Detection of Dense Molecular Gas in Inter-Arm Spurs in M51

TL;DR

This study maps molecular gas in inter-arm spurs of M51, revealing their potential role in star formation and challenging previous notions of star formation delay between spiral arms and inter-arm regions.

Contribution

First detection and characterization of molecular gas in inter-arm spurs of M51, highlighting their significance in star formation processes.

Findings

Spurs contain molecular gas with mass ~3 million solar masses.

Spurs are extended and contain enough fuel for star formation.

Spurs may be the sites of inter-arm star formation, reducing delay assumptions.

Abstract

Spiral arm spurs are prominent features that have been observed in extinction and 8$\mu$m emission in nearby galaxies. In order to understand their molecular gas properties, we used the Owens Valley Radio Observatory to map the CO(J=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$_{H2} \sim3\times10^6$ M$_{\sun}$ and $\Sigma_{H2} \sim$50 M$_{\sun}$ 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 $\sim$500 pc and contain adequate fuel for star formation, they may be the birthplace for observed inter-arm HII regions. This reduces the requirement for the significant time delay that would be otherwise needed if the inter-arm 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 CARMA and future ALMA telescopes and can be compared to several recent numerical simulations that have been examining the evolution of spiral arm spurs.