Broadband Studies of Compact Sources

Abstract

Progress is reported on a program to study the polarization, spec tral and variability properties of compact sources at centimeter and millimeter wavelengths. Source characteristics divide according to the broadband shapes of their spectra.We report here progress on an extensive program to study the phys ical properties of compact radio sources through their polarization, spectral and variation characteristics. The program includes simul taneous polarimetry at centimeter and millimeter wavelengths for a sample of 20 active sources. When possible, optical and infrared pho tometry and polarimetry have been obtained, as well.In addition, we have obtained VLA observations (at 2, 6 and 20cm) of an unbiased sample of 40 flat spectrum (a > -.5, S ~ v a ) sources in the S4 survey and a sample of 20 strong nuclear components of classical doubles. This ob servational work is supported by theoretical calculations of the appear ance and evolution of compact sources. An analysis of the spectral and variability characteristics of the active sources has been reported elsewhere (Jones et al. 1981).A preliminary study of the S4 sources also has been reported (Rudnick and Jones 1982) .The study has shown a strong relationship between the polarization and variability of sources and their spectral shape.If we divide the S4 sources into groups with straight or power spectra (\, e.g., 0954+ 55), simple convex or "humped" spectra (s*\> e.g., 0923+39), and complex spectra e.g., 1641+39), we find that only the complex sources com monly exhibit large amplitude variability on timescales of a few years. The simple convex and straight spectrum sources appear relatively con stant, although they are mostly unresolved at 6cm on the VLA (<0V2) and, therefore, of kiloparsec or smaller dimensions. The straight spectrum sources have a median polarization ~3% at 2cm, but the value decreases to -0.3% at 20cm indicating considerable Faraday depolarization. This can be explained if these sources have internal electron densities ~10 -2 cm~3 as if, for example, they are confined in the inner regions of 329 D. S. Heeschen and C. M. Wade (eds.), Extragalactic Radio Sources, 329-330.