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Seyfert galaxies

spectrum, while other compact emission regions close to the nucleus show the spectrum of HII regions. Studies of Sérsic–Pastoriza galaxies have been aimed at clarifying a possible link between intense, localized, star formation, as in the hot spots, and the presence of non-stellar, nuclear activity.

seston The total animate and inanimate particulate matter in natural waters.

setdown The radiation stress associated with breaking water waves results in a setup, or superelevation of the mean water level, in the vicinity of the mean water line (where the water meets the shore), and a setdown, or drop in mean water level, in the vicinity of the wave breakpoint.

settling speed (velocity) The speed at which a particular type of particle will fall through a column of still fluid (generally water).

settling tube An experimental apparatus for measurement of settling velocity.

setup The radiation stress associated with breaking water waves results in a setup, or superelevation of the mean water level, in the vicinity of the mean water line (where the water meets the shore). See also setdown.

Seyfert-1 galaxies Seyfert galaxies showing two systems of emission lines, i.e., broad Balmer lines (and, in general, broad recombination lines, which may include strong emission from singly ionized iron), and narrower forbidden lines in the optical and UV spectrum of their nuclei. The width of the Balmer lines can reach 30,000 km s−1 at the line base, and it is typically several thousand km s−1 at half maximum. The width of the forbidden lines is usually restricted to several hundred km s−1 at half maximum. The presence of broad lines is a defining feature of “type-1” active galactic nuclei, a more general class that includes quasars and QSOs since they show emission line properties very similar to that of Seyfert-1 nuclei. Broad and narrow lines are emitted in different regions, called the “Broad Line Region” and the “Narrow Line Region”, respectively. The 8th Edition of A Cat-

alogue of Quasars and Active Nuclei by M.-P. Véron–Cétty and P. Véron lists more than 1100 Seyfert-1 galaxies known as of early 1998.

Seyfert-2 galaxies Seyfert galaxies showing permitted and forbidden emission lines with similar width, typically several hundred km s−1 in the optical and UV spectrum of their nuclei. In other words, Seyfert-2 galaxies do not show broad permitted lines as observed in Seyfert-1 galaxies. Seyfert-2 galaxies are of lower luminosity than Seyfert-1, and they are believed to be roughly two to three times more frequent than Seyfert-1 in a volume-limited sample. The discovery of broad Balmer lines in polarized light in the prototype Seyfert-2 galaxy NGC 1068, and in other nearby Seyfert-2 nuclei, has led to a model in which Seyfert-2 galaxies are actually Seyfert-1, whose Broad Line Region is obscured from view by a thick torus of molecular gas. Free electrons slightly above the torus should scatter the broad line photons into the line of sight, letting them be detected only in polarized light. The applicability of this model to most Seyfert-2 galaxies is as yet a subject of debate. Several works have pointed out that Seyfert-2 galaxies are a heterogeneous class, and that several Seyfert-2 galaxies could be genuinely different from Seyfert-1 galaxies, with features in their spectral energy distributions that could be related to intense star formation. The 8th Edition of A Catalogue of Quasars and Active Nuclei by M.-P. Véron–Cétty and P. Véron lists about 560 Seyfert-2 galaxies known as of early 1998.

Seyfert galaxies Galaxies showing a bright, star-like nucleus, whose optical and UV spectrum shows prominent emission lines. Seyfert galaxies are typically identified by their optical spectrum, which shows emission lines of the Balmer series, along with strong forbidden lines such as the nebular lines of the oxygen twice ionized, and of singly ionized nitrogen. Singled out in 1943 by C. Seyfert as an independent class of galaxies, the importance of Seyfert galaxies was largely unappreciated until the discovery of quasars. Quasars and Seyfert galaxies are now considered part of the broader class of active galactic nuclei. In this view, Seyfert galaxies are radio-quiet, low luminosity active

© 2001 by CRC Press LLC

sferic

galactic nuclei, distinct from quasars since the host galaxy is visible and since their luminosity is lower. It is customary, albeit arbitrary, as there is no discontinuity between nearby active galactic nuclei and quasars, to distinguish between Seyferts and quasars, defining Seyfert nuclei as having luminosity lower than 1011 solar luminosity, the luminosity of a large galaxy.

sferic The abbreviated term for an occurrence of atmospheric radio noise. See atmospheric noise.

shadow bands Bright and dark bands that are observed running along the ground just before the onset of totality in a solar eclipse. They are thought to arise from the refraction of the light from a thin crescent of the sun behind the moon, varying due to turbulence only tens or hundreds of meters above the ground. They can have wavelengths of centimeters to close to 1 m, and move at rates of meters per second (determined by the rate of motion of the atmospheric inhomogeneities). Their contrast is typically low, about 1%.

