Effective mass loading factor f′ _o vs. radius. f′ _o is calculated from Equation (10) and is shown to increase with radius. The measured gas fraction and metallicity of J1148 are unphysical at R < 1 kpc in the context of our chemical evolution model, and so we also show the results of perturbing the gas fraction by multiples of the uncertainty σ( f _gas) = 0.15. We note that the overall shape and normalization of f′ _o ( R) are relatively constant over a range Δ f _gas = 4σ = 0.6; clearly Equation (10) is relatively insensitive to gas fraction in this regime. Dotted lines show the result from the simple analytic approximation described in Section 5.1 (see discussion in the text). For J1148 the analytic prediction applies to the case f _gas − σ.