NGC 3646. - This galaxy has been discussed by Burbidge et al. (1961c), who note that there appears to be a considerable amount of dust between the inner and outer ellipses of this large object. The 100-inch (2.5-m) and 90-inch (2.3-m) blue photographs strongly suggest the presence of dust as distributed in the sketch. The bright HII regions identified on the ON and Off H plates are clearly associated with the more obvious dust lanes. For this galaxy, the redshift of about 4000 km s-1 is second only to NGC 2532 (v 5200 km s-1) in the sample chosen for this study. These two galaxies have several similarities; their inner spiral arms are similar in size and structure, they each have complex outer structure of very great extent (outer bright HII regions at distances of the order of 18 kpc from their respective nuclei if H = 75 km s-1 Mpc-1), and there is evidence for the presence of extensive dust lanes defining a spiral pattern throughout each galaxy.
There is, in fact, even a suggestion of a wavy segment of a spiral arm in the outer region of each galaxy. This wave structure in NGC 3646 is on the NE side of the galaxy. Burbidge et al. (1961c) have suggested that the wave represents an instability (on a scale of about 900 pc). A comparison of the ON and OFF H photographs suggests that the composition of the wave is not a young population of dust and HII regions but rather that it is a mixture of stars whose combined light is of a color red enough to be recorded in the red continuum as well as in the blue.