Delayed star formation in high-redshift stream-fed galaxies

We propose that star formation (SF) is delayed relative to the inflow rate in rapidly accreting galaxies at very high redshift (z > 2) because of the energy conveyed by the accreting gas. Accreting gas streams provide fuel for SF, but they stir the disc and increase turbulence above the usual levels compatible with gravitational instability, reducing the SF efficiency in the available gas. After the specific inflow rate has sufficiently decreased - typically at z < 3 - galaxies settle in a self-regulated regime with efficient SF. An analytic model shows that this interaction between infalling gas and young galaxies can significantly delay SF and maintain high gas fractions (> 40 per cent) down to z approximate to 2, in contrast to other galaxy formation models. Idealized hydrodynamic simulations of infalling gas streams on to primordial galaxies confirm the efficient energetic coupling at z > 2 and suggest that this effect is largely under-resolved in existing cosmological simulations.