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

Abstract

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 $10^4$ cm${}^{-3}$ < n(H${}_2$) < $10^6$ cm${}^{-3}$- this is the first time a dense gas mass fraction has been measured over a range of densities with a single data set. The DGMF in W51A is high,f >~70% above $n>10^4$ cm${}^{-3}$, 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. The data set has been made public at http://dx.doi.org/10.7910/DVN/26818. 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 ~ 1x10${}^5$ cm${}^{-3}$ gas. Gas with n > 10${}^4$ cm${}^{-3}$ is weakly correlated with low and moderate mass star formation, but does not strongly correlate with high-mass star formation. (3) The nondetection of H${}_2$CO 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.

Accepted to A&A. 31 pages, 59 figures. Data available from http://dx.doi.org/10.7910/DVN/26818 and source code available from http://github.com/keflavich/w51_singledish_h2co_maps

References in corpus (14)

• The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey
• The VLA Galactic Plane Survey
• A Minimum Column Density of 1 g cm^-2 for Massive Star Formation
• On the Star Formation Rate - Brightest Cluster Relation: Estimating the peak SFR in post-merger galaxies
• CO-dark gas and molecular filaments in Milky Way type galaxies
• The On The Fly Imaging Technique
• Displacement of the Sun from the Galactic Plane
• A Sino-German lambda6 cm polarization survey of the Galactic plane I. Survey strategy and results for the first survey region
• Molecular Line Emission from Massive Protostellar Disks: Predictions for ALMA and the EVLA
• The effect of stellar winds on the formation of a protocluster
• Galactic H2CO Densitometry I: Pilot survey of Ultracompact HII regions and methodology
• The Comparison of Physical Properties Derived from Gas and Dust in a Massive Star-Forming Region
• Enhancement of CO(3-2)/CO(1-0) Ratios and Star Formation Efficiencies in Supergiant HII Regions
• The Star Formation Relation for Regions in the Galactic Plane: The Effect of Spatial Resolution

Cited by in corpus (30)

• Large-scale filaments associated with Milky Way spiral arms
• Interferometric observations of magnetic fields in forming stars
• ALMA-IMF I -- Investigating the origin of stellar masses: Introduction to the Large Program and first results
• A Census of Large-Scale ($\ge$ 10 pc), Velocity-Coherent, Dense Filaments in the Northern Galactic Plane: Automated Identification Using Minimum Spanning Tree
• Thermal Feedback in the high-mass star and cluster forming region W51
• The Role of Outflows, Radiation Pressure, and Magnetic Fields in Massive Star Formation
• GMC Collisions as Triggers of Star Formation. V. Observational Signatures
• Molecular-Cloud-Scale Chemical Composition I: Mapping Spectral Line Survey toward W51 in the 3 mm Band
• Kinetic temperature of massive star forming molecular clumps measured with formaldehyde
• Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde. III. The Orion Molecular Cloud 1
• Kinetic temperature of massive star forming molecular clumps measured with formaldehyde. II. The Large Magellanic Cloud
• Mapping Jet-ISM Interactions in X-ray Binaries with ALMA: A GRS 1915$+$105 Case Study
• A Massive Star is Born: How Feedback from Stellar Winds, Radiation Pressure, and Collimated Outflows Limits Accretion onto Massive Stars
• Surveying the Giant HII Regions of the Milky Way with SOFIA: I. W51A
• ALMA-IMF. VII. First release of the full spectral line cubes: Core kinematics traced by DCN J=(3-2)
• Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde IV. The ALMA view of N113 and N159W in the LMC
• Jet-ISM Interactions near the Microquasars GRS 1758-258 and 1E 1740.7-2942
• A global view on star formation: The GLOSTAR Galactic plane survey VIII. Formaldehyde absorption in Cygnus~X
• The Bolocam Galactic Plane Survey. XIII. Physical Properties and Mass Functions of Dense Molecular Cloud Structures
• A Multi-Scale Picture of Magnetic Field and Gravity from Large-Scale Filamentary Envelope to Core-Accreting Dust Lanes in the High-Mass Star-Forming Region W51
• Near-infrared spectroscopy of the massive stellar population of W51: evidence for multi-seeded star formation
• Disentangling the radio emission of the supernova remnant W51C
• QUIJOTE scientific results -- V. The microwave intensity and polarisation spectra of the Galactic regions W49, W51 and IC443
• Long-term Variability of H${}_2$CO Masers in Star-forming Regions
• First detection of CS masers around a high-mass young stellar object, W51 e2e
• Revisiting rotationally excited CH at radio wavelengths: A case study towards W51
• Near-Infrared Spectroscopy of Ultracompact \ion{H}{2} regions in W51A with NIFS/ALTAIR
• GTC/CanariCam Mid-IR Polarimetry of Magnetic Fields in Star-Forming Region W51 IRS2
• Rethinking Habitability using Biogenic Precursors: Formaldehyde in Millimeter Molecular Clouds of the Inner Galaxy
• Clustered Star Formation in the center of NGC 253 Contributes to Driving the Ionized Nuclear Wind