Response of gravitationally coupled gaseous and stellar components to asymmetric warp in disc galaxies

Abstract

ABSTRACT The outer disc region of most spiral galaxies (approximately 50 per cent of all disc galaxies) shows warping above the galactic mid-plane and is primarily asymmetric by nature. In this work, we explore analytically the effect of the gas component on asymmetric warps in a realistic self-gravitating collision-less disc residing in a cold oblate dark matter halo’s potential field. We consider the disc to be composed of gravitationally coupled stars and gas components. The quadratic eigenvalue equation describing the shape and frequency of the bending mode is formulated and solved numerically. Two stable ground-state bending modes m = 0 and m = 1, representing the U-shape and the mostly observed S-shaped warp in the galactic disc are superimposed linearly to generate and examine the asymmetric warps in the disc. The resulting asymmetry in warp is measured by asymmetric index (Aasym) by varying physical parameters such as the mass of the gas components and the halo flattening parameter. It is shown that the gas fraction in the disc has a negligible contribution to the generation of asymmetric warp in the disc. The disc residing in a spherical dark matter halo is found to be more asymmetry than that in the counterpart oblate halo.