A 850 \mu m map of the interacting spiral galaxy M 51 shows well-defined spiral arms , closely resembling the structures seen in CO and HI emission .
However , most of the 850 \mu m emission originates in an underlying exponential disk , a component that has not been observed before in a face-on galaxy at these wavelengths .
The scale-length of this disk is 5.45 kpc , which is somewhat larger than the scale-length of the stellar disk , but somewhat smaller than that of atomic hydrogen .
Its profile can not be explained solely by a radial disk temperature gradient but requires the underlying dust to have an exponential distribution as well .
This reinforces the view that the submm emission from spiral galaxy disks traces total hydrogen column density , i.e .
the sum of H _ { 2 } and H i .
A canonical gas-to-dust ratio of 100 \pm 26 is obtained for \kappa _ { 850 } = 1.2 g ^ { -1 } cm ^ { 2 } , where \kappa _ { 850 } is the dust opacity at 850 \mu m .