Molecular Line Emission as a Tool for Galaxy Observations (LEGO)

Abstract

Trends observed in galaxies, such as the Gao & Solomon relation, suggest a linear relationship between the star formation rate and the mass of dense gas available for star formation. Validation of such trends requires the establishment of reliable methods to trace the dense gas in galaxies. One frequent assumption is that the HCN (J = 1-0) transition is unambiguously associated with gas at H 2 densities 10 4 cm −3 . If so, the mass of gas at densities 10 4 cm −3 could be inferred from the luminosity of this emission line, L HCN (1-0) . Here we use observations of the Orion A molecular cloud to show that the HCN (J = 1-0) line traces much lower densities ∼10 3 cm −3 in cold sections of this molecular cloud, corresponding to visual extinctions A V ≈ 6 mag. We also find that cold and dense gas in a cloud like Orion produces too little HCN emission to explain L HCN (1-0) in star forming galaxies, suggesting that galaxies might contain a hitherto unknown source of HCN emission. In our sample of molecules observed at frequencies near 100 GHz (also including 12 CO, 13 CO, C 18 O, CN, and CCH), N 2 H + is the only species clearly associated with relatively dense gas.

Young massive star cluster formation in the Galactic Centre is driven by global gravitational collapse of high-mass molecular clouds

“…10 However, despite their high critical densities, the HCN and HCO + maps show little correspondence to the features identifiable within the dust continuum maps (compare to overlaid contours on Figures 5 and 6), as is typically observed within Galactic Disc star forming regions (e.g. Kauffmann et al 2017c). Indeed, Rathborne et al (2015) found a similar result for the majority of the molecular line transitions identified in ALMA Band 3 observations towards the Brick molecular cloud (see Figure 1).…”

Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde

“…Unlike ammonia, the fractional abundance of H 2 CO is stable at various stages of star formation (Mangum et al 1990;Caselli et al 1993;Johnstone et al 2003;Gerner et al 2014;Tang et al 2017a,b,c). OMC-1 has been a valuable target for measuring lines of H 2 CO because of its high densities and temperatures (e.g., Mangum et al 1990Bergin et al 1994Bergin et al , 1996Peng et al 2012;Gong et al 2015b;Kauffmann et al 2017) and subsequently large surface brightnesses (e.g., Lis et al 1998;Johnston & Bally 1999;Megeath et al 2012;Lombardi et al 2014). Previous observations show that H 2 CO has a spatially extensive distribution in the OMC-1 region including Orion KL, Orion south, the Orion Bar, and the northern part of the OMC-1 region (e.g., Batrla et al 1983;Bastien et al 1985;Mangum et al 1990van der Wiel et al 2009;Leurini et al 2010).…”

Electron Excitation of High Dipole Moment Molecules Re-examined

“…Significant additional theoretical work is therefore needed before a satisfying explanation can be given for the extended emission from molecules like HCN in low density environments (Kauffmann et al 2017). …”

“…Since this emission is 1 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena CA, 91109, USA; paul.f.goldsmith@jpl.nasa.gov 2 Max Planck Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany generally not spatially resolved, excitation by electrons could be contributing, especially in the outer regions of clouds subject to high radiation fields. Kauffmann et al (2017) have studied the density associated with HCN emission in the Orion molecular cloud, and find that a large fraction of the flux is produced in regions having n(H 2 ) ≈ 10 3 cm −3 , well below the range ≥ 3×10 4 cm −3…”

“…Electron excitation is, for example, unimportant for CO and C + . Electron excitation may be responsible for the surprisingly large spatial extent of the emission from dense gas tracers in some molecular clouds (Pety et al 2017;Kauffmann, Goldsmith et al 2017). The enhanced estimates for HCN abundances and HCN/CO and HCN/HCO + ratios observed in the nuclear regions of luminous galaxies may be in part a result of electron excitation of high dipole moment tracers.…”