The dense gas mass fraction in the W51 cloud and its protoclusters

Context. The density structure of molecular clouds determines how they will evolve.
Aims: We map the velocity-resolved density structure of the most vigorously star-forming molecular cloud in the Galactic disk, the W51 giant molecular cloud.
Methods: We present new 2 cm and 6 cm maps of H₂CO, radio recombination lines, and the radio continuum in the W51 star forming complex acquired with Arecibo and the Green Bank Telescope at ~ 50″ resolution. We use H₂CO absorption to determine the relative line-of-sight positions of molecular and ionized gas. We measure gas densities using the H₂CO densitometer, including continuous measurements of the dense gas mass fraction (DGMF) over the range 10⁴cm^-3<n(H₂) < 10⁶cm^-3 - this is the first time a dense gas mass fraction has been measured over a range of densities with a single data set.
Results: The DGMF in W51 A is high, f ≳ 70% above n> 10⁴cm^-3, while it is low, f< 20%, in W51 B. We did not detect any H₂CO emission throughout the W51 GMC; all gas dense enough to emit under normal conditions is in front of bright continuum sources and therefore is seen in absorption instead.
Conclusions: (1) The dense gas fraction in the W51 A and B clouds shows that W51 A will continue to form stars vigorously, while star formation has mostly ended in W51 B. The lack of dense, star-forming gas around W51 C indicates that collect-and-collapse is not acting or is inefficient in W51. (2) Ongoing high-mass star formation is correlated with n ≳ 1 × 10⁵cm^-3 gas. Gas with n> 10⁴cm^-3 is weakly correlated with low and moderate mass star formation, but does not strongly correlate with high-mass star formation. (3) The nondetection of H₂CO emission implies that the emission detected in other galaxies, e.g. Arp 220, comes from high-density gas that is not directly affiliated with already-formed massive stars. Either the non-star-forming ISM of these galaxies is very dense, implying the star formation density threshold is higher, or H ii regions have their emission suppressed.

The data set has been made public at http://dx.doi.org/10.7910/DVN/26818Appendices are available in electronic form at http://www.aanda.org