Detection of dense molecular gas in interarm spurs in M51

Spiral arm spurs are prominent features that have been observed in extinction and 8 m emission in nearby galaxies. In order to understand their molecular gas properties, we used the Owens Valley Radio Observatory (OVRO) to map the CO(1-0) emission in three spurs emanating from the inner northwestern spiral arm of M51. We report CO detections from all three spurs. The molecular gas mass and surface density are M_H_2 ~ 3 X 10^6 M_☉ and Σ_H_2 ~ 50 M_☉ pc^(-2). Thus, relative to the spiral arms, the spurs are extremely weak features. However, since the spurs are extended perpendicular to the spiral arms for ~500 pc and contain adequate fuel for star formation, they may be the birthplace for observed interarm H ii regions. This reduces the requirement for the significant time delay that would be otherwise needed if the interarm star formation was initiated in the spiral arms. Larger maps of galaxies at similar depth are required to further understand the formation and evolution of these spurs and their role in star formation— such data should be forthcoming with the new Combined Array for Research in Millimeter Astronomy (CARMA) and future Atacama Large Millimeter/submillimeter Array (ALMA) telescopes, and can be compared to several recent numerical simulations that have been examining the evolution of spiral arm spurs.