Star Formation Rates in Molecular Clouds and the Nature of the Extragalactic Scaling Relations

Abstract. In this paper, we investigate scaling relations between star formation rates and molecular gas masses for both local Galactic clouds and a sample of external galaxies. We specifically consider relations between the star formation rates and measurements of dense, as well as total, molecular gas masses. We argue that there is a fundamental empirical scaling relation that directly connects the local star formation process with that operating globally within galaxies. Specifically, the total star formation rate in a molecular cloud or galaxy is linearly proportional to the mass of dense gas within the cloud or galaxy. This simple relation, first documented in previous studies, holds over a span of mass covering nearly nine orders of magnitude and indicates that the rate of star formation is directly controlled by the amount of dense molecular gas that can be assembled within a star formation complex. We further show that the star formation rates and total molecular masses, characterizing both local clouds and galaxies, are correlated over similarly large scales of mass and can be described by a family of linear star formation scaling laws, parameterized by f_DG, the fraction of dense gas contained within the clouds or galaxies. That is, the underlying star formation scaling law is always linear for clouds and galaxies with the same dense gas fraction. These considerations provide a single unified framework for understanding the relation between the standard (nonlinear) extragalactic Schmidt-Kennicutt scaling law, that is typically derived from CO observations of the gas, and the linear star formation scaling law derived from HCN observations of the dense gas.

SFR-molecular-mass diagram for local molecular clouds and galaxies from the Gao & Solomon (2004) sample. The solid symbols correspond to measurements of dense cloud masses either from extinction observations of the galactic clouds or HCN observations of the galaxies. The open symbols correspond to measurements of total cloud masses of the same clouds and galaxies, either from extinction measurements for the galactic clouds or CO observations for the galaxies. For the galaxies, pentagons represent the locations of normal spirals, while the positions of starburst galaxies are represented by squares (LIRGs) and inverted triangles (ULIRGs). Triangles represent high-z BzK galaxies.