SIMBAD references

We investigate the effect of a global change in the ionizing continuum level on the behavior of the strong optical broad emission lines seen in spectra of the nuclear emission-line regions of active galactic nuclei (AGNs), including the Balmer lines, He I λ5876, and He II λ4686. Unlike most of the prominent heavy-element lines found in the UV, the optical hydrogen and helium recombination lines' emissivities are strongly dependent on the incident continuum flux, since these lines arise out of excited states whose optical depths depend on the incident flux of photons. Using photoionization calculations, we determine the luminosity-dependent responsivities, η(r,L(t))=ΔlogL_line/ΔlogL_cont, of these lines for a general model of the broad emission line region (BLR), with the purpose of establishing them as important probes of the physical conditions within the BLRs of AGNs. The dependence of these lines' emissivities on the incident photon flux invokes a dependence in their responsivities on distance from the central continuum source. In particular, the responsivities of these lines are generally anticorrelated with the incident photon flux. Thus, their responsivities vary with distance within the BLR for a fixed continuum luminosity and change with time as the continuum source varies. Consequently, after correcting for light-travel-time effects, the response of the Balmer and optical helium lines should generally be strongest during low continuum luminosity states. Responsivity that depends on photon flux and continuum state may explain a number of outstanding problems currently under investigation in broad-line variability studies of these and other emission lines. These include the origin of the intrinsic Baldwin effect, measurements of luminosity-dependent lags (a ``breathing'' BLR), and luminosity-dependent variations in integrated broad emission line flux ratios (including He II λ4686/Hβ), broad-line profile shapes, and radial velocity-dependent intensity ratios. The broad Hα/Hβ and He I/Hβ flux ratios and the Balmer emission line responsivity are observed to decrease from the line center to the line wings. These, along with our findings, lead to the conclusion that the BLR velocity field diminishes with increasing distance from the central continuum source. This is consistent with recent reverberation studies that find a relationship between the emission-line lag and rms profile width for multiple lines in individual AGNs, which implies that the velocity field is dominated by a central massive object. Finally, the responsivity of ionization-bounded clouds can account for much of the observed behavior of the optical recombination lines (e.g., the weak response of the Balmer line wings) previously attributed to a substantial contribution from matter-bounded clouds at small BLR radii.