Disk heating and bending instability in galaxies with counterrotation

Abstract

With the help of high-resolution long-slit and integral-field spectroscopy observations, the number of confirmed cases of galaxies with counterrotation is increasing rapidly. The evolution of such counterrotating galaxies remains far from being well understood. In this paper we study the dynamics of counterrotating collisionless stellar disks by means of $N$-body simulations. We show that, in the presence of counterrotation, an otherwise gravitationally stable disk can naturally generate bending waves accompanied by strong disk heating across the disk plane, that is in the vertical direction. Such conclusion is found to hold even for dynamically warm systems with typical values of the initial vertical-to-radial velocity dispersion ratio ${\sigma }_{\mathrm{z}}/{\sigma }_{\mathrm{R}}\approx 0.5$, for which the role of pressure anisotropy should be unimportant. We note that, during evolution, the ${\sigma }_{\mathrm{z}}/{\sigma }_{\mathrm{R}}$ ratio tends to rise up to values close to unity in the case of locally Jeans-stable disks, whereas in disks that are initially Jeans-unstable it may reach even higher values, especially in the innermost regions. This unusual behavior of the ${\sigma }_{\mathrm{z}}/{\sigma }_{\mathrm{R}}$ ratio in galaxies with counterrotation appears not to have been noticed earlier. Our investigations of systems made of two counterrotating components with different mass-ratios suggest that even apparently normal disk galaxies (i.e., with a minor counterrotating component so as to escape detection in current observations) might be subject to significant disk heating especially in the vertical direction.

13 pages, 10 figures (accepted for publication in Astronomy & Astrophysics)

References in corpus (7)

• The Structures of Distant Galaxies I: Galaxy Structures and the Merger Rate to z~3 in the Hubble Ultra-Deep Field
• Age velocity dispersion relations and heating histories in disc galaxies
• The difference in age of the two counter-rotating stellar disks of the spiral galaxy NGC 4138
• Numerical code for multi-component galaxies: from N-body to chemistry and magnetic fields
• Response of the integrals in the Tremaine-Weinberg method to multiple pattern speeds: a counter-rotating inner bar in NGC 2950?
• Kelvin-Helmholtz Instability of Counter-Rotating Discs
• Local stability of self-gravitating fluid disks made of two components in relative motion