The energetics of starburst-driven outflows at z ∼ 1 from KMOS

We present an analysis of the gas outflow energetics from KMOS observations of ∼ 529 main-sequence star-forming galaxies at z ∼ 1 using broad, underlying H α and forbidden lines of [N II] and [S II]. Based on the stacked spectra for a sample with median star-formation rates and stellar masses of SFR = 7 M_⊙ yr^-1 and M_⋆ = (1.0 ± 0.1) × 10¹⁰ M_⊙, respectively, we derive a typical mass outflow rate of \dot{M}_wind = 1-4 M_⊙ yr^-1 and a mass loading of \dot{M}_wind / SFR = 0.2-0.4. By comparing the kinetic energy in the wind with the energy released by supernovae, we estimate a coupling efficiency between the star formation and wind energetics of ɛ ∼ 0.03. The mass loading of the wind does not show a strong trend with star-formation rate over the range ∼ 2-20 M_⊙ yr^-1, although we identify a trend with stellar mass such that dM / dt / SFR ∝ M_\star ^{0.26± 0.07}. Finally, the line width of the broad H α increases with disc circular velocity with a sub-linear scaling relation FWHM_broad ∝ v^{0.21 ± 0.05}. As a result of this behaviour, in the lowest mass galaxies (M_⋆ ≲ 10¹⁰ M_⊙), a significant fraction of the outflowing gas should have sufficient velocity to escape the gravitational potential of the halo whilst in the highest mass galaxies (M_⋆ ≳ 10¹⁰ M_⊙) most of the gas will be retained, flowing back on to the galaxy disc at later times.