Really Cool Stars and the Star Formation History at the Galactic Center

We present λ /Δ λ = 550 to 1200 near infrared H and K spectra for a magnitude limited sample of 79 asymptotic giant branch and cool supergiant stars in the central ≈5 pc (diameter) of the Galaxy. Using a set of similar spectra obtained for solar neighborhood stars with the same range in T eff and M bol for the Galactic center (GC) sample as calibrators, we construct the Hertzsprung–Russell diagram for the GC sample. Using an automated maximum likelihood routine, we derive a coarse star formation history of the GC. We find that roughly 75 % of the stars that have formed in the central few pc are older than 5 Gyr. In particular, the best fitting star formation history yields star formation rates in four age bins of 0.01 to 0.10 Gyr, 0.10 to 1.0 Gyr, 1.0 to 5.0 Gyr, and 5 Gyr to 12 Gyr, of 39.5 ± 11 × 10 −4 M ⊙ yr −1 , 3.6 ± 2 × 10 −4 M ⊙ yr −1 , 5.7 ± 2 × 10 −4 M ⊙ yr −1 , and 13.4 ± 5 × 10 −4 M ⊙ yr −1 , respectively. The GC has been forming stars over the last 100 Myr at a rate roughly four times higher than the average rate over 12 Gyr, though the total mass formed in this recent epoch is less than three percent of the total mass formed over all times.