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

Abstract

We predict the evolution of giant clumps undergoing star-driven outflows in high-z gravitationallyunstable disc galaxies.We find that themass-loss is expected to occur through a steadywind over many tens of free-fall times (tff ~ 10 Myr) rather than by an explosive disruption inone or a few tff. Our analysis is based on the finding from simulations that radiation trapping isnegligible because it destabilizes the wind (Krumholz & Thompson 2012, 2013). Each photoncan therefore contribute to the wind momentum only once, so the radiative force is limitedto 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. Theadiabatic phase of supernovae and main-sequence winds can double this rate. The resultingoutflow mass-loading factor is of order unity, and if the clumps were to deplete their gas, thetime-scale would have been a few disc orbital times, to end with half the original clump massin 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 fromthe disc and clump-clump mergers, so their mass actually grows in time and with decreasingradius. From the six to seven giant clumps with observed outflows, five are consistent withthese predictions, and one has a much higher mass-loading factor and momentum injectionrate. The latter either indicates that the estimated outflow is an overestimate (within the 1serror), that the star formation rate has dropped since the time when the outflow was launchedor that the driving mechanism is different, e.g. supernova feedback in a cavity generated bythe other feedbacks. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.