Fit with all six free parameters (blue long-dashed lines), fit with a_z = a_Rleft as a free parameter (cyan short-dashed lines), fit with a_z fixed at 6.6 kpc (twice the disk scale-length) and a_R free (magenta dotted lines), fit with a_z fixed at 6.6 kpc (twice the disk scale-length) and a_R fixed at the disk scale-length, 3.3 kpc (black solid lines), and fit it with a_z = a_R left as a free parameter, and with σ_z/σ_R fixed at 0.63 (red dot-dashed lines). Note that the bottom panel has a different horizontal scale from the top three panels. In the bottom panel the Hα data (full circles) come from Palunas & Williams (2000).