Accelerating galaxy winds during the big bang of starbursts

Abstract

ABSTRACT We develop a new method to infer the temporal, geometric, and energetic properties of galaxy outflows, by combining stellar spectral modelling to infer starburst ages, and absorption lines to measure velocities. If winds are accelerated with time during a starburst event, then these two measurements enable us to solve for the wind radius, similarly to length-scales and the Hubble parameter in big bang cosmology. This wind radius is the vital, but hard-to-constrain parameter in wind physics. We demonstrate the method using spectra of 87 starburst galaxies at z = 0.05 − 0.44, finding that winds accelerate throughout the starburst phase and grow to typical radii of ≈1 kpc in ≈10 Myr. Mass flow rates increase rapidly with time, and the mass-loading factor exceeds unity at about 10 Myr – while still being accelerated, the gas will likely unbind from the local potential and enrich the circumgalactic medium. We model the mechanical energy available from stellar winds and supernovae, and estimate that a negligible amount is accounted for in the cool outflow at early times. However, the energy deposition increases rapidly and ∼10 per cent of the budget is accounted for in the cool flow at 10 Myr, similar to some recent hydrodynamical simulations. We discuss how this model can be developed, especially for high-redshift galaxies.