High-velocity outflows from young star-forming galaxies in the UKIDSS Ultra-Deep Survey

We investigate galactic-scale outflows in the redshift range 0.71 z 1.63, using 413 K-band selected galaxies observed in the spectroscopic follow-up of the UKIDSS Ultra-Deep Survey (UDSz). The galaxies have an average stellar mass of 10 9.5 M and span a wide range in restframe colours, representing typical star-forming galaxies at this epoch. We stack the spectra by various galaxy properties, including stellar mass, [O II] equivalent width, star formation rate, specific star formation rate and rest-frame spectral indices. We find that outflows are present in virtually all spectral stacks, with velocities ranging from 100 to 1000 km s -1 , indicating that large-scale outflowing winds are a common property at these redshifts. The highest velocity outflows (>500 km s -1 ) are found in galaxies with the highest stellar masses and the youngest stellar populations. Our findings suggest that high-velocity galactic outflows are mostly driven by star-forming processes rather than active galactic nuclei, with implied mass outflow rates comparable to the rates of star formation. Such behaviour is consistent with models required to reproduce the high-redshift mass-metallicity relation.