Another Flattened Dark Halo: Polar Ring Galaxy A01360801

Knowledge of the shape of dark matter halos is critical to our understanding of galaxy formation, dynamics, and of the nature of dark matter itself. Polar ring galaxies (PRGs) | early-type galaxies deened by their outer rings of gas, dust and stars on orbits nearly perpendicular to those of the central host | provide a rare probe of the vertical-to-radial axis ratio (q = c=a) of dark halos. We present a Fabry-Perot velocity eld for the H gas in the kinematically-connrmed PRG A01360801. By comparing ring orbits evolved in a generalized mass model to the observed ring velocity eld and morphology of A01360801, we conclude that q 0:5 and rule out a spherical geometry. 1. WHY POLAR RINGS? Many PRG, including A01360801, appear to be at or near equilibrium, so that the ring gas can be assumed to follow closed orbits. The shape and velocity structure of closed orbits contain information about the shape of the underlying mass distribution: orbits are elongated along the short axis of a attened potential and have speeds that vary with ring azimuth (slower over the poles). Given a viewing geometry, the unique relationship between orbit position and line-of-sight velocity allows one to use the 2-D velocity eld of a polar ring to constrain the shape of its dark halo. A01360801 is particularly suitable since its ring rotation curve is at to more than 12 disk scale lengths above the plane of its galactic disk (Schweizer et al. 1983, SWR), indicating that the ring dynamics are dominated by the dark halo. We can therefore expect that the attening we infer from our Fabry-Perot data reeects the attening of the dark mass. If the mass distribution is not spherical and the polar axis does not lie in the sky plane, the kinematic major and minor axes of the ring will be twisted relative to the morphological axes (Sackett 1991). This signature is in our data (Fig. 1), and has the opposite sense as would be expected for deviations in the velocity eld due to innow. Our goal is to determine the value of the axis ratio of the density distribution, q , that produces the best t to the geometry and velocity eld of the ring. 2. THE TWO-DIMENSIONAL VELOCITY FIELD We used the Rutgers CCD Fabry-Perot spectrograph on the CTIO 4m