Variations in the Galactic star formation rate and density thresholds for star formation

Abstract: The conversion of gas into stars is a fundamental process in astrophysics and cosmology. Stars are known to form from the gravitational collapse of dense clumps in interstellar molecular clouds, and it has been proposed that the resulting star formation rate is proportional to either the amount of mass above a threshold gas surface density, or the gas volume density. These star-formation prescriptions appear to hold in nearby molecular clouds in our Milky Way Galaxy's disk as well as in distant galaxies where the star formation rates are often much larger. The inner 500 pc of our Galaxy, the Central Molecular Zone (CMZ), contains the largest concentration of dense, high-surface density molecular gas in the Milky Way, providing an environment where the validity of star-formation prescriptions can be tested. Here we show that by several measures, the current star formation rate in the CMZ is an order-of-magnitude lower than the rates predicted by the currently accepted prescriptions. In particular, the region 1 deg < l < 3.5 deg, |b| < 0.5 deg contains ~10^7 Msun of dense molecular gas -- enough to form 1000 Orion-like clusters -- but the present-day star formation rate within this gas is only equivalent to that in Orion. In addition to density, another property of molecular clouds, such as the amplitude of turbulent motions, must be included in the star-formation prescription to predict the star formation rate in a given mass of molecular gas.

• The paper demonstrates that the observed star formation rate in the Galactic Center is roughly an order of magnitude lower than predicted by standard Schmidt-Kennicutt relations.
• It employs a comparative analysis of the CMZ and Galactic disk using extensive survey data to challenge traditional density thresholds in star formation models.
• The study proposes that turbulence and unique local environmental conditions suppress star formation, calling for refined models to better predict stellar birth rates.

Variations in the Galactic Star Formation Rate and Density Thresholds for Star Formation

Longmore et al. investigate the observed variations in the star formation rate (SFR) across the Milky Way, focusing on dense molecular gas regions and their correlation to established star formation models. The study hinges on a comparative analysis of the Galactic Central Molecular Zone (CMZ) and the Galactic disk to assess the adequacy of proposed star formation prescriptions, particularly the Schmidt-Kennicutt (SK) relations and volumetric star formation theories.

The research provides a detailed examination of the SFR in the CMZ, characterized by high concentrations of dense gas within the inner 500 pc of the Galaxy. Despite the abundance of dense molecular gas in the CMZ, the current SFR is conspicuously lower than predictions based on standard SK relations. The authors establish that the SFR in this region is roughly an order of magnitude lower than theoretically expected, prompting a re-evaluation of the conditions necessary for star formation beyond the conventional density thresholds.

Here are several critical points from the paper:

1. Discrepancy in SFR Predictions:
   • The CMZ houses approximately $10^7$ M$_\odot$ of dense molecular gas, potentially forming 1000 Orion-like clusters. Yet, the observed SFR matches only that of a single Orion cluster, contradicting SK-based predictions.
2. Potential Influence of Turbulence:
   • The authors suggest that parameters such as turbulent motion amplitudes, not traditionally included in star formation models, could impact the SFR. This observation highlights a critical area where current models may be insufficiently comprehensive.
3. Impact of Environmental Conditions:
   • The study emphasizes the significance of local environmental characteristics specific to the CMZ, such as its unique gas dynamics, which might artificially suppress the SFR despite an abundance of resources.
4. Observational Consistency:
   • Using extensive survey data (HOPS, HiGAL), the study confirms that observational biases do not account for the SFR discrepancy, thereby reinforcing the need to explore theoretical modifications.

The implications of these findings are twofold. Practically, they call for updated models incorporating factors like turbulence to accurately predict star formation in regions with non-standard environmental conditions. Theoretically, this study provides grounds for reconsidering the universality of current star formation laws, which may not sufficiently account for specific galactic components such as the CMZ.

Expected future developments in this field might involve refining the integration of turbulence-related metrics and environmental factors into star formation models. As observational capabilities advance, it is prudent to anticipate further observational tests of these revised models on a galactic and extragalactic scale, potentially recalibrating the assumptions underlying star formation theories across diverse astrophysical contexts.