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.

💡 Research Summary

The authors present a comprehensive 21‑cm neutral‑hydrogen (HI) study of the nearby, bright galaxy NGC 3718 and its companion NGC 3729. NGC 3718 is a striking system: a straight dust lane bisects its centre, its optical spiral arms appear diffuse and irregular, and an extended HI disk surrounds the stellar body. Earlier work had identified a strongly warped gas disk that appears edge‑on where the dust lane is seen, but the new observations dramatically improve spatial coverage and sensitivity, revealing a bisymmetric HI structure that stretches to a projected radius of 7 arcmin (≈35 kpc) and a faint streamer that reaches a cloud 12 arcmin (≈60 kpc) north of the galaxy.

The data were obtained with the VLA and WSRT, providing a resolution of ~15″ and a velocity resolution of a few km s⁻¹. The resulting data cube was examined with channel maps, moment maps, and position‑velocity slices, confirming that the gas disk is highly twisted and that no HI is found in the plane of the stellar disk. To quantify the geometry, the authors employed the INSPECTOR task in the GIPSY software package, fitting a tilted‑ring model interactively to slices through the cube. Each ring is described by its radius, inclination (i), position angle of the line of nodes (Ω), and rotation speed (V_rot).

The best‑fit model shows a dramatic radial variation in both inclination and line‑of‑nodes angle. At a radius of 30″ (≈1.5 kpc) the gas orbits are nearly polar (i≈80°) and the line of nodes points roughly along the galaxy’s minor axis. Moving outward, the inclination steadily declines to 30–40° at the outermost measured radius, while the line of nodes swings through about 100° in azimuth, producing a twist of roughly 90° about the pole when the geometry is expressed in the reference frame of the stellar disk. The rotation curve is fairly flat, with an orbital period of ~1 Gyr at the outer edge, indicating that the warped structure has survived for many dynamical times.

To interpret this geometry, the authors consider the gravitational potential of the galaxy, dominated by a dark‑matter halo. Assuming the halo shares the same mid‑plane as the stellar disk, they calculate the differential precession rates of inclined rings in a flattened potential. A nearly spherical halo (axis ratio q≈0.9–1.0) yields precession rates that vary slowly with radius, allowing the observed tilt and node profiles to approximate the condition for “rigid precession” – a configuration in which the entire warped disk precesses as a solid body without changing its shape. In such a scenario the warp can be long‑lived, persisting for several gigayears, which explains the remarkable coherence of the observed structure.

The presence of the northern streamer and the distant HI cloud suggests ongoing or recent accretion of external gas, possibly the source of the warp’s initial misalignment. However, the main warped disk appears dynamically decoupled from the stellar component, implying that the gas settled into its present configuration after being torqued by the halo’s potential rather than by internal stellar torques.

In summary, the paper provides a detailed observational characterization of NGC 3718’s HI disk, demonstrates that its extreme warp can be understood as a consequence of differential precession in a fairly round dark halo, and argues that the near‑rigid precession condition accounts for the warp’s longevity. These results have broader implications for the formation and persistence of warped gas disks in galaxies, offering a concrete example of how halo shape and external gas accretion can shape galactic structure over cosmological timescales.