7.8.3 Seyfert Galaxies
Seyfert galaxies are one of the most active spiral galaxies. They are characterized by
the existence of very luminous nuclei with strong emission lines. Their active
behaviors were first remarked by C. Seyfert of the Mt. Wilson Observatory in 1943.
In Japan, Seyfert galaxies were observed widely in optical and infrared regions in
the 1970 - 1980s. Kaneko Noboru, Ohtani Hiroshi and their group (in 1968–1972)
made spectroscopic analysis of emission lines and infrared continuum from the
central region of NGC 1068. They found a star-like nucleus emitting a hot blackbody radiation. Kodaira Keiichi and Iye Masanori and others (1979) observed a
cluster of galaxies, VV 150, and Seyfert galaxy NGC 3227. The brightest member of
VV 150 (VV150 A) exhibited Seyfert-type spectrum, resembling to NGC 3227 in its
spectral feature.
Taniguchi Yoshiaki, Kameya Osamu and Nakai Naomasa (in 1990–1992) carried
out CO molecular-line observations of nuclear regions of starburst galaxies and
Seyfert galaxies using the Nobeyama 45-m telescope. They showed that the CO gas
made a ring structure both in starburst galaxies and Seyfert galaxies. Based on these
observations, Taniguchi proposed a starburst-implosion model, that is, molecular
gas rings are active SFRs where the high velocity gas is formed by successive
supernova-explosions inside these rings. The ejected gas partly falls into the galactic
center as the steady energy source of active galactic nuclei (Taniguchi 1992).
Seyfert galaxies were classified into two types, Seyfert 1 and 2, based on the
presence or absence of broad emission lines in the optical spectra. This classification,
however, soon becomes questionable. In 1985, J. R. Antonucci (National Radio
Astronomical Observatory) and J. S. Miller (Lick Observatory) carried out optical
polarimetric observations of an archetypical Seyfert-2 galaxy NGC 1068, and found
broad emission lines in the wing parts of Hα and SII lines. These wings are highly
polarized, as compared to the central narrow parts in low polarization which is a
typical feature of Seyfert 2 galaxy. This discovery gave a motivation to the proposal
of the unified model of Seyfert galaxies with a single parameter, that is, the viewing
angle of the active galactic nucleus hidden within a torus of narrow-line emission
region. In 1991, Awaki Hisamitsu, Koyama Katsuji and their group observed
16 Seyfert galaxies, including 9 Seyfert-2 galaxies, with GINGA satellite. They
showed that the X-ray properties can be explained by the unified model of accretiontorus as shown in Fig. 7.42. In this model strong X-ray radiation is beamed within a
small solid angle toward the rotation axis of the torus, and weak soft X-ray is emitted
from the dotted region in the Figure. The dark region represents the torus emitting
narrow optical lines. The variety of Seyfert galaxies is possibly explained by the
different view angle. The size and density parameters can take different values from
galaxy to galaxy (Awaki et al. 1991).
In 1992, Mouri Hideaki and his group conducted observations of infrared [Fe II]
1.644 μ m line emission in Seyfert galaxies, NGC 1068 and NGC 4151, and they
found the stratification of emission regions from inner ionized region to outer neutral
228
7 Postwar Development of Astrophysics, 1946–2000 (Part II: Astrophysics)
Seyfert galaxies are one of the most active spiral galaxies. They are characterized by
the existence of very luminous nuclei with strong emission lines. Their active
behaviors were first remarked by C. Seyfert of the Mt. Wilson Observatory in 1943.
In Japan, Seyfert galaxies were observed widely in optical and infrared regions in
the 1970 - 1980s. Kaneko Noboru, Ohtani Hiroshi and their group (in 1968–1972)
made spectroscopic analysis of emission lines and infrared continuum from the
central region of NGC 1068. They found a star-like nucleus emitting a hot blackbody radiation. Kodaira Keiichi and Iye Masanori and others (1979) observed a
cluster of galaxies, VV 150, and Seyfert galaxy NGC 3227. The brightest member of
VV 150 (VV150 A) exhibited Seyfert-type spectrum, resembling to NGC 3227 in its
spectral feature.
Taniguchi Yoshiaki, Kameya Osamu and Nakai Naomasa (in 1990–1992) carried
out CO molecular-line observations of nuclear regions of starburst galaxies and
Seyfert galaxies using the Nobeyama 45-m telescope. They showed that the CO gas
made a ring structure both in starburst galaxies and Seyfert galaxies. Based on these
observations, Taniguchi proposed a starburst-implosion model, that is, molecular
gas rings are active SFRs where the high velocity gas is formed by successive
supernova-explosions inside these rings. The ejected gas partly falls into the galactic
center as the steady energy source of active galactic nuclei (Taniguchi 1992).
Seyfert galaxies were classified into two types, Seyfert 1 and 2, based on the
presence or absence of broad emission lines in the optical spectra. This classification,
however, soon becomes questionable. In 1985, J. R. Antonucci (National Radio
Astronomical Observatory) and J. S. Miller (Lick Observatory) carried out optical
polarimetric observations of an archetypical Seyfert-2 galaxy NGC 1068, and found
broad emission lines in the wing parts of Hα and SII lines. These wings are highly
polarized, as compared to the central narrow parts in low polarization which is a
typical feature of Seyfert 2 galaxy. This discovery gave a motivation to the proposal
of the unified model of Seyfert galaxies with a single parameter, that is, the viewing
angle of the active galactic nucleus hidden within a torus of narrow-line emission
region. In 1991, Awaki Hisamitsu, Koyama Katsuji and their group observed
16 Seyfert galaxies, including 9 Seyfert-2 galaxies, with GINGA satellite. They
showed that the X-ray properties can be explained by the unified model of accretiontorus as shown in Fig. 7.42. In this model strong X-ray radiation is beamed within a
small solid angle toward the rotation axis of the torus, and weak soft X-ray is emitted
from the dotted region in the Figure. The dark region represents the torus emitting
narrow optical lines. The variety of Seyfert galaxies is possibly explained by the
different view angle. The size and density parameters can take different values from
galaxy to galaxy (Awaki et al. 1991).
In 1992, Mouri Hideaki and his group conducted observations of infrared [Fe II]
1.644 μ m line emission in Seyfert galaxies, NGC 1068 and NGC 4151, and they
found the stratification of emission regions from inner ionized region to outer neutral
228
7 Postwar Development of Astrophysics, 1946–2000 (Part II: Astrophysics)