An Intermittent Star Formation History in a 'Normal' Disk Galaxy: The Milky Way

The star formation rate history of the Milky Way is derived using the chromospheric age distribution for 552 stars in the solar neighborhood. The stars sample birthsites distributed over a very large range of distances because of orbital diffusion, and so give an estimate of the global star formation rate history. The derivation incorporates the metallicity dependence of chromospheric emission at a given age, and corrections to account for incompleteness, scale height-age correlations, and stellar evolutionary effects. The results show that the Milky Way has undergone fluctuations in the global star formation rate with amplitudes greater than a factor of 2-3 on timescales less than 0.2-1 Gyr. The actual history is likely to be more bursty than found here because of the smearing effect of age uncertainties. There is some evidence for a slow secular increase in the star formation rate, perhaps a record of the accumulation history of our galaxy. A smooth nearly-constant star formation rate history is strongly ruled out, confirming the result first discovered by Barry (1988) using a smaller sample and a different age calibration. This result suggests that galaxies can fluctuate coherently on large scales.