Excess 2 centimeter emission - A new continuum component in the spectra of radio-quiet quasars

Abstract

We are studying the centimeter and millimeter regions of the spectra of radio-quiet quasars and luminous Seyfert 1 nuclei. For most objects, steep-spectrum optically thin synchrotron radiation dominates in the 20 cm to 6 cm region. However, in at least nine cases out of our sample of 17, a flat or rising component becomes dominant by 2 cm. We identify this as a new continuum component, which we call the high-frequency excess (HFE). In this Letter, we highlight the luminous quasar PG 1634 + 706, for which the HFE dominates throughout the centimeter region. For this particular object, extrapolation of this new component underpredicts the infra-red fluxes and predicts the "big blue bump" approximately correctly. Therefore it is unrelated to the former and could possibly be the tail of the latter. We can't yet determine the spectral indices of the HFE in the other objects. We consider several interesting possibilities for the cause or causes of the HFE: self-absorbed synchrotron sources, ~10 6 K nuclear free-free sources, and ~10 4 K free-free sources from associated nuclear starbursts. Although we cannot yet say which explanation is correct, the brightness temperature predictions are very distinct , so future observations should provide the answer.