Steady Outflows in Giant Clumps of High-z Disk Galaxies During Migration and Growth by Accretion

TL;DR

This paper predicts that giant clumps in high-redshift disk galaxies experience steady, wind-driven outflows over many tens of millions of years, with implications for their evolution and mass growth during migration.

Contribution

It provides a theoretical model for steady outflows in giant clumps, incorporating multiple feedback mechanisms and predicting their impact on clump evolution and migration.

Findings

Outflows are steady over tens of Myr, not explosive.

Mass-loading factor is approximately unity, with clumps growing via accretion.

Most observed outflows are consistent with model predictions.

Abstract

We predict the evolution of giant clumps undergoing star-driven outflows in high-z gravitationally unstable disk galaxies. We find that the mass loss is expected to occur through a steady wind over many tens of free-fall times (t_ff ~ 10 Myr) rather than by an explosive disruption in one or a few t_ff. 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 timescale would have been a few disk orbital times, to end with half the original clump mass in stars. However, the clump migration time to the disk center is on 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 disk orbital time by accretion from the disk and clump-clump mergers, so their mass actually grows in time and with decreasing radius. From the 6-7 giant clumps with observed outflows, 5 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 1-sigma error), that the SFR 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.