Measuring Star Formation Rates in the Milky Way from Hi-GAL 70 μm Observations

Abstract

Three methods for computing the total star formation rate (SFR) of the Milky Way agree well with a reference value of 1.65 ± 0.19 M⊙ yr−1. They are then used to determine the radial dependence of the SFR and face-on map for the Milky Way. First, the method based on a model of star formation in Hi-GAL-defined dense clumps, adjusted for an increase in the gas-to-dust ratio with Galactocentric radius, predicts 1.65 ± 0.61 M⊙ yr−1. Second, the method using the 70 μm emission, commonly used in other galaxies, with a technique to assign distances to the extended emission, predicts M⊙ yr−1. Finally, a method based on theoretical predictions of star formation efficiency as a function of virial parameter, with masses corrected for metallicity dependence, applied to a catalog of molecular clouds also predicts a value in agreement at 1.47 M⊙ yr−1. The three methods predict the radial variation of the SFR, with remarkably good agreement from the Central Molecular Zone out to about 20 kpc. More differences were seen in face-on maps with a resolution of 0.5 kpc made with the three approaches and in comparisons to the local (within 3 kpc) SFR, indicating limitations of the methods when applied to smaller scales. The 70 μm SFR follows very closely the surface density of molecular gas, corrected for a metallicity-dependent CO conversion factor. A molecular gas depletion time of 1 Gyr is consistent with the data, as is a molecular Kennicutt–Schmidt relation with a power-law slope of 1.10 ± 0.06.