Near-infrared constraints on the driving mechanisms for spiral structure

Abstract

We have imaged a sample of 17 inclined spiral galaxies with measured Hα rotation curves in the K band, in order to determine the morphology of the old stellar population that dominates the mass in the disc. The K-band images of the galaxies have been used to determine the radial extent of grand-design spiral structure and compare this with the turnover radius in their rotation curves, where the rotation curve transforms from solid-body rotation to differential rotation. If the arms do not extend past this radius, the winding problem is solved. We find in all 17 cases that the arms extend past this radius, with the radius of grand-design spiral structure being a factor of 1.3–9.6 times larger than the rotation curve turnover radius. Of these galaxies, four have near neighbours and central bars and a further seven have a central bar. These bars or near neighbours may be the cause of the driving of the spiral potential in the discs of these galaxies. Of the remaining six galaxies, five show some evidence for a bar or oval distortion in their K-band images. The remaining galaxy (UGC 14) shows no evidence for a central bar and has no near neighbours. Finally, we also find that the spiral structure of these galaxies in the near-infrared is extremely regular, although some range in the regularity of spiral structure is found. To quantify this range in spiral structure, we introduce the dust-penetrated arm class, which is analogous to the dust-penetrated classification scheme of Block & Puerari and is based on the optical arm class of Elmegreen & Elmegreen.