Does the HCN/CO ratio trace the star-forming fraction of gas? I. A comparison with analytical models of star formation

Abstract

We use archival ALMA observations of the HCN and CO $J=1-0$ transitions, in addition to the radio continuum at 93 GHz, to assess the relationship between dense gas, star formation, and gas dynamics in ten, nearby (U)LIRGs and late-type galaxy centers. We frame our results in the context of turbulent and gravoturbulent models of star formation to assess if the HCN/CO ratio tracks the gravitationally-bound, star-forming gas in molecular clouds ($f_{\mathrm{grav}}$) at sub-kpc scales in nearby galaxies. We confirm that the HCN/CO ratio is a tracer of gas above $n_{\mathrm{SF}}\approx 10^{4.5}$ cm${}^{-3}$, but the sub-kpc variations in HCN/CO do not universally track $f_{\mathrm{grav}}$. We find strong evidence for the use of varying star formation density threshold models, which are able to reproduce trends observed in $t_{\mathrm{dep}}$ and ${\epsilon }_{\mathrm{ff}}$ that fixed threshold models cannot. Composite lognormal and powerlaw models outperform pure lognormal models in reproducing the observed trends, even when using a fixed powerlaw slope. The ability of the composite models to better reproduce star formation properties of the gas provides additional indirect evidence that the star formation efficiency per free-fall time is proportional to the fraction of gravitationally-bound gas.

