| Abstract | H I synthesis observations and optical surface photometry of the nearly face-on ScI galaxy NGC 3938 are reported. The rotation curve and the radial variation of the H I surface density optical surface brightness are derived, and indicate that the galaxy is a normal system of its type with no evidence of a systematic pattern of z-motions in the H I layer in excess of 5 km/s. The first three moments of the profile are calculated over the range of velocity in which a signal appears, and Gaussians are fitted, which indicate that the method can produce serious bias. A constancy is observed and related to the sharp increase in the cooling rate of neutral gas at 10,000 K, which is evidence that the dark mass indicated by the flat rotation curves of spiral galaxies resides in the halos of these systems. |
