Radio spectra of radio quiet quasars

Radio continuum spectra for 32 radio quiet quasars (RQQs) and 7 luminous Seyfert 1 galaxies, obtained with the VLA and typically covering four frequencies in the observed range 1.5 to 14.9 GHz, are presented. Two samples are considered, one selected from IRAS detections and on average at low redshift (16 objects), and the other selected by radio and 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 quite heterogeneous, and categorize the spectra into four general classes similar to those defined by Kuhr et al. [A&AS, 45, 367 (1981)] for radio loud objects: straight, concave, convex, and complex. The relative frequencies of these classifications in our radio quiet samples are similar to those seen in radio loud quasars and radio galaxies. Based on this and other considerations we conclude that radio quiet and radio loud quasars have similar spectral shapes. About 40% of the RQQs studied 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 these variable sources show flat or inverted radio spectra, and none have straight, steep spectra. We have detected one of our sources with VLBI, and essentially all the flux resides in a compact, subparsec core. This source has an inverted spectrum and is variable, strongly indicating partially opaque synchrotron emission. Thus the cores of many radio quiet quasars appear to be scaled-down versions of those found in radio loud objects. However, we cannot rule out free-free emission for the flat spectrum component in all cases. Radio quiet quasar spectra probably 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 synchrotron from an extended (perhaps jet-like) component powered by the active nucleus; and partially opaque synchrotron from a compact, VLBI-scale core. All three of these components may play a role in the radio spectra of low-redshift quasars, but only the latter two are significant contributors in luminous, high-redshift objects. (C) 1996 American Astronomical Society.