χ2 obtained by comparing the ne function from the model to that obtained from the data (see Fig. A.1) as a function of the two free parameters, α and NHII. The minimum value (4.64) is obtained for α = −0.15 and NHII = 15 000. The thick contour corresponds to χ2 exceeding the minimum value by 2.3, as recommended by Lampton et al. (1976) to estimate the 1σ confidence level.
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