Observational study of the spiral gaalxy NGC 6946. I. HI and radio continuum observations.

We present H I and radio continuum observations of the Scd galaxy NGC 6946 at ~25" resolution, obtained with the Westerbork Synthesis Radio Telescope and the Penticton interferometer. The data from the two interferometers have been combined to measure the emission of the galaxy on all angular scales down to 25". These observations provide a detailed view at the structure of the radio-continuum and at the distribution and kinematics of the H I gas. We show that about 1/3 of the 21 and 50 cm radio-continuum emission comes from the spiral arms; this emission which has an average spectral index of 0.55 is mainly non-thermal. The other 2/3 of the emission comes from an exponential disk which has a spectral index of 0.8 and a scale- length of 1.7' similar to the one measured for the blue light. The highest concentrations of H I emission coincide with the spiral arms of the galaxy. The spiral structure which is rather chaotic within the optically bright part of the galaxy, has much contrast in the outer parts where two well delineated spiral arms can be followed over several tens of kpc. Departures from the general movement of rotation of ~10-20 km s^-1^ are seen in the spiral arms. The H I spiral arms are fragmented in giant complexes with masses of a few 10^7^ M_sun_ and lengths of a few kpc. Several holes with sizes up to 2 kpc and missing masses a few 10^7^ M_sun_ are seen in the distribution of the HI gas. They could be the end result of intense star formation similar to the one seen to happen in several giant H II regions detected in radio-continuum. The velocity dispersion of the gas, σ_v_, decreases systematically with galacto-centric distance from 16 km s^-1^ in the inner disk to 8 km s^-1^ at the edge of the optical disk. The spectra are shown to have non-gaussian profiles suggesting the existence of a fast gas component sharing a wider velocity dispersion than the bulk of the gas. This fast component accounts for about 40% of the total kinetic energy present in the relative motions of the atomic gas on scales smaller than 1 kpc. A few isolated clouds of ~10^6^ M_sun_, spatially unresolved, detected at velocities differing by about 40 km s^-1^ from the main component of the emission, appear as extreme examples of gas with fast motions.