THE FLATTENED DARK HALO OF POLAR RING GALAXY NGC 4650A - A CONSPIRACYOF SHAPES

Kinematics and photometry of the polar ring galaxy NGC 4650A, includin g new observations of the rotation and velocity dispersion of its cent ral 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 ro tation curves from detailed mass and photometric modeling rule out a s pherical dark halo. The best-fit models have halos with isodensity sur faces that are flattened to a shape between E6 and E7 (axis ratios bet ween 0.4 and 0.3); the asymptotic equatorial speeds of these models ar e in excellent agreement with the I-band Tully-Fisher relation. This d egree of dark halo flattening is larger than that expected from N-body collapse simulations of dissipationless dark matter. Since the kinema tics and surface brightness profile of the central luminous body indic ate that its light has an intrinsic axis ratio c/a less than or equal to 0.4, in NGC 4650A the radial ''conspiracy'' between the dark and lu minous components that leads to flat rotation curves may extend to the shape of the mass distribution as well.