Measuring Star Formation Rates in the Milky Way from Hi-GAL 70 <i>μ</i>m 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 μ m emission, commonly used in other galaxies, with a technique to assign distances to the extended emission, predicts <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>1.4</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.44</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.63</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> 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 μ m 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.