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Star formation driven outflows are a critically important phenomenon in theoretical treatments of galaxy evolution, despite the limited ability of observational studies to trace galactic winds across cosmological time‐scales. It has been suggested that the strongest Mg  ii  absorption‐line systems detected in the spectra of background quasars might arise in outflows from foreground galaxies. If confirmed, such ‘ultrastrong’ Mg  ii  (USMg  ii ) absorbers would represent a method to identify significant numbers of galactic winds over a huge baseline in cosmic time, in a manner independent of the luminous properties of the galaxy. To this end, we present the first detailed imaging and spectroscopic study of the fields of two USMg  ii  absorber systems culled from a statistical absorber catalogue, with the goal of understanding the physical processes leading to the large velocity spreads that define such systems.  Each field contains two bright emission‐line galaxies at similar redshift ( Δ v ≲ 300  km s  −1  ) to that of the absorption. Lower limits on their instantaneous star formation rates (SFRs) from the observed [O  ii ] and H β  line fluxes, and stellar masses from spectral template fitting indicate specific SFRs among the highest for their masses at these redshifts. Additionally, their 4000‐Å break and Balmer absorption strengths imply they have undergone recent ( ∼ 0.01–1 Gyr) starbursts. The concomitant presence of two rare phenomena – starbursts and USMg  ii  absorbers – strongly implies a causal connection. We consider these data and USMg  ii  absorbers in general in the context of various popular models, and conclude that galactic outflows are generally necessary to account for the velocity extent of the absorption. We favour starburst‐driven outflows over tidally stripped gas from a major interaction, which triggered the starburst as the energy source for the majority of systems. Finally, we discuss the implications of these results and speculate on the overall contribution of such systems to the global SFR density at   z ≃ 0.7 .

Large‐scale outflows from z ≃ 0.7 starburst galaxies identified via ultrastrong Mg ii quasar absorption lines