Star Formation Rates in Molecular Clouds and the Nature of the Extragalactic Scaling Relations (1112.4466v1)

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 (non-linear) 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.

• The paper demonstrates a linear scaling relation between star formation rates and dense molecular gas over nine orders of magnitude.
• It uses the dense gas fraction (fDG) as a key parameter to explain variations in star formation efficiencies across different environments.
• The study integrates the Schmidt-Kennicutt law with HCN-based observations, offering a unified framework for both galactic and extragalactic star formation.

Star Formation Rates in Molecular Clouds and Extragalactic Scaling Relations

The paper conducted by Lada et al. explores fundamental relationships governing star formation rates (SFR) in both local molecular clouds and entire galaxies. It primarily focuses on the relation between star formation processes and molecular gas masses, highlighting empirical scaling relations applicable across vast spatial scales. This analysis emphasizes the critical role of dense molecular gas in determining the rates at which stars form, suggesting a bridging concept that seamlessly connects galactic incidents of star formation with extragalactic dynamics.

Core Findings

The authors present several key insights:

1. Linear Scaling Relation: The paper posits a linear scaling relation between the total SFR and the dense molecular gas mass in both molecular clouds and galaxies. This relation extends over nine orders of magnitude, implying that the star formation process, whether occurring in a local cloud or a distant galaxy, is predominantly controlled by the amount of dense molecular gas available in a given star-forming region.
2. Dense Gas Fraction ($f_{DG}$): The quantity $f_{DG}$ denotes the fraction of dense gas within molecular clouds or galaxies. Lada et al. find that the star formation scaling law remains linear as long as the systems being compared maintain a consistent dense gas fraction. Variations in $f_{DG}$ account for discrepancies in star formation efficiencies across different environments, such as those seen in starburst galaxies compared to others.
3. Unified Framework: The authors propose a unified framework integrating the standard extragalactic Schmidt-Kennicutt law with the linear star formation laws based on HCN observations. While the former often appears super-linear, Lada et al. suggest that its apparent steepness can be attributed more to the varied dense gas fractions in studied galaxies.

Implications and Future Directions

The implications of these results are twofold:

• Theoretical Understanding: This paper provides a simplified yet robust conceptual model for understanding star formation across different cosmic scales. It emphasizes the dense molecular gas component as a central factor in predicting star formation rates, simplifying theoretical models that treat star formation as a complex interplay of numerous variables.
• Practical Applications: For observational astronomers, these findings suggest that using dense gas tracers like HCN might yield more accurate predictions for star formation rates compared to more conventional CO-based total mass estimations. This could refine methodologies in observing and interpreting data on both local and extragalactic scales.

Looking forward, further investigation might focus on elucidating the processes that govern the formation and concentration of dense gas within molecular clouds. Understanding these mechanisms could help explain why certain environments, such as starburst galaxies, exhibit higher dense gas fractions. Additionally, exploring the influence of dynamic and variable conditions, like galaxy mergers or stellar feedback processes, on the dense molecular gas fraction could further refine our understanding of star formation across diverse galactic conditions. The challenge thus remains to decipher these intrinsic processes and their signatures across cosmic time scales, enhancing our predictive capability concerning galactic evolution and star formation histories in the universe.