Steady outflows in giant clumps of high-z disc galaxies during migration and growth by accretion

Abstract

We predict the evolution of giant clumps undergoing star-driven outflows in high-z gravitationally unstable disc galaxies. We find that the mass-loss is expected to occur through a steady wind over many tens of free-fall times (tff ∼ 10 Myr) rather than by an explosive disruption in one or a few tff. Our analysis is based on the finding from simulations that radiation trapping is negligible because it destabilizes the wind (Krumholz & Thompson 2012, 2013). Each photon can therefore contribute to the wind momentum only once, so the radiative force is limited to L/c. When combining radiation, protostellar and main-sequence winds, and supernovae, we estimate the total direct injection rate of momentum into the outflow to be 2.5 L/c. The adiabatic phase of supernovae and main-sequence winds can double this rate. The resulting outflow mass-loading factor is of order unity, and if the clumps were to deplete their gas, the time-scale would have been a few disc orbital times, to end with half the original clump mass in stars. However, the clump migration time to the disc centre is of the order of an orbital time, about 250 Myr, so the clumps are expected to complete their migration prior to depletion. Furthermore, the clumps are expected to double their mass in a disc orbital time by accretion from the disc and clump–clump mergers, so their mass actually grows in time and with decreasing radius. From the six to seven giant clumps with observed outflows, five are consistent with these predictions, and one has a much higher mass-loading factor and momentum injection rate. The latter either indicates that the estimated outflow is an overestimate (within the 1σ error), that the star formation rate has dropped since the time when the outflow was launched or that the driving mechanism is different, e.g. supernova feedback in a cavity generated by the other feedbacks.