The origin of polar ring galaxies: Evidence for galaxy formation by cold accretion

Polar ring galaxies are flattened stellar systems with an extended ring of gas and stars rotating in a plane almost perpendicular to the central galaxy. We show that their formation can occur naturally in a hierarchical universe where most low-mass galaxies are assembled through the accretion of cold gas infalling along megaparsecscale filamentary structures. Within a large cosmological hydrodynamical simulation, we find a system that closely resembles the classic polar ring galaxy NGC 4650A. How galaxies acquire their gas is a major uncertainty in models of galaxy formation, and recent theoretical work has argued that cold accretion plays a major role. This idea is supported by our numerical simulations and the fact that polar ring galaxies are typically low-mass systems.