The State of the Gas and the Relation Between Gas and Star Formation at Low Metallicity: the Small Magellanic Cloud

Abstract: We compare atomic gas, molecular gas, and the recent star formation rate (SFR) inferred from H-alpha in the Small Magellanic Cloud (SMC). By using infrared dust emission and local dust-to-gas ratios, we construct a map of molecular gas that is independent of CO emission. This allows us to disentangle conversion factor effects from the impact of metallicity on the formation and star formation efficiency of molecular gas. On scales of 200 pc to 1 kpc we find a characteristic molecular gas depletion time of ~1.6 Gyr, similar to that observed in the molecule-rich parts of large spiral galaxies on similar spatial scales. This depletion time shortens on much larger scales to ~0.6 Gyr because of the presence of a diffuse H-alpha component, and lengthens on much smaller scales to ~7.5 Gyr because the H-alpha and H2 distributions differ in detail. We estimate the systematic uncertainties in our measurement to be a factor of 2-3. We suggest that the impact of metallicity on the physics of star formation in molecular gas has at most this magnitude. The relation between SFR and neutral (H2+HI) gas surface density is steep, with a power-law index ~2.2+/-0.1, similar to that observed in the outer disks of large spiral galaxies. At a fixed total gas surface density the SMC has a 5-10 times lower molecular gas fraction (and star formation rate) than large spiral galaxies. We explore the ability of the recent models by Krumholz et al. (2009) and Ostriker et al. (2010) to reproduce our observations. We find that to explain our data at all spatial scales requires a low fraction of cold, gravitationally-bound gas in the SMC. We explore a combined model that incorporates both large scale thermal and dynamical equilibrium and cloud-scale photodissociation region structure and find that it reproduces our data well, as well as predicting a fraction of cold atomic gas very similar to that observed in the SMC.

Analyzing Gas and Star Formation in the Small Magellanic Cloud at Low Metallicity

The study conducted by Bolatto et al. focuses on the relationship between gas compositions and star formation activities in the Small Magellanic Cloud (SMC), particularly emphasizing the effects of low metallicity. This intricate analysis disentangles molecular gas properties from atomic gas distributions, utilizing sophisticated methods including far-infrared dust emission data for deriving molecular gas content independent of traditional CO emission observations.

Molecular Gas and Star Formation

The study reveals that the SMC's molecular gas depletion time, $\tau_{\text{dep}}$, at resolutions ranging from 200 pc to 1 kpc, is approximately 1.6 Gyr, aligning with values observed in dense parts of spiral galaxies. At larger scales, this depletion time is reduced to about 0.6 Gyr, yet extends to nearly 7.5 Gyr at smaller scales due to distribution differences between molecular gas and star formation. This highlights the relative inefficiency of star-forming processes in the SMC compared to spirals, whereas the mechanism within molecular gas seems consistent across different environments and is marginally impacted by metallicity variations.

Total Gas Density and Star Formation

Distinct from the molecular gas findings, the correlation between total gas density (i.e., combined molecular and atomic gas) and star formation rates in the SMC presents a steep slope, implying intensively higher total gas surface densities are needed to achieve star formation levels seen in large spirals. This observation suggests that the molecular fraction and consequently star formation in the SMC is substantially lower, attributed to both the high atomic gas surface density and low conversion efficiency from atomic to molecular gas.

Theoretical Modeling and Implications

The research evaluates two theoretical frameworks—KMT09 and OML10—against its empirical findings. KMT09 suggests that star formation is heavily tied to molecular gas content and accurately reflects observed molecular depletion times. However, it assumes cold atomic envelopes around molecular clouds, which isn't supported in the SMC context. Conversely, OML10 underscores the significance of thermal and dynamical equilibrium influencing gas partitioning into diffuse and gravitationally-bound phases. The study postulates a modified OML10 model (OML10h) where enhanced FUV photon escape at lower metallicities raises the diffuse phase pressure, explaining discrepancies seen in standard models about the high atomic gas surface density in the SMC.

Future Directions and Concluding Remarks

The research posits an intriguing avenue for understanding star formation dynamics in low-metallicity environments, suggesting that while star formation efficiency in molecular gas appears resilient to metallicity changes, atomic-to-molecular gas processes are indeed sensitive to metallicity. Further observations, particularly of diffuse ISM cooling mechanisms and extensive surveys of cold atomic gas structures, may yield a deeper understanding of these complex interactions. This work not only enriches existing star formation theories by juxtaposing empirical data from a chemically primitive context but also challenges researchers to refine models that account for varied environmental conditions across different galaxy types.