On the density regime probed by HCN emission

Abstract

HCN J$=$1$-$0 emission is commonly used as a dense gas tracer, thought to mainly arise from gas with densities $\sim 10^4-10^5\mathrm{c}{\mathrm{m}}^{-3}$. This has made it a popular tracer in star formation studies. However, there is increasing evidence from observational surveys of `resolved' molecular clouds that HCN can trace more diffuse gas. We investigate the relationship between gas density and HCN emission through post-processing of high resolution magnetohydrodynamical simulations of cloud-cloud collisions. We find that HCN emission traces gas with a mean volumetric density of $\sim 3\times 10^3\mathrm{c}{\mathrm{m}}^{-3}$ and a median visual extinction of $\sim 5\mathrm{mag}$. We therefore predict a characteristic density that is an order of magnitude less than the "standard" characteristic density of $\mathrm{n}\sim 3\times 10^4\mathrm{c}{\mathrm{m}}^{-3}$. Indeed, we find in some cases that there is clear HCN emission from the cloud even though there is no gas denser than this standard critical density. We derive luminosity-to-mass conversion factors for the amount of gas at $A_{\mathrm{V}}>8$ or at densities $n>2.85\times 10^3\mathrm{c}{\mathrm{m}}^{-3}$ or $n>3\times 10^4\mathrm{c}{\mathrm{m}}^{-3}$, finding values of ${\alpha }_{\mathrm{HCN}}=6.79,8.62$ and $27.98{\mathrm{M}}_{\odot }(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^2)$, respectively. In some cases, the luminosity to mass conversion factor predicted mass in regions where in actuality there contains no mass.

References in corpus (51)

• Star Formation in the Milky Way and Nearby Galaxies
• E pur si muove: Galiliean-invariant cosmological hydrodynamical simulations on a moving mesh
• An atomic and molecular database for analysis of submillimetre line observations
• The Star Formation Rate and Dense Molecular Gas in Galaxies
• Modelling Accretion in Protobinary Systems
• Structure of Stationary Photodissociation Fronts
• On the Star Formation Rates in Molecular Clouds
• The Relationship Between Gas Content and Star Formation in Molecule-Rich Spiral Galaxies
• Slow Star Formation in Dense Gas: Evidence and Implications
• The Critical Density and the Effective Excitation Density of Commonly Observed Molecular Dense Gas Tracers
• HCN Survey of Normal Spiral, IR-luminous and Ultraluminous Galaxies
• Probing the evolution of molecular cloud structure: From quiescence to birth
• The Arepo public code release
• Rotational quenching of CO due to H${}_2$ collisions
• Modeling Collapse and Accretion in Turbulent Gas Clouds: Implementation and Comparison of Sink Particles in AMR and SPH
• Extremely Inefficient Star Formation in the Outer Disks of Nearby Galaxies
• Variations in the Galactic star formation rate and density thresholds for star formation
• Connecting Dense Gas Tracers of Star Formation in our Galaxy to High-z Star Formation
• Star Formation Rates in Molecular Clouds and the Nature of the Extragalactic Scaling Relations
• Magnetohydrodynamics on an unstructured moving grid
• What controls star formation in the central 500 pc of the Galaxy?
• Simulating the formation of molecular clouds. II. Rapid formation from turbulent initial conditions
• Inefficient star formation through turbulence, magnetic fields and feedback
• The anatomy of the Orion B Giant Molecular Cloud: A local template for studies of nearby galaxies
• Star formation laws in Luminous Infrared Galaxies. New observational constraints on models
• Modeling CO Emission: I. CO as a Column Density Tracer and the X-Factor in Molecular Clouds
• Modeling the Warm Ionized Interstellar Medium and Its Impact on Elemental Abundance Studies
• The Relationship Between Molecular Gas Tracers and Kennicutt-Schmidt Laws
• TreeCol: a novel approach to estimating column densities in astrophysical simulations
• GMC formation by agglomeration and self gravity
• Molecular Line Emission as a Tool for Galaxy Observations (LEGO). I. HCN as a tracer of moderate gas densities in molecular clouds and galaxies
• Gas phase Elemental abundances in Molecular cloudS (GEMS) I. The prototypical dark cloud TMC 1
• HCN Observations of Dense Star-Forming Gas in High Redshift Galaxies
• Is molecular gas necessary for star formation?
• HCN/HNC intensity ratio: a new chemical thermometer for the molecular ISM
• Tracing the formation of molecular clouds via [CII], [CI] and CO emission
• Simulations of the star-forming molecular gas in an interacting M51-like galaxy
• Millimeter-Wave Line Ratios and Sub-beam Volume Density Distributions
• Formation sites of Population III star formation: The effects of different levels of rotation and turbulence on the fragmentation behavior of primordial gas
• Dense gas is not enough: environmental variations in the star formation efficiency of dense molecular gas at 100pc scales in M51
• Electron-impact rotational and hyperfine excitation of HCN, HNC, DCN and DNC
• LEGO II: A 3 mm molecular line study covering 100 pc of one of the most actively star-forming portions within the Milky Way Disc
• Indications of a sub-linear and non-universal Kennicutt-Schmidt relationship
• First Detection of HCO+ Emission at High Redshift
• Numerical Calibration of the HCN$-$Star Formation Correlation
• Characterizing the line emission from molecular clouds. Stratified random sampling of the Perseus cloud
• Star Formation Occurs in Dense Gas, but What Does "Dense" Mean?
• Do Spectroscopic Dense Gas Fractions Track Molecular Cloud Surface Densities?
• Molecular-Cloud-Scale Chemical Composition III: Constraints of Average Physical Properties through Chemical Models
• Molecular line signatures of cloud-cloud collisions
• Synthetic line and continuum observations of simulated turbulent clouds: the apparent widths of filaments