Molecular Star Formation Rate Indicators in Galaxies

Abstract

We derive a physical model for the observed relations between star formation rate (SFR) and molecular line (CO and HCN) emission in galaxies, and show how these observed relations are reflective of the underlying star formation law. We do this by combining 3D non-LTE radiative transfer calculations with hydrodynamic simulations of isolated disk galaxies and galaxy mergers. We demonstrate that the observed SFR-molecular line relations are driven by the relationship between molecular line emission and gas density, and anchored by the index of the underlying Schmidt law controlling the SFR in the galaxy. Lines with low critical densities (e.g. CO J=1-0) are typically thermalized and trace the gas density faithfully. In these cases, the SFR will be related to line luminosity with an index similar to the Schmidt law index. Lines with high critical densities greater than the mean density of most of the emitting clouds in a galaxy (e.g. CO J=3-2, HCN J=1-0) will have only a small amount of thermalized gas, and consequently a superlinear relationship between molecular line luminosity and mean gas density. This results in a SFR-line luminosity index less than the Schmidt index for high critical density tracers. One observational consequence of this is a significant redistribution of light from the small pockets of dense, thermalized gas to diffuse gas along the line of sight, and prodigious emission from subthermally excited gas. At the highest star formation rates, the SFR-Lmol slope tends to the Schmidt index, regardless of the molecular transition. The fundamental relation is the Kennicutt-Schmidt law, rather than the relation between SFR and molecular line luminosity. We use these results to make imminently testable predictions for the SFR-molecular line relations of unobserved transitions.

ApJ Accepted - Results remain same as previous version. Content clarified with Referee's comments

References in corpus (13)

• Highly-excited CO emission in APM08279+5255 at z=3.9
• The Relationship Between Molecular Gas Tracers and Kennicutt-Schmidt Laws
• Molecular gas in extreme star-forming environments: the starbursts Arp220 and NGC6240 as case studies
• Dense gas in luminous infrared galaxies
• HCN Observations of Dense Star-Forming Gas in High Redshift Galaxies
• Giant Molecular Clouds in M31: I - Molecular Cloud Properties
• HCN versus HCO+ as dense molecular gas mass tracer in Luminous Infrared Galaxies
• The Role of Galactic Winds on Molecular Gas Emission from Galaxy Mergers
• Modeling the Dust Properties of z ~ 6 Quasars with ART^2 -- All-wavelength Radiative Transfer with Adaptive Refinement Tree
• HCN to HCO^+ Millimeter Line Diagnostics of AGN Molecular Torus I : Radiative Transfer Modeling
• The Nature of CO Emission From z~6 Quasars
• A broadband spectroscopic search for CO line emission in HDF850.1: the brightest submillimetre object in the Hubble Deep Field North
• The coupling of dynamics and molecular chemistry in galaxies

Cited by in corpus (3)

• The Role of Galactic Winds on Molecular Gas Emission from Galaxy Mergers
• The Star Formation Rate - Dense Gas Relation in Galaxies as Measured by HCN (3-2) Emission
• The Properties of Dense Molecular Gas in the Milky Way and Galaxies