DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

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-
© 2001 by CRC Press LLC
Seyfert galaxies
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
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