Optical depth at 9.7 μm, τ_9.7 versus logarithmic of the hydrogen column density at X-rays, N_H. N_H in units of cm^-2. Spitzer/IRS data of our sample are removed here for clarity. Edge-on galaxies are shown as big red squares, galaxies with dust lanes are shown with big yellow stars, and mergers are shown as big green circles. Edge-on galaxies are shown only when dust-lanes are not present for clarity of the plot.