Recent Radio Studies of Bright Galaxies

Abstract

Some results are discussed of recent – still largely unpublished – high-resolution studies of bright spirals, both in the radio continuum and in the 21-cm line radiation. Special emphasis is given to observations of M51, M81 and NGC 4258.The most important new data are (1) Estimates of the contrast in gas density between arms and interarm regions (M81 and M101); the contrast appears to be quite strong. It provides an important parameter for the density wave. (2) Evidence for a pronounced decrease of gas density along the arms when these are followed towards the centre. Considerable line radiation is observed from outer regions where the optical intensity of the arms is very low. (3) Extension of rotation curves to larger distances from the centre. (4) Data on the motions in the spiral waves. (5) Determination of the shift between synchrotron and optical arms, providing direct evidence for the formation of stars as a consequence of the passage of the interstellar gas through the spiral density wave. From the shift measured in M51 the formation time is found to be roughly ten million years. If the data for (2) are ascribed to the depletion of the gas by star formation, the average net fraction of the gas consumed in star formation is found to be between 2% and 3% per passage through the spiral wave (Table I). (6) Separation of nuclear and disk synchrotron radiation in spirals. (7) Evidence for a recent expulsion of about 108 solar masses from the nuclear region of NGC 4258.