shadow zone Seismic rays passing the core of the Earth are refracted due to the rapid decrease in seismic wave speed in the outer core, such that direct P waves cannot be detected in a zone of about 105◦ to 140◦ epicentral distance. This zone is called the shadow zone.

shallow water A relative measure of water depth used in the study of water waves. Shallow water is (arbitrarily) defined as a ratio of water depth to wavelength, h/L, of 1/25 (sometimes 1/10).

shallow water wave A wave in water more shallow than 1/25 of a wavelength; in this case, the speed of propogation is c = √gh, where g is the acceleration of gravity and h is the water depth.

shatter cone In geology, a distinctively striated conical rock structure ranging from a few centimeters to a few meters in length, along which fracturing has occurred, believed to have been formed by the passage of a shock wave following meteorite impact. The mechanism is

postulated to be a focusing of the shock wave behind obstacles, concentrating its effect in conical regions.

shear The mathematical quantity (a symmetric rank-2, 3-dimensional tensor) that determines the rate of change of shape of a given portion of a continuous medium surrounding a given particle after factoring out the overall change in volume (see also acceleration, expansion, kinematical invariants, rotation, tidal forces). Consider a fluid in which acceleration, expansion, and rotation all vanish while shear is nonzero. An observer could examine a parallelepiped whose one vertex is at his position and the other vertices are at neighboring particles of the medium. With time, this parallelepiped would change into another one whose volume is the same, but its shape is different (sheared) from the initial one. In real astronomical bodies (and in all fluid mediums in everyday life) shearing motion is typical.

shear modulus Also called rigidity. The proportionality constant G between shear stress τij and shear strain εij for a linearly elastic medium: τij = 2Gεij . G is expressed in terms of Young’s modulus E and Poisson’s ratio v as

G =

E

 

.

 

 

 

2(1

+

v)

 

 

 

 

shear probe See airfoil probe.

shear stress A stress (force per unit area) exerted along a surface, as opposed to a stress normal to a surface. In viscous fluids the shear stress τ is τ = µ(du/dz), where du/dz is a velocity gradient, u is the velocity in the x- direction, and µ is the viscosity of the fluid.

shear velocity A measure of shear stress used in the study of sediment transport. Defined as u = √τ/ρ, where τ is the shear stress on the bed, and ρ is density.

shear wave A seismic body wave in which the displacement is perpendicular to the direction of propagation. Shear waves can propagate through solids but not through liquids or gases.

© 2001 by CRC Press LLC

shock normal

shear wave splitting When shear waves propagate in an elastic medium that is anisotropic in the normal plane of the common ray path, waves with different particle vibration directions have different speeds. As a result, waves propagating in the same direction but vibrating in different directions are separated and arrive at the same receiver at different times. This phenomenon is called shear wave splitting.

sheet pile A sheet, generally made of steel, that is driven into the ground to hold soil back.

shelf wave See continental shelf waves.

shepherding moon See shepherd satellite.

shepherd satellite A satellite that constrains the extent of a planetary ring in the radial direction through gravitational forces. Saturn’s F-ring and the rings of Uranus and Neptune are kept very narrow by shepherd moons which orbit just inside and outside the rings and by their gravitation influence on the small particles of the rings. Their presence produces “wakes” and waves in the rings which tend to keep the ring particles together.

shergottite A class of achondritic stony meteorites that are geologically young, and have solidified from molten rock in an extraterrestrial setting, whose prototype was (seen to fall) found in Shergotty, India in 1865. Shergottites have distinct textures and mineralogies indicative of igneous processes. These meteorites have a higher volatile content than other igneous meteorites, and their composition shows evidence of a basaltic origin. In addition, shergottite may contain maskelynite, a plagioclase glass formed by preterrestrial impact. This shock-melted glass has gas abundances and isotope ratios very similar to gases in Mars’ atmosphere. Because of this, it is now widely accepted that shergottites and other SNC meteorites were formed on Mars, thrown off, and later captured by the Earth.

shields diagram A graph for assessment of critical motion of sediment particles subjected to flow of water. A dimensionless plot of nondi-

mensional shear stress vs. boundary Reynolds number.

shield volcano A large, broad, low-sloped volcanic cone built up by successive flows of low-viscosity (i.e., fluid) lavas. The flanks of shield volcanos often have slopes of no more than 2◦ to 3◦. Lava is erupted from both the central caldera as well as from fissures along the flanks, but the eruptions are generally nonexplosive in nature. Shield volcanos are common features on many planets — examples include the Hawaiian volcanos on Earth, Olympus Mons on Mars, and Sif Mons on Venus.

