Figure 12

Figure 12: The spiral galaxy NGC 6946 as it appears in the optical (color composite from the BV R bands, left; image obtained by SSM with Rachel Kuzio de Naray using the Kitt Peak 2.1 m telescope), near-infrared (J HK bands, middle [209]), and in atomic gas (21 cm radiaiton, right [481]). The images are shown at the same physical scale, illustrating how the atomic gas typically extends to greater radii than the stars. Images like these are used to construct mass models representing the observed distribution of baryonic mass.