A Constant Molecular Gas Depletion Time in Nearby Disk Galaxies (1102.1720v1)

Abstract: We combine new sensitive, wide-field CO data from the HERACLES survey with ultraviolet and infrared data from GALEX and Spitzer to compare the surface densities of H2, Sigma_H2, and recent star formation rate, Sigma_SFR, over many thousands of positions in 30 nearby disk galaxies. We more than quadruple the size of the galaxy sample compared to previous work and include targets with a wide range of galaxy properties. Even though the disk galaxies in this study span a wide range of properties, we find a strong and approximately linear correlation between Sigma_SFR and Sigma_H2 at our common resolution of 1kpc. This implies a roughly constant median H2 consumption time, tau_H2 = Sigma_H2 / Sigma_SFR, of ~2.35Gyr (including heavy elements) across our sample. At 1kpc resolution, there is only a weak correlation between Sigma_H2 and tau_H2 over the range Sigma_H2~5-100M_sun/pc^2, which is probed by our data. We compile a broad set of literature measurements that have been obtained using a variety of star formation tracers, sampling schemes and physical scales and show that overall, these data yield almost exactly the same results, although with more scatter. We interpret these results as strong, albeit indirect evidence that star formation proceeds in a uniform way in giant molecular clouds in the disks of spiral galaxies.

• The paper reveals that the molecular gas depletion time is consistently around 2.35 Gyr across diverse disk galaxies.
• It leverages comprehensive CO, UV, and IR datasets to correlate molecular gas surface densities with star formation rates.
• The constant depletion time suggests uniform conditions in giant molecular clouds, supporting homogeneous star formation models.

Summary on Molecular Gas Depletion Time in Disk Galaxies

The paper "A Constant Molecular Gas Depletion Time in Nearby Disk Galaxies" by F. Bigiel and colleagues offers significant insights into the relationship between molecular gas surface densities and recent star formation rates in nearby disk galaxies. By leveraging a dataset that encompasses a more extensive and diverse range of galaxies than previous studies, the authors seek to reinforce the notion of uniform star formation processes within giant molecular clouds (GMCs).

Key Findings

The paper taps into CO data from the HERACLES survey combined with ultraviolet (UV) and infrared (IR) data from GALEX and Spitzer telescopes to map the correlation between molecular gas ($\Sigma_{\rm H2}$) and star formation rate ($\Sigma_{\rm SFR}$) surface densities across numerous positions in 30 nearby galaxies. A characteristic feature of the data is its consistency in terms of molecular gas depletion time, where the median hydrogen molecule (H$_2$) consumption time, $$\tau_{\rm Dep}^{\rm H2} = \Sigma_{\rm H2}/\Sigma_{\rm SFR}$$, is observed to be approximately 2.35 Gyr. This timeframe encompasses heavy elements and remains constant across a range of surface densities: $\Sigma_{\rm H2} \approx 5$–100 M$_\odot$ pc$^{-2}$.

Contrary to what might be expected, the correlation between $\Sigma_{\rm H2}$ and $\tau_{\rm Dep}^{\rm H2}$ is weak, implying a near-uniform consumption time of molecular gas irrespective of its surface density level. These findings suggest that star formation in the disk galaxies occurs in a largely consistent manner, echoing a uniformity in the physical conditions and processes in GMCs.

Implications

The demonstrated consistency in galactic molecular gas depletion times has several implications. Practically, it indicates a uniformity in the conditions and efficiency of star formation across different environments within galaxies. Theoretically, it lends credence to models that assume a homogeneous model for star-forming regions in galaxies.

The linear correlation found between $\Sigma_{\rm SFR}$ and $\Sigma_{\rm H2}$ aligns with previous studies, reinforcing the idea that the star formation efficiency per unit of molecular gas remains constant within these environments. This constancy could facilitate more precise predictions in galaxy evolution models about where and how stars will form based on available molecular gas reservoirs.

Future Developments

While the results underline a prevalent pattern, there is scope for future work to explore deviations at higher $\Sigma_{\rm H2}$ or examine conditions in more extreme environments such as those in luminous infrared galaxies or ultra-luminous infrared galaxies. Additionally, direct observations and simulations of individual GMCs could further clarify the microphysics that uphold the observed constancy in these star formation processes.

In conclusion, this research draws a consistent picture of star formation in disk galaxies, with implications reaching far into how galactic disks assemble over time. Such consistency holds potential for advances in the field of astrophysics, specifically in refining models of galactic evolution and star formation.