Close Encounters between Galaxies of Different Masses: some Dynamic Effects in a Simple Scheme

Extract

When a small galaxy of mass M₂ passes in the vicinity of a triaxial or oblate primary galaxy of mass M₁M₂, some gas could be stripped from the secondary falling down in the gravitational field of the companion and/or lifted from the less bound regions of the last one. These gaseous particles will point out the presence of a torque induced by the disturbing object on the primary. Assuming a homogeneous ellipsoid for the main body, analytical evaluations are made as function of the geometry on the possible co/counter-rotation or polar rings for the gas involved. An application is considered to the well studied galaxy A0136-0801 (Schweizer et al 1983), testing the possibility to form the observed polar ring. We assume the primary is a normal S0 disk and the ring's gas is coming from the disk plane. Due to the axial symmetry, for two symmetric elements of mass ΔM/2 lying on the x-axis at a distance l/2 from the center of mass, the quadrupole torque due to M₂ passing at distance r with colatitude θ is: where are the frame vectors and G the gravitational constant. Comparing it with the restoring torque due to a homogeneous spheroid of major semi-axis a, miming the main body of the galaxy, we obtain for the ratio: where ∊ is the semi-axis ratio, z the height on the equatorial plane and k' the positive solution of the equation for the cofocal spheroid touching the ΔM/2 particle positions. We obtain that the static configuration considered is consistent with the possibility to send mass to the pole assuming: M₁ = 4.5 10¹⁰ M_O, M₁/M₂ = 2.25, l/2 = 10kpc, a=4kpc, θ=45° and r=15kpc, if (i) Γ_qlasts for about a revolution period of the ΔM/2 particle and (ii) the mass of the main body is enclosed in a not too much small or large scale length, this last being one of the more strict conditions. The weight of the higher harmonic of multipole has been considered.