arXiv comment23 pages, 11 figures, 1 appendix, accepted for publication in ApJ

References in corpus (83)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• Array Programming with NumPy
• Astropy: A Community Python Package for Astronomy
• The Astropy Project: Building an inclusive, open-science project and status of the v2.0 core package
• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• The CO-to-H2 Conversion Factor
• The Star Formation Law in Nearby Galaxies on Sub-Kpc Scales
• The Spitzer c2d Legacy Results: Star Formation Rates and Efficiencies; Evolution and Lifetimes
• A General Theory of Turbulence-Regulated Star Formation, From Spirals to ULIRGs
• The Star Formation Rate and Dense Molecular Gas in Galaxies
• Calibrating Extinction-Free Star Formation Rate Diagnostics with 33GHz Free-Free Emission in NGC6946
• The Star Formation Rate of Turbulent Magnetized Clouds: Comparing Theory, Simulations, and Observations
• On the Star Formation Rates in Molecular Clouds
• The Dark Molecular Gas
• Re-examining Larson's Scaling Relationships in Galactic Molecular Clouds
• MAMBO Mapping of Spitzer c2d Small Clouds and Cores
• Determining Star Formation Rates for Infrared Galaxies
• A Universal, Local Star Formation Law in Galactic Clouds, Nearby Galaxies, High-Redshift Disks, and Starbursts
• 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
• The Star Formation Rate of Supersonic MHD Turbulence
• Common-Resolution Convolution Kernels for Space- and Ground-Based Telescopes
• Probing the evolution of molecular cloud structure: From quiescence to birth
• On the Star Formation Efficiency of Turbulent Magnetized Clouds
• Connecting Dense Gas Tracers of Star Formation in our Galaxy to High-z Star Formation
• A Herschel [CII] Galactic plane survey I: the global distribution of ISM gas components
• Maximally Star-Forming Galactic Disks I. Starburst Regulation Via Feedback-Driven Turbulence
• Bias-free Measurement of Giant Molecular Cloud Properties
• CMasher: Scientific colormaps for making accessible, informative and 'cmashing' plots
• The Big Problems in Star Formation: the Star Formation Rate, Stellar Clustering, and the Initial Mass Function
• Analytical star formation rate from gravoturbulent fragmentation
• What determines the density structure of molecular clouds ? A case study of Orion B with Herschel
• Cloud-Scale Molecular Gas Properties in 15 Nearby Galaxies
• On the Density Probability Function of Galactic Gas. I. Numerical Simulations and the Significance of the Polytropic Index
• Supersonic turbulence in the ISM: stellar extinction determinations as probes of the structure and dynamics of dark clouds
• The "True" Column Density Distribution in Star-Forming Molecular Clouds
• Gravity or turbulence? II. Evolving column density PDFs in molecular clouds
• When Feedback Fails: The Scaling and Saturation of Star Formation Efficiency
• Variations in the Star Formation Efficiency of the Dense Molecular Gas across the Disks of Star-Forming Galaxies
• A Herschel [C II] Galactic plane survey II: CO-dark H2 in clouds
• High-dynamic-range extinction mapping of infrared dark clouds: Dependence of density variance with sonic Mach number in molecular clouds
• Observational Evidence of Dynamic Star Formation Rate in Milky Way Giant Molecular Clouds
• Cloud Scale ISM Structure and Star Formation in M51
• Dynamical Equilibrium in the Molecular ISM in 28 Nearby Star-Forming Galaxies
• The Two States of Star Forming Clouds
• EMPIRE: The IRAM 30-m Dense Gas Survey of Nearby Galaxies
• Molecular Line Emission as a Tool for Galaxy Observations (LEGO). I. HCN as a tracer of moderate gas densities in molecular clouds and galaxies
• Molecular clouds have power-law probability distribution functions
• Revisiting the Integrated Star Formation Law. II. Starbursts and the Combined Global Schmidt Law
• The Turbulent Warm Ionized Medium: Emission Measure Distribution and MHD Simulations
• The physical origin of long gas depletion times in galaxies
• The Star Formation Rate in the Gravoturbulent Interstellar Medium
• The density structure and star formation rate of non-isothermal polytropic turbulence
• Characterizing filaments in regions of high-mass star formation: High-resolution submilimeter imaging of the massive star-forming complex NGC 6334 with ArTéMiS
• The Column Density Variance-Sonic Mach Number Relationship
• Dense Gas, Dynamical Equilibrium Pressure, and Star Formation in Nearby Star-Forming Galaxies
• Millimeter-Wave Line Ratios and Sub-beam Volume Density Distributions
• Supernova Driving. IV. The Star Formation Rate of Molecular Clouds
• Understanding star formation in molecular clouds II. Signatures of gravitational collapse of IRDCs
• High-resolution radio continuum survey of M33: III. Magnetic fields
• Dense Gas in the Milky Way
• Understanding star formation in molecular clouds III. Probability distribution functions of molecular lines in Cygnus X
• The shapes of column density PDFs - The importance of the last closed contour
• Density probability distribution functions of diffuse gas in the Milky Way
• The Kennicutt-Schmidt Law and Gas Scale Height in Luminous and Ultra-Luminous Infrared Galaxies
• A method for reconstructing the PDF of a 3D turbulent density field from 2D observations
• The Razor's Edge of Collapse: The Transition Point from Lognormal to Powerlaw in Molecular Cloud PDFs
• Synthesis Imaging of Dense Gas in Nearby Galaxies
• The Self-gravitating Gas Fraction and The Critical Density for Star Formation
• The Lognormal Probability Distribution Function of the Perseus Molecular Cloud: A Comparison of HI and Dust
• Star formation in a high-pressure environment: An SMA view of the Galactic centre dust ridge
• The Anatomy of the Column Density Probability Distribution Function (N-PDF)
• An Evolutionary Model for Collapsing Molecular Clouds and Their Star Formation Activity. II. Mass Dependence of the Star Formation Rate
• First Results from the $Herschel$ and ALMA Spectroscopic Surveys of the SMC: The Relationship Between [CII]-bright Gas and CO-bright Gas at Low Metallicity
• Numerical Calibration of the HCN$-$Star Formation Correlation
• The CO-dark molecular gas mass in 30 Doradus
• Spatially-Resolved Dense Molecular Gas and Star Formation Rate in M51
• The HI Probability Distribution Function and the Atomic-to-Molecular Transition in Molecular Clouds
• Dense Gas Fraction and Star Formation Efficiency Variations in the Antennae Galaxies
• The Effects of Radiative Transfer on the PDFs of Molecular MHD Turbulence
• Relationship between turbulence energy and density variance in the Solar neighbourhood molecular clouds
• Kiloparsec-scale Variations in the Star Formation Efficiency of Dense Gas: the Antennae Galaxies (NGC 4038/39)
• ALMA Observations of Atomic Carbon [C I] (${{}^3\mathrm{P}}_1\to {{}^3\mathrm{P}}_0$) and Low-$J$ CO Lines in the Starburst Galaxy NGC 1808

Cited by in corpus (3)

• Molecular gas and star formation in nearby starburst galaxy mergers
• Dense gas scaling relations at kiloparsec scales across nearby galaxies with the ALMA ALMOND and IRAM 30m EMPIRE surveys
• Does the HCN/CO ratio trace the star-forming fraction of gas? II. Variations in CO and HCN Emissivity