On stability and spiral patterns in polar disks

¹ Institute of Theoretical Physics and Astrophysics, University of Kiel, 24098 Kiel, Germany
² Institute of Astronomy, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria e-mail: theis@astro.univie.ac.at
³ Max-Planck-Institute for Astrophysics, Postfach 1317, 85741 Garching, Germany
⁴ University of Wisconsin, Department of Astronomy, 475 N. Charter St., Madison, WI 53706, USA e-mail: [sparke;jsg]@astro.wisc.edu

Received: 17 May 2005
Accepted: 3 October 2005

Abstract

To investigate the stability properties of polar disks we performed two-dimensional hydrodynamical simulations for flat polytropic gaseous self-gravitating disks which were perturbed by a central S0-like component. Our disk was constructed to resemble that of the proto-typical galaxy NGC 4650A. This central perturbation induces initially a stationary two-armed tightly-wound leading spiral in the polar disk. For a hot disk (Toomre parameter ), the structure does not change over the simulation time of 4.5 Gyr. In case of colder disks, the self-gravity of the spiral becomes dominant, it decouples from the central perturbation and grows, until reaching a saturation stage in which an open trailing spiral is formed, rather similar to that observed in NGC 4650A. The timescale for developing non-linear structures is 1–2 Gyr; saturation is reached within 2–3 Gyr. The main parameter controlling the structure formation is the Toomre parameter. The results are surprisingly insensitive to the properties of the central component. If the polar disk is much less massive than that in NGC 4650A, it forms a weaker tightly-wound spiral, similar to that seen in dust absorption in the dust disk of NGC 2787. Our results are derived for a polytropic equation of state, but appear to be generic as the adiabatic exponent is varied between (isothermal) and (very stiff).