High-Resolution Observations of Compact Radio Sources at 6 and 18 Centimeters

Abstract

The small-scale structure of extragalactic radio sources has been studied at 18 and 6 cm wavelengths by using a tape-recording interferometer. At the longest antenna spacing, 10536 km, the baseline at 6 cm was 176 million wavelengths and the resolution was about 0'.'0004 for the stronger sources. Many sources, including optically identified galaxies and QSOs, are found to have several distinct components of widely differing size in the range from a few hundredths of a second of arc to the limit of our resolution. In general, the smallest components are strongest at the shortest wavelengths, and the dimensions are in good agreement with those expected if the low-frequency cutoffs are due to synchrotron self-absorption. The magnetic field strengths deduced from our observations and the sell- absorption cutoff frequency are typically of the order of 10- gauss. The maximum brightness temperatures observed are 10 0 K. Many sources, including 0106+01, 3C 273, 3C 279,1555+00, 2145+06, 3C 446, 3C 454.3, and 2345-16 all contain components which are unresolved on the longest baseline and are less than 010004 in diameter. Observations at shorter wavelengths are required to resolve these sources. Because the maxirnum brightness temperature is limited to 10 0 K by inverse Compton scattering, baselines no greater than the diameter of the Earth are probably adequate to resolve all of the stronger extragalactic sources. One source, 3C 84, has shown an apparent increase in angnlar size of about 35 percent in one year.