7.8.2.2 Edge-on Galaxies
Edge-on galaxies are the spiral galaxies seen from the plane of spiral arms. These
galaxies provide the information on the vertical structure of spiral galaxies. Observations had been made mainly by Sofue Yoshiaki, Handa Toshihiro and their group
in the early 1990s with the 45-m telescope using the CO lines.
In the edge-on galaxy NGC 891, Sofue’s group found a prominent spur of
molecular gas emerging vertically from the disk, at 5.5 kpc from the center of the
galaxy. The height of the spur from the galactic disk was 520 pc, and the velocity
width was less than 20 km s
À1 . On the origin of spur ejection, they supposed the
possibilities of a supper-bubble in the disk, or of some instability in the gas-disk,
though not conclusive (Handa et al. 1992). On the structure of gas disk, they found
the presence of a molecular gas ring at 3.5 kpc and a rigidly rotating inner-disk or a
bar of radius of 3 kpc. From these properties they concluded that NGC 891 is a SBb
galaxy in the Hubble type. In addition, they found that the thickness of the CO disk is
larger than that of our Galaxy by factor 2. They suggested that the geometrically
thick CO disk of NGC 891 might be due to active star formation in the disk (Sofue
and Nakai 1993; Handa et al. 1994).
Other edge-on galaxies, NGC 3079, NGC4565, NGC 4631 and NGC 5907, were
similarly observed by the same group in the CO molecular lines in the early 1990s.
Fig. 7.41 Photograph of
Spiral-arm structure of M31.
A two-armed spiral pattern
is recognized in the outer
part (Sawa et al. 1981)
7.8 Galaxies
227
Edge-on galaxies are the spiral galaxies seen from the plane of spiral arms. These
galaxies provide the information on the vertical structure of spiral galaxies. Observations had been made mainly by Sofue Yoshiaki, Handa Toshihiro and their group
in the early 1990s with the 45-m telescope using the CO lines.
In the edge-on galaxy NGC 891, Sofue’s group found a prominent spur of
molecular gas emerging vertically from the disk, at 5.5 kpc from the center of the
galaxy. The height of the spur from the galactic disk was 520 pc, and the velocity
width was less than 20 km s
À1 . On the origin of spur ejection, they supposed the
possibilities of a supper-bubble in the disk, or of some instability in the gas-disk,
though not conclusive (Handa et al. 1992). On the structure of gas disk, they found
the presence of a molecular gas ring at 3.5 kpc and a rigidly rotating inner-disk or a
bar of radius of 3 kpc. From these properties they concluded that NGC 891 is a SBb
galaxy in the Hubble type. In addition, they found that the thickness of the CO disk is
larger than that of our Galaxy by factor 2. They suggested that the geometrically
thick CO disk of NGC 891 might be due to active star formation in the disk (Sofue
and Nakai 1993; Handa et al. 1994).
Other edge-on galaxies, NGC 3079, NGC4565, NGC 4631 and NGC 5907, were
similarly observed by the same group in the CO molecular lines in the early 1990s.
Fig. 7.41 Photograph of
Spiral-arm structure of M31.
A two-armed spiral pattern
is recognized in the outer
part (Sawa et al. 1981)
7.8 Galaxies
227