shingle Coarse, weathered gravel on a beach.

shoal A bump in the floor of a sea, lake, or river that results in a local reduction in water depth. Also used as a verb in the discussion of water wave transformation.

shoaling The process of a water wave changing height due to changes in water depth.

shoaling coefficient A non-dimensional coefficient used to calculate the change in height of a water wave due to a change in water depth. Defined as Ks = Cg1 /Cg2 , where Ks is the shoaling coefficient, and Cg1 and Cg2 are the wave group velocities at the two points of interest.

shock normal Direction perpendicular to the shock front. If the shock normal is known, in magnetohydrodynamics the angle θBn between the magnetic field direction and the shock normal can be calculated. In magnetohydrodynamics, this angle is crucial for shock formation and particle acceleration and can be used to classify shocks in quasi-parallel and quasi-perpendicular and to determine the shock speed (see quasiparallel shock, quasi-perpendicular shock). The direction n of the shock normal can be calculated from the magnetic field measurements using the coplanarity theorem (see coplanarity theorem).

n = (Bu × Bd) × (Bu − Bd) .

|(Bu × Bd) × (Bu − Bd)|

Here Bu and Bd are the magnetic field vectors in the upstream and downstream medium.

© 2001 by CRC Press LLC

shock spike

The shock speed then can be calculated from the mass conservation across the shock (see Rankine–Hugoniot relations) as

v

s =

9dud − 9uuu

·

n

9d − 9u

 

 

with 9 being the plasma density, u being the plasma speed, and the indices u and d indicating the upstream and downstream medium, respectively.

shock spike In magnetospheric physics, a short intensity increase, lasting for some 10 min, around the time of shock passage. Shock spikes are observed at quasi-perpendicular shocks only; they are a typical feature of shock drift acceleration.

shock wave A shock wave is associated with a sudden change in properties of a continuous medium, in particular a sudden increase in gasdynamic pressure. A shock forms when a propagation speed exceeds the typical signal speed in the medium: a body might move through a medium faster than the signal speed (e.g., a supersonic jet, traveling shock) or a supersonic flow might be slowed down at an obstacle (e.g., the solar wind at a planetary magnetosphere, standing shock). Another shock wave is the blast wave following an explosion. General characteristics of shock waves are: (1) the disturbance propagates faster than the signal speed, (2) at the shock front, the properties of the medium change abruptly, and (3) behind the shock, a transition back to the state of the undisturbed medium must occur. Here a reverse shock might form. The jump of properties at the shock front is described by the Rankine– Hugoniot relations. Shocks transport information; however, since their speed exceeds the signal speed no information can propagate ahead of the shock. The passage of a shock changes the properties of the medium irreversibly.

Shoemaker–Levy A comet that impacted on the planet Jupiter after breaking into at least 21 major pieces. The impacts lasted from July 16 through July 22, 1994. The impacts created fireballs and raised plumes of up to 3000 km above the Jovian cloudtops. Remnants left on

Jupiter for days included circular and arcate dark smudges.

shoreline A loosely used term indicating the line separating water from land. This line actually moves back and forth as wave and tide conditions change. More precise definitions involve description of the mean water line, still water line, mean high tide line, etc.

short wave fadeout (SWF) A solar flare is accompanied by an increase in X-ray emissions. The increased X-ray flux ionizes the dayside D region of the ionosphere leading to an increase in absorption of radio waves passing through it. A fadeout occurs when a radio frequency becomes too weak to use as a result of this absorption. The event is called a short-wave fade because all short-wave frequencies passing through the daytime ionosphere will be affected. In the past, when communications depended more heavily on HF radio, this meant that all short-wave HF radio transmissions could simultaneously cease to be heard. Most M-class and all X-class flares will cause short-wave fades. The increased ionization leads to a number of other effects. See magnetic crochet, solar flare, sudden frequency deviation (SFD).

side-cast dredge A dredge that discharges dredged material off to one side of the dredge centerline. Often used to dredge navigation channels.

sidereal day The interval of time required for a given right ascension to return to a particular observer’s meridian; 23h56m4s.

sidereal month See month.

sidereal time Time measured in terms of the rotation of the Earth with respect to the stars. Because the Earth’s sense of rotation is in the same direction as its revolution around the sun, it must rotate through more than 360◦ from noon to noon. Hence, there is one more revolution with respect to the stars, than with respect to the sun, in a given year. Sidereal time equals the right ascension of objects on the observer’s meridian. Equivalently it is the hour angle of the autumnal equinox. Sidereal time includes the

© 2001 by CRC Press LLC

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