The Systematic Properties of the Warm Phase of Starburst-Driven Galactic Winds

Using ultraviolet absorption lines, we analyze the systematic properties of the warm ionized phase of starburst-driven winds in a sample of 39 low-redshift objects that spans broad ranges in starburst and galaxy properties. Total column densities for the outflows are ∼10²¹ cm^-2. The outflow velocity (v_out) correlates only weakly with the galaxy stellar mass ({M}_*), or circular velocity (v_cir), but strongly with both SFR and SFR/area. The normalized outflow velocity ({v}_{out}/{v}_{cir}) correlates well with both SFR/area and SFR/{M}_*. The estimated outflow rates of warm ionized gas (\dot{M}) are ∼1-4 times the SFR, and the ratio \dot{M}/{SFR} does not correlate with v_out. We show that a model of a population of clouds accelerated by the combined forces of gravity and the momentum flux from the starburst matches the data. We find a threshold value for the ratio of the momentum flux supplied by the starburst to the critical momentum flux needed for the wind to overcome gravity acting on the clouds (R_crit). For {R}_{crit} \gt 10 (strong-outflows) the outflow’s momentum flux is similar to the total momentum flux from the starburst and the outflow velocity exceeds the galaxy escape velocity. Neither of these is the case for the weak outflows ({R}_{crit} \lt 10). For the weak-outflows, the data severely disagree with many prescriptions in numerical simulations or semi-analytic models of galaxy evolution. The agreement is better for the strong outflows, and we advocate the use of R_crit to guide future prescriptions.