Radio Continuum Observations of the Nucleus of our Galaxy and Other Normal Galaxies

Abstract

The radio properties of the nuclei of normal galaxies have been reviewed by Ekers (1974, 1978a, 1978b) and of the Galactic Centre by Oort (1977). In this report I will concentrate on material available since then. The following paper by Van der Hulst extends this review with more information on the structure of the central sources in normal galaxies.No complete high resolution maps of Sgr A have been made since the “WORST” map (Ekers et al. 1975) so I will use it as a reference for the new information which is available. At the lower frequency of 327 MHz there have been a series of occultation measurements by Gopal-Krishna and Swarup (1976). These again show the separation into two components; a nonthermal Sgr A East and a thermal Sgr A West. In addition they show some spatial variation in the nonthermal spectral index and also suggest that the nonthermal source extends over the entire region of Sgr A. From observations using the RATAN telescope Parijskij (private communication) also suggests that the Sgr A East component might just be a peak on a more symmetrical nonthermal source which is centred on Sgr A West. RATAN observations are in progress in the wavelength range from 2-6 cm and these confirm the thermal nature of Sgr A West. As the VLA elements come into operation along more than one arm of the Y it becomes possible to use it to map Sgr A. 5 and 15 GHz maps (Brown, Lo and Johnston 1978 and private communication) with a resolution of a few seconds of arc resolve the Sgr A West source into a number of components with an overall size of ˜l’ × 30’ elongated along the galactic plane and centred on the Sgr A West point source (Balick and Brown 1974). The VLA maps of Sgr A West agree well with the inner contours of the WORST map and this distribution of emission is similar to that seen in the 53μ map (Harvey, Cambell and Hofmann’ 1976). Such an association is common in HII regions and is consistent with the thermal nature of Sgr A West.