The Radio Continuum Morphology of Spiral Galaxies

Abstract

Once Jansky realized that the extraterrestial microwave radiation which he had discovered was not coming from the Sun but from a direction near to that of the Galactic center (Jansky 1933) and after he established that the emission was associated with the Milky Way (Jansky 1935), he naturally concluded that either stars in the disk or interstellar matter in the Galaxy was responsible for the emission ofthis radiation. Reber ( 1940) confirmed this and was the first to try to detect similar emission from the nearest external spiral galaxy M31 in Andromeda. He gives a positive result at 160 MHz of about 3800 Jy, 1 corresponding to a main-beam bright­ ness temperature of � lOooK assuming that the true beam width of his telescope was about 12°. In a later paper (Reber 1944) he gives an inconclusive detection of about 960 Jy corresponding to � 25°K. We know now that the latter value of flux density is an order of magnitude too high to be a detection; the brightness temperature indicates that what Reber observed was probably part of the complicated structure in the galactic background [see for example the all-sky map of Landecker & Wielebinski ( 1970)]. However, with the Galaxy dominating the radio sky it is under­ standable that the nearest spiral galaxies were expected to be detectable. Ryle, Smith & Elsmore (1950) were therefore not surprised that four ofthe weakest sources in their first Cambridge Survey appeared to be associated with the brightest spiral galaxies. Their quoted flux densities are now known also to be in error; in their case it is most likely that their observations were adversely affected by confusion with neighboring sources. Detection of spiral galaxies turned out to be possible only with telescopes that were not confused at their detection limit and that had a resolving power comparable with the size of the galaxies. It was with such an instrument that Hanbury Brown & Hazard ( 1951) made the first incontrovertible observation of the