Kinematic modelling of disk galaxies

Abstract

This is the second of a series of papers in which the kinematics of disk galaxies is studied by analysing spectroscopic data. Making use of the methods presented in Paper I, this work focuses on the large-scale distribution of disk and Dark Matter halo material by analysing the structure of warped galaxies. observations of three disk galaxies showing regular large-scale (“grand-design”) warps are presented and their structure is analysed. The kinematics and geometry of the galaxies are modelled by fitting a tilted-ring model directly to the data cube, thereby avoiding problems due to orbit crowding or beam smearing. The galaxies follow the previously known “rules for the behaviour of warps”. Furthermore, at large radii the disk settles into a relatively coplanar disk that is inclined with respect to the inner one. With increasing radius, the measured rotation velocity shows a rise. In one of the cases, NGC 5204, the rotation curve shows a jump at the transition radius. While the best fits in the other two cases also suggest that such a jump is present, the change in rotation velocity is compatible with a gradual rise. In two cases, the surface density of the warped layer shows a clear drop at the transition radius and decreases only very slowly further out, while this feature is not as clear for UGC 3580. A literature search shows that these properties are shared by a number of other galaxies. In all cases, an additional, slowly rotating gas layer is found to be present. The change in the rotation velocity and the remarkable geometry of the disks together indicate that the radius at which the warp starts marks the transition between two dynamical regimes, an inner one governed by the stellar disk and an outer one determined by the Dark Matter halo.