A STUDY OF CENTAURUS A AT 31 CENTIMETERS

Abstract

covered, and shortly after its discovery it was identified with the peculiar galaxy NGC 5128. Interferometry by Mills1 showed that there was a central source of dimensions 6' X 3', and Bolton, Westfold, Stanley, and Slee2 found that there was an associated extended source of dimensions of the order of 2°. The extended object was also observed at 400 Mc/s by McGee, Slee, and Stanley, with a pencil-beam antenna,3 and by Piddington and Trent at 600 Mc/s.4 The latter investigators suggested that the extension might form a bridge between NGC 5128 and our own galaxy. The true extent of this object has only really become apparent since the studies by Sheridan5 with the 85 Mc/s Mills cross (beam width 50') and by Shain6 with the 19.7 Mc/s Mills cross (beam width 1?4). These observations show an elongated object nearly 10° in length lying almost along a line of constant right ascension. The present study was carried out with a 90-foot equatorially mounted radio telescope at a frequency of 960 Mc/s. At this wavelength the beam width is about 50 minutes of arc. The receiver used had a conventional crystal mixer with an excess noise temperature of about 300° K. No image rejection was employed ; thus the signal was received in two bands at 930 and 990 Mc/s, each 10 Mc/s wide. The receiver input was switched at 400 cycles per second with a diode switch between the main horn feed pointed at the reflector and a reference horn directed away from the reflector. The 400 cycles per second difference signal was recorded. The observations consisted of a series of drift curves taken with the telescope fixed on a particular declination. Due to the low altitude of this object, only three observations (at different hour angles), each lasting about 40 minutes, were made on each night. One of these was for calibration purposes at declination 42°40'. The maximum signal received from the direction of the central source corresponded to an antenna temperature of 50° K, but the