Twisting of X-ray isophotes in triaxial galaxies

Abstract

We investigate X-ray isophote twists created by triaxiality differences between the luminous stellar distributions and the dark halos in elliptical galaxies. For a typically oblate luminous galaxy embedded in a more prolate halo formed by dissipationless collapse, the triaxiality difference of ΔT = ~0.7 leads to typical isophote twists of <Δψ_X> = ~16 deg +/- 19 deg at 3 stellar effective radii. In a model which includes baryonic dissipation the effect is smaller, with ΔT = ~0.3 and <Δψ_X> = ~5 deg +/- 8 deg. Thus, accurate measurements of X-ray isophote twists may be able to set constraints on the interactions between baryons and dissipationless dark matter during galaxy formation. The 30-deg X-ray isophote twist in the E4 galaxy NGC 720 cannot be reproduced by our model, suggesting an intrinsic misalignment between the halo and the stars rather than a projection effect.