Formaldehyde Densitometry of Starburst Galaxies: Density-Independent Global Star Formation

Abstract

Accurate techniques which allow for the derivation of the spatial density in star formation regions are rare. A technique which has found application for the derivation of spatial densities in Galactic star formation regions utilizes the density-sensitive properties of the K-doublet transitions of formaldehyde (H2CO). In this paper, we present an extension of our survey of the formaldehyde 1(10)-1(11) (lambda = 6.2 cm) and 2(11)-2(12) (lambda = 2.1 cm) K-doublet transitions of H2CO in a sample of 56 starburst systems (Mangum etal. 2008). We have extended the number of galaxies in which both transitions have been detected from 5 to 13. We have improved our spatial density measurements by incorporating kinetic temperatures based upon NH3 measurements of 11 of the galaxies with a total of 14 velocity components in our sample (Mangum etal. 2013). Our spatial density measurements lie in a relatively narrow range of from 10^(4.5) to 10^(5.5) cm^(-3). This implies that the Schmidt-Kennicutt relation between L_(IR) and M_(dense): (1) Is an indication of the dense gas mass reservoir available to form stars, and (2) Is not directly dependent upon a higher average density driving the star formation process in the most luminous starburst galaxies. We have also used our H2CO measurements to derive two separate measures of the dense gas mass which are generally smaller, in many cases by a factor of 10^2-10^3, than those derived using HCN. This disparity suggests that H2CO traces a denser, more compact, component of the giant molecular clouds in our starburst galaxy sample. We also report measurements of the rotationally-excited lambda = 6.3 cm 2P_(1/2) J=1/2 state of OH and the H111alpha radio recombination line taken concurrently with our H2CO 1(10)-1(11) measurements.

27 pages, 11 figures, 8 tables (sheesh...what a long paper!). Accepted for publication in ApJ. Updated with proof corrections. arXiv admin note: text overlap with arXiv:0710.2115

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• A hidden molecular outflow in the LIRG Zw 049.057
• Dense-gas tracers and carbon isotopes in five 2.5<z<4 lensed dusty star forming galaxies from the SPT SMG sample
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• Discovery of Methanimine Megamasers Toward Compact Obscured Galaxy Nuclei
• Extreme star formation in the Milky Way: Luminosity distributions of young stellar objects in W49A and W51
• Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde V. The massive filament DR21
• APEX and NOEMA observations of H${}_2$S in nearby luminous galaxies and the ULIRG Mrk~231 -- Is there a relation between dense gas properties and molecular outflows?
• H2CO and H110α observations towards NH3 sources
• An Imaging and Spectroscopic Exploration of the Dusty Compact Obscured Nucleus Galaxy Zw~049.057
• A Spectral Analysis of the Centimeter Regime of Nearby Galaxies: RRLs, Excited OH, and NH${}_3$
• Molecular Gas Density Measured with H${}_2$CO and CS toward a Spiral Arm of M51
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• The 107 GHz methanol transition is a dasar in G0.253+0.016
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