A radio and optical study of the galaxy PKS 1718-649.

Results are presented for radio continuum measurements (408 to 5000 MHz), H I 21-cm line observations, and optical spectrophotometry of the 15.5-magnitude D galaxy identified with the radio source PKS 1718-649. It is shown that this galaxy has spiral arms of very low surface brightness and an unusually high content of neutral hydrogen corresponding to about 6% of the indicative mass (about 450 billion solar masses), which is significantly higher than is normal for early-type spirals. This galaxy is classified as type SAB(s)bP in the de Vaucouleurs and de Vaucouleurs (1964) system. The radio continuum is found to have an inverted power-law spectrum from 408 to 2700 MHz and to be coincident with the optical centroid of the galaxy within the accuracy of the radio observations. It is suggested that the strong optical emission lines are emitted by a gas ionized by moderate-velocity shock waves, that the kinetic-energy content of mass motions in the ionized gas could provide the observed emission-line luminosity for less than 1300 yr, and that these motions are likely to be driven continuously by a source of energy that may be associated with the compact radio source.