ATLASGAL -- Star forming efficiencies and the Galactic star formation rate

The ATLASGAL survey has characterised the properties of approximately 1000 embedded HII regions and found an empirical relationship between the clump mass and bolometric luminosity that covers 3-4 orders of magnitude. Comparing this relation with simulated clusters drawn from an initial mass function and using different star formation efficiencies we find that a single value is unable to fit the observed luminosity to mass ($L/M$) relation. We have used a Monte Carlo simulation to generate 200,000 clusters using the $L/M$-ratio as a constraint to investigate how the star formation efficiency changes as a function of clump mass. This has revealed that the star formation efficiency decreases with increasing clump mass with a value of 0.2 for clumps with masses of a few hundred solar masses and dropping to 0.08 for clumps with masses of a few thousand solar masses. We find good agreement between our results and star formation efficiencies determined from counts of embedded objects in nearby molecular clouds. Using the star formation efficiency relationship and the infrared excess time for embedded star formation of $2\pm1$, Myr we estimate the Galactic star formation rate to be approximately $0.9\pm0.45$ Msun yr$^{-1}$, which is in good agreement with previously reported values. This model has the advantage of providing a direct means of determining the star formation rate and avoids the difficulties encountered in converting infrared luminosities to stellar mass that affect previous galactic and extragalactic studies.