STRUCTURE OF DARK HALOS - MODEL-INDEPENDENT INFORMATION FROM HI ROTATION CURVES

Although the approximately flat rotation curves of gas in the outskirt s of spirals are generally taken as strong evidence for spherical, iso thermal dark matter halos, this conclusion is often incorrect and alwa ys model-dependent. A re-examination of the old and (nearly) model-ind ependent inversion technique for determining the surface mass density of galaxies from their kinematics is presented. The method is shown to be relatively insensitive to noise in the kinematics. Due to incomple te: kinematical knowledge at large radius, however, the surface mass d ensity is reliable only in the inner half of the galaxy, a result that also applies to traditional rotation curve fitting techniques.