Can galaxy warps be used to provide constraints on halo properties?

Abstract

The prevalence of flat rotation curves in external galaxies¹ strongly suggests an extensive but as yet unseen halo component. However, as direct observations of the implied matter distribution have proved very elusive, more indirect methods are needed to constrain its properties. At the same time, the explanation of observed warps in galaxies^3–7 has been intensively studied by dynamicists, resulting in the appearance of many rival mechanisms (see ref. 8 for a review). For galaxies with companions, the tidal interaction theory is the most attractive, but fails to account for the observed bending of our Galaxy “by a factor approaching 3” (ref. 9). The problem of the rapid kinematical disruption which results if one follows particle orbits in a self-consistent disk is avoided if one accepts the existence of a dominant spherical (halo) component in the region of the warps^8,10. Here, we explore the consequence of separating disk and halo mass distributions using a recently developed theory of bending waves². In this theory, the height of bending allows the functional form of the disk density distribution to be determined. With proper normalization this determination allows the halo density to be inferred from the rotation curve. We will discuss the warps in our Galaxy and in galaxies NGC2841 and M33, for which a relatively complete set of reduced data is available.