Constraints from²⁶Al Measurements on the Galaxy's Recent Global Star Formation Rate and Core‐Collapse Supernovae Rate

Gamma rays from the decay of 26 Al offer a stringent constraint on the Galaxy's global star formation rate over the past million years, supplementing other methods for quantifying the recent Galactic star formation rate, such as equivalent widths of Hα emission. Advantages and disadvantages of using 26 Al gamma-ray measurements as a tracer of the massive star formation rate are analyzed. Estimates of the Galactic 26 Al mass derived from COMPTEL measurements are coupled with a simple, analytical model of the 26 Al injection rate from massive stars and restrict the Galaxy's recent star formation rate to 5 ± 4 M ☉ yr -1 . In addition, we show that the derived 26 Al mass implies a present-day Type II + Ib supernovae rate of 3.4 ± 2.8 per century, which seems consistent with other independent estimates of the Galactic core-collapse supernova rate. If some independent measure of the massive star initial mass function or star formation rate or Type II + Ib supernovae rate were to become available (perhaps through estimates of the Galactic 60 Fe mass), then a convenient way to restrain, or possibly determine, the other parameters is presented.