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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. These results confirm the claim that spiral galaxies are transparent, at least in the outer parts, and clearly contradict the claim by Valentijn (1991) that the dark matter in spiral galaxies resides in the disk, in the form of absorbing clouds particularly abundant in the outer parts.
Bosma et al. (Sun,) studied this question.