Witnessing the Earliest Phases of Galactic Wind Formation in Superluminous Supernova Hosts: Implications for Star-formation-driven Outflows in Low- and High-redshift Galaxies

Direct observational constraints on the earliest phases of galactic outflows remain scarce. We present Very Large Telescope/X-shooter spectroscopy of seven Type I superluminous supernova (SLSN-I) host galaxies at z = 0.15−0.51, exploiting the bright SLSN continua as down-the-barrel probes of the host interstellar medium. Five of the seven hosts exhibit narrow, blueshifted Mg II λλ 2796, 2803 absorption, indicative of the presence of low-ionized outflows. Voigt modeling of the Mg II absorption yields bulk outflow velocities of 45−99 km s −1 and terminal velocities of v max ≃ 103 − 164 km s − 1 , the latter being below the ones reported for more evolved galaxies of similarly low mass and star formation rate. Given the expected short lifetimes of massive SLSN progenitors, we argue that these outflows are most naturally explained as originating during the preceding stellar-wind-dominated phase. We derive wind masses of M wind ≃ 10 4 −10 5 M ⊙ , mass outflow rates comparable to or lower than the star formation rates, and kinetic energies of 10 51 −10 52 erg. These energies represent only a modest fraction (≃10%) of the mechanical energy expected from stellar winds during the first few Myr, demonstrating that the observed outflows can be powered entirely by presupernova feedback. Together with their low terminal velocities, these results support a scenario in which stellar winds and radiation pressure dominate the early acceleration of cold outflows, before core-collapse supernovae become dynamically important. Our work provides one of the first empirical constraints on early feedback phases relevant for high-redshift galaxies and for time-dependent implementations of stellar feedback in galaxy formation simulations.