Thickening of galactic disks through clustered star formation

The building blocks of galaxies are star clusters. These form with low-star formation efficiencies and, consequently, loose 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 sub-populations. 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 characterised by larger cluster masses, rather than as yet unknown stellar-dynamical heating mechanisms, suggests that the star formation rate in the MW disk has been quietening down, or at least shifting towards less-massive star-forming units. Thin-disk 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 disk 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 (≈ 10M⊙), even under the extreme assumption that early star formation occurred in a thin gas-rich disk. This enhanced star-formation episode in an early thin Galactic disk could have been triggered by passing satellite galaxies, but direct satellite infall into the disk may not be required for disk heating.