The remarkable warped and twisted gas disk in NGC 3718
  We have mapped NGC 3718, a nearby bright galaxy in a loose group, and its
companion NGC 3729 in the 21cm line of neutral hydrogen. NGC 3718 is a
strikingly unusual galaxy with a strong straight dust lane across the center,
peculiar diffuse spiral arms, and an extended disk of neutral hydrogen. Earlier
work showed the gas disk to be strongly twisted, warping through edge-on where
we see the straight dust lane; stars formed in this gas appear to make up the
'spiral arms'. Our improved maps show a twisted but bisymmetric disk of gas
extending to 7arcmin or 35kpc, where the orbital period is roughly 1Gyr. It is
surrounded by fragmentary spiral features, and a streamer of gas extending to a
cloud lying 12arcmin or 60kpc to the north. We use INSPECTOR, a task in GIPSY,
to fit a tilted-ring model interactively to slices through the HI data cube.
The apparent major axis swings through 100 degrees from the innermost gas
orbits at 30arcsec radius to the outer edge. When viewed in the reference frame
of the galaxy's stellar disk, the innermost gas orbits are nearly polar, while
the outer rings of gas are tilted at 30-40 degrees. The line of nodes, where
the gas orbits pass through the plane of the stellar disk, twists by roughly 90
degrees about the pole. We do not see gas orbiting in the plane of the stellar
disk. If we assume that the galaxy's dark halo shares the same midplane, then
the observed twist can be explained by differential precession in a dynamical
model in which the dark halo is fairly round. The run of tilt with radius is
close to what is required for the warped gas disk to precess rigidly in the
galaxy's gravitational field without changing its shape. This fact probably
accounts for the longevity of the twisted structure.
