2002ApJ...577L.103S

We have detected four giant molecular cloud associations (sizes ≤6".6~430 pc) in the western and eastern regions of the polar ring in NGC 2685 (the Helix galaxy) using the Owens Valley Radio Observatory millimeter interferometer. Emission from molecular gas is found close to the brightest Hα and H I peaks in the polar ring and is confirmed by new IRAM 30 m single-dish observations. The CO and H I line velocities are very similar, providing additional kinematic confirmation that the CO emission emerges from the polar ring. For the first time, the total molecular mass within a polar ring is determined [M_H2~(8-11)x10⁶ M_☉, using the standard Galactic conversion factor]. We detect about M_H2∼4.4x10⁶ M_☉in the nuclear region with the single dish. Our upper limit derived from the interferometric data is lower (M_H2≤0.7x10⁶ M_☉), suggesting that the molecular gas is distributed in an extended (≥1.3 kpc) diffuse disk. These new values are an order of magnitude lower than in previous reports. The total amount of molecular gas and the atomic gas content of the polar ring are consistent with formation due to accretion of a small gas-rich object, such as a dwarf irregular. The properties of the NGC 2685 system suggest that the polar ring and the host galaxy have been in a stable configuration for a considerable time (a few gigayears). The second (outer) H I ring within the disk of NGC 2685 is very likely at the outer Lindblad resonance of the ∼11 kpc long stellar bar.