A Molecular Star Formation Law in the Atomic Gas Dominated Regime in Nearby Galaxies (1105.4605v2)

Abstract: We use the IRAM HERACLES survey to study CO emission from 33 nearby spiral galaxies down to very low intensities. Using atomic hydrogen (HI) data, mostly from THINGS, we predict the local mean CO velocity from the mean HI velocity. By renormalizing the CO velocity axis so that zero corresponds to the local mean HI velocity we are able to stack spectra coherently over large regions as function of radius. This enables us to measure CO intensities with high significance as low as Ico = 0.3 K km/s (H2_SD = 1 Msun/pc2), an improvement of about one order of magnitude over previous studies. We detect CO out to radii Rgal = R25 and find the CO radial profile to follow a uniform exponential decline with scale length of 0.2 R25. Comparing our sensitive CO profiles to matched profiles of HI, Halpha, FUV, and IR emission at 24um and 70um, we observe a tight, roughly linear relation between CO and IR intensity that does not show any notable break between regions that are dominated by molecular (H2) gas (H2_SD > HI_SD) and those dominated by atomic gas (H2_SD < HI_SD). We use combinations of FUV+24um and Halpha+24um to estimate the recent star formation rate (SFR) surface density, SFR_SD, and find approximately linear relations between SFR_SD and H2_SD. We interpret this as evidence for stars forming in molecular gas with little dependence on the local total gas surface density. While galaxies display small internal variations in the SFR-to-H2 ratio, we do observe systematic galaxy-to-galaxy variations. These galaxy-to-galaxy variations dominate the scatter in relations between CO and SFR tracers measured at large scales. The variations have the sense that less massive galaxies exhibit larger ratios of SFR-to-CO than massive galaxies. Unlike the SFR-to-CO ratio, the balance between HI and H2 depends strongly on the total gas surface density and radius. It must also depend on additional parameters.

• The paper introduces a novel stacking technique using HI velocity predictions to reduce the CO detection threshold by an order of magnitude.
• The paper reveals a linear relationship between star formation rate and molecular gas surface density, consistent across atomic and molecular gas regimes.
• The paper reports significant galaxy-to-galaxy variations, with lower mass galaxies showing higher star formation efficiencies compared to more massive spirals.

The Molecular Star Formation Law in the Atomic Gas Dominated Regime

The paper by Schruba et al. explores the assessment of the molecular star formation laws within atomic gas-dominated environments in nearby spiral galaxies. By using data from extensive astronomical surveys, including the IRAM HERACLES survey for CO (carbon monoxide) emission and the THINGS survey for atomic hydrogen (HI), the authors analyze star formation processes across a wide spectrum of galactic environments.

Study Approach and Methodology

The paper examines CO emissions from 33 nearby spiral galaxies to low intensities. Utilizing a novel methodology, the researchers predict the local mean CO velocity via HI velocity, allowing coherent stacking of spectra over large areas. This technique significantly lowers the CO intensity detection threshold to $I_{\rm CO} \approx 0.3$ K km s$^{-1}$ (corresponding to $\Sigma_{\rm H2} \approx 1$ M$_{\odot}$ pc$^{-2}$) - an order of magnitude improvement over previous abilities.

Key Findings

1. Radial CO Profiles: The paper finds that CO radial profiles display a consistent exponential decline with a scale length of approximately $0.2~r_{25}$. This uniformity in decline suggests a continuous decrease in molecular gas supply, which impacts star formation rates across different regions of a galaxy.
2. Star Formation and Molecular Gas: A roughly linear relationship exists between the star formation rate (SFR) surface density ($\Sigma_{\rm SFR}$) and the molecular gas surface density ($\Sigma_{\rm H2}$). This relationship persists in both molecular gas-dominated and atomic gas-dominated regions, indicating molecular gas to be a primary component for star formation regardless of the local gas type.
3. Galaxy Variability: While minor variations across internal SFR-to-H$_2$ ratios were observed, significant galaxy-to-galaxy changes were noted, with less massive galaxies demonstrating higher SFR-to-CO ratios compared to their massive counterparts.
4. Interplay Between Atomic and Molecular Gas: The shift between atomic and molecular gases correlates strongly with the total surface gas density and radial positioning within galaxies, suggesting additional parameters affecting this balance.
5. Implications for Star Formation Laws: The research supports the notion that star formation in galaxies can be broken into two distinct processes: the aggregation of molecular clouds and the conversion of H$_2$ into stars. These findings reinforce observed gas-SFR relations with variable slopes and previously identified star formation thresholds.

Implications and Future Prospects

The findings have significant implications for theoretical models of galaxy evolution and star formation. The paper affirms the critical role of molecular gas in star formation, extending observations to lower density regimes not previously explored effectively. The implications of these findings suggest that future observational efforts and models should incorporate both the aggregation of molecular material and star formation efficiencies when exploring galactic evolution processes.

Future research may continue to refine the understanding of molecular-to-atomic gas transition regions and quantify the variability in star formation efficiency across different galactic environments. These efforts will be pivotal in advancing astrophysical models and understanding how galaxies convert gas into stars across cosmic timescales.