The flattened dark halo of polar ring galaxy NGC 4650A: A conspiracy of shapes?

Kinematics and photometry of the polar ring galaxy NGC 4650A, including new observations of the rotation and velocity dispersion of its central stellar disk, are used to infer the presence of a dark matter halo and to measure its shape. Fits to the observed disk and polar ring rotation curves from detailed mass and photometric modeling rule out a spherical dark halo. The best-fit models have halos with isodensity surfaces that are flattened to a shape between E6 and E7 (axis ratios between 0.4 and 0.3); the asymptotic equatorial speeds of these models are in excellent agreement with the I-band Tully-Fisher relation. This degree of dark halo flattening is larger than that expected from N-body collapse simulations of dissipationless dark matter. Since the kinematics and surface brightness profile of the central luminous body indicate that its light has an intrinsic axis ratio c/a ≲ 0.4, in NGC 4650A the radial "conspiracy" between the dark and luminous components that leads to flat rotation curves may extend to the shape of the mass distribution as well.