COLD GASS, an IRAM Legacy Survey of Molecular Gas in Massive Galaxies: II. The non-universality of the Molecular Gas Depletion Timescale

Abstract: We study the relation between molecular gas and star formation in a volume-limited sample of 222 galaxies from the COLD GASS survey, with measurements of the CO(1-0) line from the IRAM 30m telescope. The galaxies are at redshifts 0.025<z<0.05 and have stellar masses in the range 10.0<log(M*/Msun)<11.5. The IRAM measurements are complemented by deep Arecibo HI observations and homogeneous SDSS and GALEX photometry. A reference sample that includes both UV and far-IR data is used to calibrate our estimates of star formation rates from the seven optical/UV bands. The mean molecular gas depletion timescale, tdep(H2), for all the galaxies in our sample is 1 Gyr, however tdep(H2) increases by a factor of 6 from a value of ~0.5 Gyr for galaxies with stellar masses of 10^10 Msun to ~3 Gyr for galaxies with masses of a few times 10^11 Msun. In contrast, the atomic gas depletion timescale remains contant at a value of around 3 Gyr. This implies that in high mass galaxies, molecular and atomic gas depletion timescales are comparable, but in low mass galaxies, molecular gas is being consumed much more quickly than atomic gas. The strongest dependences of tdep(H2) are on the stellar mass of the galaxy (parameterized as log tdep(H2)= (0.36+/-0.07)(log M* - 10.70)+(9.03+/-0.99)), and on the specific star formation rate. A single tdep(H2) versus sSFR relation is able to fit both "normal" star-forming galaxies in our COLD GASS sample, as well as more extreme starburst galaxies (LIRGs and ULIRGs), which have tdep(H2) < 10^8 yr. Normal galaxies at z=1-2 are displaced with respect to the local galaxy population in the tdep(H2) versus sSFR plane and have molecular gas depletion times that are a factor of 3-5 times longer at a given value of sSFR due to their significantly larger gas fractions.

• The paper reveals that molecular gas depletion times vary with galaxy mass and specific star formation rates, challenging the notion of a universal timescale.
• It uses CO line measurements from the IRAM 30m telescope combined with HI and optical data to precisely quantify star formation efficiency.
• The findings encourage a re-evaluation of galaxy evolution models, showing that star formation efficiency is influenced by intrinsic galactic properties.

Overview of "COLD GASS: Variable Molecular Gas Depletion Times"

The paper entitled "COLD GASS, an IRAM Legacy Survey of Molecular Gas in Massive Galaxies: II. The Non-Universality of the Molecular Gas Depletion Timescale" provides an in-depth analysis of the relation between molecular gas content and star formation in a diverse sample of 222 galaxies. Using CO line measurements from the IRAM 30m telescope, complemented by HI observations and SDSS/GALEX photometry, the study examines star formation efficiencies and challenges the notion of a universal molecular gas depletion timescale.

Key Findings

The study highlights several critical points about molecular gas depletion timescales (dep) across galaxies and their varying relationship with other galaxy properties:

1. Molecular Gas Depletion Timescale: The mean dep for the surveyed galaxies is approximately 1 Gyr. However, dep is found to increase significantly with stellar mass, from about 0.5 Gyr in lower mass galaxies ($\sim 10^{10} M_{\odot}$) to roughly 3 Gyr in more massive systems (several $\times 10^{11} M_{\odot}$).
2. Atomic vs. Molecular Gas Depletion: Unlike molecular gas, the atomic gas depletion timescale remains constant at about 3 Gyr across the mass spectrum, implying that molecular gas is depleted much faster in lower mass galaxies compared to atomic gas.
3. Strong Predictors of dep: Stellar mass and specific star formation rate (sSFR) emerge as the key predictors of dep. For instance, a clear correlation exists with sSFR, where dep decreases with increasing sSFR.
4. Implications of sSFR and Mass: The paper presents a robust relationship between dep and both stellar mass and sSFR, hinting at intrinsic variations in star formation efficiency, possibly due to different star formation triggers or feedback mechanisms across galaxies.
5. Comparison with Other Studies: Contrasts with previous work from the HERACLES survey are made, emphasizing the broader dynamic range and unbiased selection of COLD GASS. Both studies agree that galaxies exhibit varied star formation efficiencies but highlight that COLD GASS uncovers a more complex and non-uniform picture of gas processing in galaxies.

Theoretical and Practical Implications

The findings of this study challenge the universality of the Kennicutt-Schmidt relation at the molecular level and suggest that the conversion of molecular gas to stars is subject to additional factors, such as galaxy mass and evolutionary state. These results have broad implications for models of galaxy evolution, indicating that star formation efficiency varies more significantly than previously considered, especially when viewed over a diverse range of galaxy types.

From a practical perspective, the variations in dep should prompt a re-evaluation of how galaxy star formation rates are modeled in simulations. These insights are particularly crucial for understanding galaxy growth and evolution over cosmic time. Future observations, potentially aided by facilities like ALMA, could further elucidate the physical processes at play, especially at higher redshifts where conditions differ markedly from the local universe.

Conclusion

Overall, the COLD GASS survey enriches our understanding of molecular gas dynamics in galaxies. By demonstrating the non-universal nature of molecular gas depletion timescales and identifying key correlating factors, this paper sets the stage for more nuanced investigations into the star formation processes governing galaxy evolution. Further research could explore the interplay between molecular gas kinetics, star formation triggers, and feedback mechanisms across different galactic environments.