The Outflowing [O II] Nebulae of Compact Starburst Galaxies at z ∼ 0.5

Abstract

High-velocity outflows are ubiquitous in compact, massive (M _* ∼ 10¹¹ M _⊙), z ∼ 0.5 galaxies with extreme star formation surface densities (Σ_SFR ∼ 2000 M _⊙ yr^‑1 kpc^‑2). We have previously detected and characterized these outflows using Mg II absorption lines. To probe their full extent, we present Keck/KCWI integral field spectroscopy of the [O II] and Mg II emission nebulae surrounding all of the 12 galaxies in this study. We find that [O II] is more effective than Mg II in tracing low surface brightness, extended emission in these galaxies. The [O II] nebulae are spatially extended beyond the stars, with radial extent R ₉₀ between 10 and 40 kpc. The nebulae exhibit nongravitational motions, indicating galactic outflows with maximum blueshifted velocities ranging from ‑335 to ‑1920 km s^‑1. The outflow kinematics correlate with the bursty star formation histories of these galaxies. Galaxies with the most recent bursts of star formation (within the last <3 Myr) exhibit the highest central velocity dispersions (σ ≳ 400 km s^‑1), while the oldest bursts have the lowest-velocity outflows. Many galaxies exhibit both high-velocity cores and more extended, slower-moving gas indicative of multiple outflow episodes. The slower, larger outflows occurred earlier and have decelerated as they propagate into the circumgalactic medium and mix on timescales ≳50 Myr.