Popping star clusters as building blocks of the Milky Way’s thick disc

Abstract

It is widely believed that star clusters form with low star formation efficiencies. With the onset of stellar winds by massive stars or finally when the first supernova blows off, the residual gas is driven out of the embedded star cluster. Due to this fact, a large number, if not all, of the stars become unbound and disperse in the gravitational potential of the galaxy. In this context, Kroupa suggested a new mechanism for the emergence of thickened galactic discs. Massive star clusters add kinematically hot components to the galactic field populations, building up, in this way, the Galactic thick disc as well. In this work, we perform, for the first time, numerical simulations to investigate this scenario for the formation of the Galactic discs of the Milky Way (MW). We find that a significant kinematically hot population of stars may be injected into the disc of a galaxy such that a thick disc emerges. For the MW, the star clusters that formed the thick disc must have had masses of about 106 M⊙.