RADIO SPECTRA OF RADIO-QUIET QUASARS

Radio continuum spectra for 32 radio quiet quasars (RQQs) and 7 lumino us Seyfert 1 galaxies, obtained with the VLA and typically covering fo ur frequencies in the observed range 1.5 to 14.9 GHz, are presented. T wo samples are considered, one selected from IRAS detections and on av erage at low redshift (16 objects), and the other selected by radio an d optical properties and covering a range of redshifts up to z=2.6 (23 objects). We find the radio spectral shapes in both samples to be qui te heterogeneous, and categorize the spectra into four general classes similar to those defined by Kuhr et al. [A&AS, 45, 367 (1981)] for ra dio loud objects: straight, concave, convex, and complex. The relative frequencies of these classifications in our radio quiet samples are s imilar to those seen in radio loud quasars and radio galaxies. Based o n this and other considerations we conclude that radio quiet and radio loud quasars have similar spectral shapes. About 40% of the RQQs stud ied here appear to have flat or inverted spectral components. Evidence is found for variability in seven sources, based on comparison with a single previous observation from the literature. At least five of the se variable sources show flat or inverted radio spectra, and none have straight, steep spectra. We have detected one of our sources with VLB I, and essentially all the flux resides in a compact, subparsec core. This source has an inverted spectrum and is variable, strongly indicat ing partially opaque synchrotron emission. Thus the cores of many radi o quiet quasars appear to be scaled-down versions of those found in ra dio loud objects. However, we cannot rule out free-free emission for t he flat spectrum component in all cases. Radio quiet quasar spectra pr obably include contributions from three components, in varying degrees : optically thin synchrotron from star-forming regions in the disk of the host galaxy and in a circumnuclear starburst; optically thin synch rotron from an extended (perhaps jet-like) component powered by the ac tive nucleus; and partially opaque synchrotron from a compact, VLBI-sc ale core. All three of these components may play a role in the radio s pectra of low-redshift quasars, but only the latter two are significan t contributors in luminous, high-redshift objects. (C) 1996 American A stronomical Society.