Polar Ring Galaxies and Warps

Polar ring galaxies, where matter is in equilibrium in perpendicular orbits around spiral galaxies, are ideal objects to probe the 3D shapes of dark matter halos. The conditions to constrain the halos are that the perpendicular system does not strongly perturb the host galaxy, or that it is possible to derive back its initial shape, knowing the formation scenario of the polar ring. The formation mechanisms are reviewed: mergers, tidal accretion, or gas accretion from cosmic filaments. The Tully-Fisher diagram for polar rings reveals that the velocity in the polar plane is higher than in the host plane, which can only be explained if the dark matter is oblate and flattened along the polar plane. Only a few individual systems have been studied in details, and 3D shapes of their haloes determined by several methods. The high frequency of warps could be explained by spontaneous bending instability, if the disks are sufficiently self-gravitating, which can put constraints on the dark matter flattening. 1 Polar rings formation scenarios Polar Ring Galaxies (PRG) are composed of an early-type host, surrounded by a gaseous and stellar perpendicular ring which has the characteristic of a latetype galaxy: large HI amount, young stars, blue colors. If the polar ring is in equilibrium, it could provide clues on the 3D-shape of dark matter in the host galaxy. For that, the polar ring must not be too massive, in order to keep the initial gravitational potential unperturbed; or in any case, we must know the formation mechanism of the polar rings, to be able to generalise the results to normal haloes. The probability to have a PRG has been estimated to ∼ 5% from the observations (Whitmore et al 1990). Several mechanisms have been proposed in the literature to form polar ring galaxies, that can be gathered into two kinds: merger between two galaxies, of which one at least is gas rich, or mass accretion by the original host galaxy. The 1 Observatoire de Paris, LERMA, 61 AV. de l’Observatoire, F-75014, Paris, France c © EDP Sciences 2008 DOI: (will be inserted later) 24 Mass Profiles and Shapes of Cosmological Structures first kind can be a major merger between two perpendicularly oriented systems, or the disruption of a small companion on a polar orbit (the debris align then on the polar ring). The second kind can be either gas transfer between two galaxies, in an hyperbolic passage, or gas accretion from cosmic filaments in a polar ring by the host. In all cases, the bulk of the stars in the polar plane form after the event, from the gas settled in the polar plane.