Thickening of galactic discs through clustered star formation

Abstract

The building blocks of galaxies are star clusters. These form with low star formation efficiencies and, consequently, lose a large part of their stars that expand outwards once the residual gas is expelled by the action of the massive stars. Massive star clusters may thus add kinematically hot components to galactic field populations. This kinematical imprint on the stellar distribution function is estimated here by calculating the velocity distribution function for ensembles of star clusters distributed as power-law or log-normal initial cluster mass functions (ICMFs). The resulting stellar velocity distribution function is non-Gaussian and may be interpreted as being composed of multiple kinematical subpopulations. The velocity dispersion of solar-neighbourhood stars increases more rapidly with stellar age than theoretical calculations of orbital diffusion predict. Interpreting this difference to arise from star formation characterized by larger cluster masses, rather than as yet unknown stellar-dynamical heating mechanisms, suggests that the star formation rate in the Milky Way disc has been quietening down, or at least shifting towards less massive star-forming units. Thin-disc stars with ages 3–7 Gyr may have formed from an ICMF extending to very rich Galactic clusters. Stars appear to be forming preferentially in modest embedded clusters during the past 3 Gyr. Applying this approach to the ancient thick disc of the Milky Way, it follows that its large velocity dispersion may have been produced through a high star formation rate and thus an ICMF extending to massive embedded clusters (≈105–6 M⊙), even under the extreme assumption that early star formation occurred in a thin gas-rich disc. This enhanced star formation episode in an early thin Galactic disc could have been triggered by passing satellite galaxies, but direct satellite infall into the disc may not be required for disc heating.