Optically thin thermal emission as the origin of the big bump in the spectra of active galactic nuclei

The nature of the "big bump," the excess of ultraviolet emission seen in many active nuclei, is a clue to the source of the energy generation in the central regions of quasars. We report observations of the Seyfert galaxy Mrk 590, which show dramatic changes in both the continuum and hydrogen lines, thus offering a clue to the origin of the "big bump." While the continuum luminosity increased by ∼0.5 dex, the core of the Balmer lines increased by a similar amount, but the broad wings of the hydrogen lines maintained constant luminosity. If hydrogen lines are produced by photoionization, nebular theory then suggests that the core of the line arises in matter which is optically thick to the ionizing continuum, while the line wings (presumably formed nearer the central object) arise in matter which is optically thin in this continuum and which nearly fully covers the continuum source. Using the ∼very broad line region∼ (VBLR) line luminosity to establish the emission measure, we then show that emission from VBLR gas can fully account for the ultraviolet continuum if it has a temperature of ∼10⁵ K. The VBLR is the likely site for the soft X-ray ∼warm absorbing∼ material as well as the radio free-free emission. If the VBLR gas lies between the source of the power-law continuum and the broad line region, then photoelectric absorption of hard radiation (hv ≥ 50 eV) by the VBLR can produce the type of soft radiation field inferred to strike the BLR by recent studies of Fairall 9. This results in a unified picture, in which the central source of power-law radiation is surrounded by ionized partially transparent gas (the VBLR) which reprocesses ∼100 eV photons into the "big bump." This in turn is surrounded by an outer, more neutral, BLR, which produces the sharp cores of the emission lines.