Regularity underlying complexity: a redshift-independent description of the continuous variation of galaxy-scale molecular gas properties in the mass-star formation rate plane

Abstract: Star-forming galaxies (SFGs) display a continuous distribution of specific star formation rates (sSFR) which can be approximated by the superposition of two log-normal distributions. The 1st of these encompasses the main sequence (MS) of SFGs, the 2nd one a rarer population of starbursts (SB). We show that the sSFR-distribution of SBs can be regarded as the result of a physical process (plausibly merging) taking the mathematical form of a log-normal boosting kernel and enhancing star formation activity. We explore the utility of splitting the star-forming population into MS and SB galaxies - an approach we term "2-Star Formation Mode" (2-SFM) framework - for understanding their molecular gas properties. Variations of star formation efficiency (SFE) and gas fractions among SFGs take a simple, redshift-independent form, once these quantities are normalized to the value of an average MS galaxy. The change in SFE for galaxies undergoing a starburst event scales supra-linearly with the SFR-increase, as expected for mergers. This implies a continuous distribution of galaxies in the Schmidt-Kennicutt plane that separates more clearly into loci for SBs and normal galaxies than observed in SFR vs. M* space. SBs with the largest deviations (>10-fold) from the MS, like many local ULIRGs, are not average SBs, but even rarer events with progenitors having larger gas fractions than typical MS galaxies. We statistically infer that the gas fractions of typical SBs decrease by a factor of 2 to 3 with respect to their direct MS progenitors, as expected to occur in short-term SFR-boosts during which internal gas reservoirs are drained more quickly than gas is accreted from the cosmic web. We predict variations of the CO-to-H2 conversion factor in the SFR-M* plane and provide evidence that the higher sSFR of distant galaxies is a direct consequence of larger gas fraction in these systems.

• The paper identifies a consistent, redshift-independent star formation law that reveals predictable molecular gas behavior across different galaxy types.
• The paper demonstrates that while main sequence galaxies show stable star formation efficiencies, starburst galaxies experience a supra-linear efficiency boost with rapid gas depletion.
• The paper reveals that variations in gas fractions, influenced by metallicity and CO-to-H₂ conversion factors, drive the cosmic evolution of star formation in massive galaxies.

Overview of "Regularity underlying complexity: a redshift-independent description of the continuous variation of galaxy-scale molecular gas properties in the mass-star formation rate plane"

The paper "Regularity underlying complexity: a redshift-independent description of the continuous variation of galaxy-scale molecular gas properties in the mass-star formation rate plane" by Sargent et al., published in the Astrophysical Journal, examines the molecular gas dynamics in star-forming galaxies (SFGs) across different redshifts. It introduces and elaborates on the concept of the "2-Star Formation Mode" (2-SFM) framework, which offers a unified description of star formation efficiencies (SFEs) and molecular gas contents in SFGs.

Key Concepts

• 2-SFM Framework: This approach classifies SFGs into two categories: those on the star-forming main sequence (MS) and starbursting galaxies (SBs). It leverages a double log-normal function to describe the specific star formation rate (sSFR) distribution, distinguishing secular growth in MS galaxies from episodic bursts in SBs.
• Star Formation and Gas Relations: The study derives a universal, redshift-invariant star formation law for massive (M_* > 10^{10} M_☉) MS galaxies, which is slightly supra-linear and remarkably tight (dispersion ~0.2 dex). The integrated Schmidt-Kennicutt (S-K) law linking total star formation rate (SFR) and molecular gas mass (M_gas) is characterized, showing a consistent power-law behavior across cosmic time.

Main Findings

1. Regularity in Molecular Gas Properties: Despite the complexity in individual galaxy properties and evolution, the paper finds simplicity in the overarching behavior of molecular gas. The integrated S-K law maintains a consistent form across a wide range of redshifts, supporting the notion that galaxy-scale molecular properties conform to predictable patterns.
2. Variation in Star Formation Efficiency: For MS galaxies, SFEs show little variation with sSFR, implying a stable efficiency in converting gas to stars. However, for SBs, SFE enhancement is shown to be supra-linear relative to SFR increases, indicating that molecular gas is consumed rapidly during burst phases.
3. Gas Fractions and Evolution: The gas-to-stellar mass ratio (f_mol) in SFGs correlates strongly with sSFR. The continuous rise of gas fractions across the MS suggests that the dispersion in sSFR and its cosmic evolution are primarily modulated by variations in gas content.
4. Starbursts and Molecular Gas Depletion: Starburst galaxies show lower gas fractions compared to their MS progenitors. This depletion occurs despite heightened SFE, due to rapid gas exhaustion before replenishment from the cosmic web is possible.
5. CO-to-H₂ Conversion Factor Variations: The paper investigates empirical recipes for the CO-to-H₂ conversion factor (α_CO), highlighting that it is significantly influenced by metallicity in MS galaxies and declines with increasing SFR boost in SBs.

Implications and Future Directions

The findings reinforce the notion of a coherent structure underlying the complexity of galaxy evolution, where molecular gas dynamics play a central role. As we advance our capacity to observe molecular lines through facilities like ALMA, the framework provided by this paper serves as a foundational model to predict and interpret the molecular gas properties of galaxies. Future work could explore the nuances in these correlations at even higher redshifts and across broader mass ranges, potentially refining the 2-SFM framework. Additionally, unraveling the physical mechanisms driving the supra-linear increase in SFE during starburst phases remains a compelling direction for theoretical and observational efforts.