12.2.3. Warps

The outer regions of the HI distribution of galaxies tend to exhibit strong departures from the regularity usually seen in the inner regions. It has been known for decades that the plane of the Milky Way displays an elegant hat-brim effect; the deviation of the centroid of the outer disk from the plane defined by the inner disk reaches 1.5 kpc at r = 18 kpc from the center. This effect was long attributed to a close tidal encounter with the Magellanic Clouds. However, perhaps the most spectacular example of a warp in another galaxy occurs in NGC 5907, displayed in Figure 12.4, which has no visible nearby neighbor that could serve as perturber. In addition, the galactocentric azimuth of the maximum height of the warp in our galaxy does not appear to depend on galactocentric distance, i.e., the warp does not precess under galactic rotation. This condition suggests the existence of self-maintaining mechanisms. A large fraction of the edge-on galaxies mapped in HI show evidence of warps. The onset of a warp usually coincides roughly with the Holmberg radius of the disk; the optical light also often falls steeply past this point. The warp of NGC 5907 shows only in the HI disk; in the case of NGC 4565, however, a slight warp is also observed in the stellar disk. While warps are easy to detect in maps of the HI distribution in edge-on galaxies, they can also be inferred from the characteristics of the velocity field in more face-on disks. Deviations from circular rotation, associated with the gas in warps, produce characteristic signatures in isovelocity contour maps. Warps thus identified are referred to as "kinematic" warps. Examples of kinematic warps are illustrated in some of the panels in Figure 12.7 and discussed in Section 12.3.2. A review of the theoretical problems associated with the interpretation of warps is given by Toomre (1983).

Figure 12.4. HI warp of NGC 5907, as observed by Sancisi (1976) with the Westerbork Synthesis Radio Telescope.