The Detection of Molecular Gas in the Outskirts of NGC 6946

We present the results of a search for molecular gas emission via the CO line in the far outer disk of the nearby spiral NGC 6946. The positions targeted were chosen to lie on or near previously identified outer-disk H II regions. Molecular gas was clearly detected out to 1.3R25, with a further tentative detection at 1.4R25. The CO detections show excellent agreement with the H I velocities and imply beam-averaged column densities of (0.3-9) × 1020 cm-2 and molecular gas masses of (2-70) × 105 M☉ per 21'' beam (560 pc). We find evidence for an abrupt decrease in the molecular fraction at the edge of the optical disk, similar to that seen previously in the azimuthally averaged areal star formation rate. Our observations provide new constraints on the factors that determine the presence and detectability of molecular gas in the outskirts of galaxies, and suggest that neither the H I column, the metallicity, nor the local heating rate alone plays a dominant role.