FREE-FREE EMISSION AND THE BIG BLUE BUMP IN ACTIVE GALACTIC NUCLEI

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 basi c rationale for the free-free hypothesis is that the spectral shape pr edicted by this mechanism matches the shapes observed in AGNs. Median continuum slopes in the optical/UV for luminous quasars are -0.2 to -0 .3 (f(nu) is-proportional-to nu(alpha), which is nearly identical to t he asymptotic slope for free-free gas at high temperature. The identif ication of the continuum with free-free relies on the BBB emission bei ng '' pure,'' i.e., not altered by extension of an infrared power law as required in fits to optically thick, geometrically thin accretion d isk models. Much recent evidence suggests that this infrared extension does not exist; a strong case can be made that the infrared is dust e mission, which cuts off shortward of 1 mum. The combination of dust in the infrared, and free-free in the optical/UV, provides good fits to non-blazar AGN spectral energy distributions. Direct evidence for the optically thin model has been seen in the form of constant ultrabroad line wings observed in some sources, which imply free-free emission st rong enough to power the BBB. Another important observation is one whi ch strongly suggests that X-rays are reprocessed into optical/UV emiss ion in the Seyfert 1 galaxy NGC 5548, and that this reprocessed emissi on has both the spectral shape and variability characteristics of opti cally thin rather than optically thick gas. It is pointed out that for densities less than or similar 10(15) cm-3, reprocessing of X-rays (i ncluding the observed Fe line emission at 6.4 keV and the Compton refl ection hump) can occur while the optical/UV spectrum is effectively op tically thin to true absorption. Thus, the free-free-emitting clouds r esponsible for the BBB could derive part of their energy from the X-ra y spectrum, while at the same time being partly or wholly responsible for the characteristic reprocessing features discovered in the X-ray r egion over the last few years. Free-free source sizes, densities, etc. , are computed for the observed luminosity range of AGNs, and for temp eratures consistent with the optical/UV/soft X-ray spectral shapes of AGNs, 10(5) less than or similar T less than or similar 10(6) K. Sourc es are likely to be composed of large numbers of small, dense cloudlet s or filaments, which are by hypothesis effectively optically thin to free-free absorption, but which can differ in their optical depth to e lectron scattering. In low-temperature sources, the model predicts a s trong Lyman emission edge, unless Compton effects are invoked to smear the edge. In high-temperature (T approximately 10(6) K) sources, the predicted Lyman edge for optically thin gas is intrinsically quite wea k. Evidence suggests that in high-luminosity sources, where the edge h as been searched for and not found, the free-free gas may in fact be s o hot that the predicted edge is already very small. The most extreme reported BBB size constraint, set by microlensing in one source, can b e accommodated by the model proposed here.