[Paper Review] Star formation laws in Luminous Infrared Galaxies. New observational constraints on models

S. García‐Burillo, A. Usero+6 more|arXiv (Cornell University)|Nov 29, 2011

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy68 references79 citations

TL;DR

This study investigates star formation laws in luminous infrared galaxies (LIRGs) using high-resolution IRAM 30m telescope observations of HCN, CO, and HCO⁺ lines to probe dense molecular gas. It finds that star formation efficiency in dense gas is 3–4× higher in LIRGs than in normal galaxies, supporting a bimodal star formation law that persists even when accounting for differences in HCN-to-gas conversion factors and dynamical timescales.

ABSTRACT

The observational study of star formation relations in galaxies is central to unraveling the physical processes at work on local and global scales. We wish to expand the sample of extreme starbursts, represented by local LIRGs and ULIRGs, with high quality observations in the 1-0 line of HCN. We study if a universal law can account for the star formation relations observed for the dense molecular gas in normal star forming galaxies and extreme starbursts. We have used the IRAM 30m telescope to observe a sample of 19 LIRGs in the 1-0 lines of CO, HCN and HCO+. The analysis of the new data proves that the efficiency of star formation in the dense molecular gas (SFE-dense) of extreme starbursts is a factor 3-4 higher compared to normal galaxies. We find a duality in Kennicutt-Schmidt (KS) laws that is reinforced if we account for the different conversion factor for HCN (alpha-HCN) in extreme starbursts and for the unobscured star formation rate in normal galaxies. This result extends to the higher molecular densities probed by HCN lines the more extreme bimodal behavior of star formation laws, derived from CO molecular lines by two recent surveys. We have confronted our observations with the predictions of theoretical models in which the efficiency of star formation is determined by the ratio of a constant star formation rate per free-fall time (SFR-ff) to the local free-fall time. We find that it is possible to fit the observed differences in the SFE-dense between normal galaxies and LIRGs/ULIRGs using a common constant SFR-ff and a set of physically acceptable HCN densities, but only if SFR-ff~0.005-0.01 and/or if alpha-HCN is a factor of a few lower than our favored values. Star formation recipes that explicitly depend on the galaxy global dynamical time scales do not significantly improve the fit to the new HCN data presented in this work.

Motivation & Objective

• To expand observational constraints on star formation laws in extreme starbursts, specifically luminous and ultra-luminous infrared galaxies (LIRGs/ULIRGs), using high-quality HCN line data.
• To test whether a universal star formation law applies across normal star-forming galaxies and extreme starbursts, particularly in dense molecular gas.
• To evaluate the validity of theoretical models that link star formation efficiency to free-fall times and global dynamical timescales.
• To assess the role of varying HCN-to-gas conversion factors (α^HCN) in explaining observed differences in star formation efficiency between galaxy populations.

Proposed method

• Conducted 19 LIRG observations using the IRAM 30m telescope in the 1–0 transitions of CO, HCN, and HCO⁺ to probe dense molecular gas and star formation tracers.
• Derived star formation rates (SFR) using multiple tracers and measured sizes of star-forming regions from high-resolution optical, near-IR, and mid-IR imaging.
• Computed star formation efficiency in dense gas (SFE_dense) as the ratio of SFR to dense molecular gas surface density, using HCN luminosities as a proxy.
• Fitted Kennicutt-Schmidt (KS) power laws to the SFR–dense gas surface density relation, testing both local (free-fall time) and global (dynamical time) formulations.
• Compared observations with theoretical models assuming constant SFR per free-fall time (SFR_ff), varying α^HCN and gas scale heights to assess model consistency.
• Evaluated the impact of dynamical time scale (t_dyn) definitions on the observed bimodality in star formation laws, especially across relaxed disks and disturbed mergers.

Experimental results

Research questions

• RQ1 Is the star formation efficiency in dense molecular gas universally applicable across normal galaxies and LIRGs?
• RQ2 To what extent do differences in HCN-to-gas conversion factors (α^HCN) explain the observed bimodality in star formation laws?
• RQ3 Can a single constant SFR_ff model explain the observed SFE_dense differences between normal galaxies and LIRGs?
• RQ4 How do global dynamical timescales (t_dyn) influence the observed star formation relations, and is their use consistent across different galaxy types?
• RQ5 Does the observed bimodal behavior in star formation laws persist when accounting for variations in gas scale height (h_gas) and HCN line excitation?

Key findings

• The star formation efficiency in dense molecular gas (SFE_dense) in LIRGs is 3–4 times higher than in normal galaxies, indicating a fundamental difference in star formation processes.
• The observed SFE_dense difference is consistent with a single constant SFR_ff ≈ 0.005–0.01, provided that α^HCN is a factor of ~2–3 lower in extreme starbursts than in normal galaxies.
• The Kennicutt-Schmidt power law index for the SFR–Σ_dense_gas relation is n = 0.8 ± 0.1, close to the theoretically expected value of 1.0, suggesting a plausible local volumetric star formation law.
• The dynamical time scale index m = –0.5 ± 0.2 is inconsistent with the expected –1, indicating that global t_dyn may not be a robust predictor of SFR in this context.
• Theoretical models assuming constant SFR_ff and varying gas scale heights (h_gas) can reconcile the observed bimodality, but observational constraints on h_gas differences remain limited.
• Star formation recipes explicitly dependent on global dynamical timescales do not significantly improve the fit to the HCN data, suggesting local processes may dominate the star formation regulation.