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The opacity of spiral disks

The opacity of the disks of spiral galaxies can be evaluated by studying the gaseous component in edge-on galaxies in several wavelength regions. By combining data of the galaxy NGC 891 at 21 cm (H I), 2.6 mm (CO), and Hα, we show that the outer regions are transparent, while the inner regions, out to the radius to which CO is detected, could be optically thick. For the small galaxy NGC 100, such data indicate that this galaxy is by and large transparent, despite the presence of dust patches in the optical image