Grand-Design Warps in Galactic Disks

Abstract

Warping, a bending of the disk, is present in all disk galaxies. It is hence a phenomenon that should not be missed to be explained in theories of galactic evolution and formation. Warps usually occur at radii where the bright stellar disk fades away. Hence, their structure is studied best by observing the neutral gas, which is the most extended detectable disk component. In most galaxies an antisymmetric "integral-sign" or "grand-design" geometry dominates the warp structure, which makes this type the most interesting target for warp studies. <br /> This work presents a detailed study of the kinematical and morphological structure of the gaseous disks of the four galaxies NGC 2541, UGC 3580, NGC 5204, and NGC 2685. The galaxies are of different type, but each contains a highly symmetric grand-design warp. <br /> Employing deep HI synthesis observations and a computer code described here, reliable tilted-ring models of the galaxies were derived. With the aid of these parametrisations it could be shown that the warps of the examined galaxies share the following features:<br /> The HI disks, while bending away from an inner preferred plane settle into a different stable orientation at large radii, the warp being the transition from one to the other preferred plane. The change of the disk's orientation comes along with a change of the rotation velocity and a change of the HI surface brightness.<br /> Once identified, these features can be found to be present in a large number of disk galaxies studied before, while having been rarely reported explicitely. Models for warp formation in which a differential precession in the outer parts of galactic disks is inferred violate these findings. The warp structure of the examined galaxies suggests that grand-design warps are long-lived common bending modes of the disk-Dark Matter halo system, in which the Dark Matter halo itself is intrinsically bent.<br /> An exceptional case, not following some of the "rules for the behaviour of warps", is the well-known "Spindle Galaxy" NGC 2685, formerly known as a prototype two-ringed polar-ring galaxy. With the new analysis method it can be reliably shown that NGC 2685 has a coherent H<font size="-1">I</font> disk that is symmetric and extremely warped. The gaseous disk has a counterpart containing dust and stars, making NGC 2685 a disk galaxy. The warp either commences very close to the centre or the inner disk is entirely warped. Besides a warp radius close to zero, the difference of its warp with respect to normal warps in high surface-brightness (HSB) galaxies is that the drop of the H<font size="-1">I</font> surface-density does not coincide with the commencement of the warp. As the stellar disk of NGC 2685 has a rather low surface-brightness in comparison to HSB galaxies, its disk can be assumed to be lightweight, a possible reason for the small warp radius: the internal gravitational coupling of the disk is weak even in the presence of a high surface-density HI component, and bending thus starts at smaller radii.