the → ground state of hydrogen and → excited states in which the single electron is freed from the atom by photons having an energy of 13.6 eV or higher.
Lyman continuum escape
goriz-e peyvastâr-e Lyman
Fr.: échappement du continuum de Lyman
The process whereby → Lyman continuum photons produced by → massive stars escape from a galaxy without being absorbed by interstellar material. Some observations indicate that the Lyman continuum escape fraction evolves with → redshift.
parhib-e Lyman (#)
Fr.: image fantôme de Lyman
In spectroscopy, a false image of a spectral line formed by irregularities in the ruling of diffraction gratings.
Fr.: limite de Lyman
The short-wavelength end of the hydrogen Lyman series, at 912 Å. Also called → Lyman continuum. It corresponds to the energy (13.6 eV) required for an electron in the hydrogen ground state to jump completely out of the atom, leaving the atom ionized.
seri-ye Lyman (#)
Fr.: séries de Lyman
A series of lines in the spectrum of hydrogen, emitted when electrons jump from outer orbits to the first orbit. The Lyman series lies entirely within the ultraviolet region. The brightest lines are Lyman-alpha at 1216 Å, Lyman-beta at 1026 Å, and Lyman-gamma at 972 Å.
Fr.: photon de Lyman-Werner
An → ultraviolet photon with an energy between 11.2 and 13.6 eV, corresponding to the energy range in which the Lyman and Werner absorption bands of → molecular hydrogen (H2) are found (→ Lyman band, → Werner band). The first generation of stars produces a background of Lyman-Werner radiation which can → photodissociate molecular hydrogen, the key → cooling agent in metal free gas below 104 K. In doing so, the Lyman-Werner radiation field delays the collapse of gaseous clouds, and thus star formation. After more massive → dark matter clouds are assembled, atomic line cooling becomes effective and H2 can begin to shield itself from Lyman-Werner radiation.