Molecular Gas and Star Formation in Nearby Disk Galaxies (1301.2328v1)

Abstract: We compare molecular gas traced by 12CO(2-1) maps from the HERACLES survey, with tracers of the recent star formation rate (SFR) across 30 nearby disk galaxies. We demonstrate a first-order linear correspondence between Sig_mol and Sig_SFR but also find important second-order systematic variations in the apparent molecular gas depletion time, t_dep^mol = Sig_mol / Sig_SFR. At our 1 kpc common resolution, CO correlates closely with many tracers of the recent SFR. Weighting each line of sight equally and using a fixed, Milky Way alpha_CO, our data yield a molecular gas depletion time, t_dep^mol=Sig_mol/Sig_SFR ~ 2.2 Gyr with 0.3 dex scatter, in good agreement with literature data. We apply a forward-modeling approach to constrain the power-law index, N, that relates the SFR surface density and the molecular gas surface density and find N=1+/-0.15 for our full data set with some variation from galaxy to galaxy. However, we caution that a power law treatment oversimplifies the topic given that we observe correlations between t_dep^mol and other local and global quantities. The strongest of these are a decreased t_dep^mol in low-mass, low-metallicity galaxies and a correlation of the kpc-scale t_dep^mol with dust-to-gas ratio, D/G. These correlations can be explained by a CO-to-H2 conversion factor that depends on D/G in the theoretically expected way. This is not a unique interpretation, but external evidence of conversion factor variations makes it a conservative one. After applying a D/G-dependent alpha_CO, some weak correlations between t_dep^mol and local conditions persist. In particular, we observe lower t_dep^mol and enhanced CO excitation associated with some nuclear gas concentrations. These appear to reflect real enhancements in the SFR/H2 and t_dep appears multivalued at fixed Sig_mol, supporting the the idea of "disk" and "starburst" modes driven by environmental factors.

• The paper quantifies a linear relationship between molecular gas surface density and star formation rate density, revealing a typical depletion time of about 2.2 Gyr.
• It employs CO(2-1) data from the HERACLES survey and star formation tracers to analyze variations in star formation efficiency across different galactic environments.
• The study shows that adjusting the CO-to-H2 conversion factor for local conditions, such as metallicity and dust content, reconciles second-order discrepancies in star formation efficiency.

Overview of Molecular Gas and Star Formation in Nearby Disk Galaxies

This essay explores the paper conducted by Leroy et al., which investigates the intricate relationship between molecular gas and star formation in 30 nearby disk galaxies. Utilizing CO(2-1) data from the HERACLES survey alongside various tracers of recent star formation, the authors aim to understand how molecular gas density correlates with star formation rate density on a kiloparsec scale.

Key Findings and Analysis

The paper reaffirms a first-order linear relationship between the molecular gas surface density, $\Sigma_{\rm mol}$, and the star formation rate surface density, $\Sigma_{\rm SFR}$. This correlation is characterized by a typical molecular gas depletion time, $\tau_{\rm dep}^{\rm mol}$, of approximately 2.2 Gyr with a scatter of 0.3 dex, assuming a constant CO-to-H$_2$ conversion factor, $\alpha_{\rm CO}$, equivalent to that of the Milky Way. These results are consistent with recent findings that suggest star formation is regulated by the molecular gas reservoir in disk galaxies.

However, the paper also uncovers significant second-order variations in $\tau_{\rm dep}^{\rm mol}$ across different galaxies and within certain galactic regions. Notably, low-mass and low-metallicity galaxies, as well as central galactic regions with concentrated molecular gas, display shorter depletion times, indicating enhanced star formation efficiencies in these environments.

Implications and Conversion Factors

The authors suggest that variations in the CO-to-H$_2$ conversion factor, $\alpha_{\rm CO}$, which is influenced by factors such as dust-to-gas ratio, can partially explain the observed differences in star formation efficiency. For example, galaxies with lower metallicities—and thus lower dust-to-gas ratios—appear to require higher $\alpha_{\rm CO}$ values to maintain consistency with a constant $\tau_{\rm dep}^{\rm mol}$.

By adopting a $\alpha_{\rm CO}$ that scales with dust shielding and the local environment, the paper is able to reconcile much of the scatter observed in $\tau_{\rm dep}^{\rm mol}$ across the sample, highlighting the necessity of accounting for such environmental dependencies in understanding star formation.

Future Prospects

Insights from this paper underscore the complexity of star formation processes in varying galactic contexts. To further dissect the nuanced differences in $\tau_{\rm dep}^{\rm mol}$, future investigations should incorporate more diverse samples and improved spatial resolution. Additionally, integrating advanced tracers of molecular gas, beyond CO, could refine understanding of how different galactic conditions influence star formation.

Moreover, exploring the interplay between molecular gas and other galactic features, such as stellar feedback and large-scale dynamics, will be essential. As astronomical instrumentation advances, enabling more precise simulations and observations, our understanding of the star formation process in various cosmic contexts is poised to deepen significantly.

In summary, the work by Leroy et al. represents a significant step in disentangling the relationship between molecular gas and star formation within disk galaxies. Through a nuanced approach to quantifying molecular gas depletion times and considering variable conversion factors, it provides a robust framework for analyzing star formation efficiency across different galactic environments.