Per-Tau Shell Radial Profile. The abscissa is the radial distance from the shell’s center and the vertical line is its modeled radius (Table 1). Top panel: the mean density (dashed), and different density percentiles (solid). Middle and bottom panels: volume and mass filling fractions for gas with different densities. In all panels, all quantities are computed within thin concentric spherical shells (see Appendix A). The shaded regions are the minimum-to-maximum variation across different dust-map samples, showing the uncertainty associated with the 3D data (see Appendix A).