Free-free emission and the big blue bump in active galactic nuclei

Abstract

This paper presents various lines of evidence and argument in favor of the free-free, or optically thin thermal, hypothesis for the optical/UV big blue bump (BBB) in active galactic nuclei (AGNs). The most basic rationale for the free-free hypothesis is that the spectral shape predicted by this mechanism matches observed in AGNs. Median continuum slopes in the optical/UV for luminous quasars are −0.2 to −0.3 (f ν ∞ν α ), which is nearly identical to the asymptotic slope for free-free gas at high temperature. The identification the continuum with free-free relies on the BBB emission being pure, i.e., not altered by extension of an infrared power law as required in fits to optically thick, geometrically thin accretion disk models