NGC-1058 - GAS MOTIONS IN AN EXTENDED, QUIESCENT SPIRAL DISK

We investigate in detail the motion of gas in the galaxy NGC 1058, using the VLA to map the emission in the 21 cm line. This galaxy is so nearly face-on that the contribution to the line width due to the variation of the rotational velocity across the D-array beam is small compared with the random z-motion of the gas. We confirm results of earlier studies, including the fact that the rotation curve drops faster than Keplerian at the outer edge of the disk, which we interpret as a fortuitous twist of the plane of rotation in the outer disk. Our very high velocity resolution (2.58 km s-1 after Hanning smoothing) allows us to measure the line width accurately throughout the disk. This width decreases with optical surface brightness, but in the extended gas disk beyond the Holmberg radius the line width is remarkably constant, with σv ≃ 5.7 km s-1. There is no correlation of line width with spiral phase or H I surface density in the outer disk. If the line width is determined by microscopic thermal motions, this implies a surprisingly cool temperature for the H I gas, while if it represents the velocity distribution of cold clouds, then its lack of spatial variation is remarkable. We see evidence for an "intermediate velocity cloud" population whose velocity width is several times broader than that of the bulk of the H I in the optically bright disk. This second Gaussian component is not present in the outer, optically very faint disk.